smtp.c

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Author : humphreyj
Date : 2018-08-12 07:51:54
Hash : 27c8a717
Message : Add Valgrind suppression file to block warnings coming from the allocation routines in OpenSSL. Fix issue reported by Valgrind where empty responses from the server were getting written to before writing to stderr.
src/smtp.c
/**
 * @file
 * @brief SMTP client library.
 * @author James Humphrey (mail@somnisoft.com)
 * @version 0.99
 *
 * This SMTP client library allows the user to send emails to an SMTP server.
 * The user can include custom headers and MIME attachments.
 *
 * This software has been placed into the public domain using CC0.
 */

/**
 * @mainpage smtp-client
 *
 * This section contains documentation generated directly from the source
 * code.
 *
 * To view the repository details, visit the main smtp-client page at
 * <a href='https://www.somnisoft.com/smtp-client'>
 * www.somnisoft.com/smtp-client
 * </a>.
 */

#if defined(_WIN32) || defined(WIN32)
# define SMTP_IS_WINDOWS
#endif /* SMTP_IS_WINDOWS */

#ifdef SMTP_IS_WINDOWS
# include <winsock2.h>
# include <ws2tcpip.h>
#else /* POSIX */
/**
 * Need to define this on some POSIX systems in order to get access to the
 * getaddrinfo and localtime_r functions.
 */
# define _POSIX_C_SOURCE 200112L
# include <netinet/in.h>
# include <sys/select.h>
# include <sys/socket.h>
# include <netdb.h>
#endif /* SMTP_IS_WINDOWS */

#include <errno.h>
#include <limits.h>
#include <stdarg.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>

#ifdef SMTP_OPENSSL
# include <openssl/bio.h>
# include <openssl/err.h>
# include <openssl/ssl.h>
# include <openssl/x509.h>
# include <openssl/x509v3.h>
#endif /* SMTP_OPENSSL */

/**
 * Get access to the @ref smtp_result_code and @ref smtp_command definitions.
 */
#define SMTP_INTERNAL_DEFINE

#include "smtp.h"

/*
 * The SMTP_TEST converts some library routines into special test seams which
 * allows the test program to control whether they fail. For example, we can
 * control when smtp_malloc fails under certain conditions with an out of
 * memory condition.
 */
#ifdef SMTP_TEST
/**
 * Declare extern linkage on some functions so we can redefine their behavior
 * in the external test suite.
 */
# define SMTP_LINKAGE extern
# include "../test/seams.h"
#else /* !(SMTP_TEST) */
/**
 * When not testing, all functions should have static linkage except for those
 * in the header.
 */
# define SMTP_LINKAGE static
#endif /* SMTP_TEST */

/**
 * Increment the read buffer size by this amount if the delimiter
 * has not been found.
 */
#define SMTP_GETDELIM_READ_SZ 1000

/**
 * Stores source and destination email addresses.
 */
struct smtp_address{
  /**
   * Specify from, to, cc, bcc.
   */
  enum smtp_address_type type;

  /**
   * Email address without any special formatting.
   *
   * For example: mail@example.com
   */
  char *email;

  /**
   * Description of the email address.
   */
  char *name;
};

/**
 * Attachment data which gets placed in the MIME email section.
 */
struct smtp_attachment{
  /**
   * Filename of the attachment.
   */
  char *name;

  /**
   * Encode all attachment data in base64 format.
   */
  char *b64_data;
};

/**
 * List of email headers to send before the mail body.
 */
struct smtp_header{
  /**
   * Header name which will get used to store the header list alphabetically.
   */
  char *key;

  /**
   * Contents to send for the corresponding header key.
   */
  char *value;
};

/**
 * Main data structure used fo holding the SMTP client context.
 */
struct smtp{
  /**
   * Bitwise list of flags controlling the behavior of this SMTP client.
   */
  enum smtp_flag flags;

  /**
   * Standard network socket connection.
   */
  int sock;

  /**
   * Read buffer and line parsing structure.
   */
  struct str_getdelimfd gdfd;

  /**
   * List of headers to print before the mail body.
   */
  struct smtp_header *header_list;

  /**
   * Number of headers in header_list.
   */
  size_t num_headers;

  /**
   * List of from, to, cc, and bcc email addresses.
   */
  struct smtp_address *address_list;

  /**
   * Number of addresses in address_list.
   */
  size_t num_address;

  /**
   * List of attachments to send.
   */
  struct smtp_attachment *attachment_list;

  /**
   * Number of attachments in attachment_list.
   */
  size_t num_attachment;

  /**
   * Status code indicating success/failure.
   *
   * This code gets returned by most of the header functions.
   */
  enum smtp_status_code status_code;

  /**
   * Timeout in seconds to wait before returning with an error.
   *
   * This applies to both writing to and reading from a network socket.
   */
  long timeout_sec;

  /**
   * Indicates if this context has an active TLS connection.
   *   - Set to 0 if TLS connection inactive.
   *   - Set to 1 if TLS connection currently active.
   */
  int tls_on;

  /**
   * Path to certificate file if using self-signed or untrusted certificate
   * not in the default key store.
   */
  const char *cafile;

#ifdef SMTP_OPENSSL
  /**
   * OpenSSL TLS object.
   */
  SSL *tls;

  /**
   * OpenSSL TLS context.
   */
  SSL_CTX *tls_ctx;

  /**
   * OpenSSL TLS I/O abstraction.
   */
  BIO *tls_bio;
#endif /* SMTP_OPENSSL */
};

/**
 * Wait until more data has been made available on the socket read end.
 *
 * @param[in] smtp SMTP client context.
 * @retval  0 If data available to read on the socket.
 * @retval -1 If the connection times out before any data appears on the
 *            socket.
 */
static int
smtp_str_getdelimfd_read_timeout(struct smtp *const smtp){
  fd_set readfds;
  struct timeval timeout;
  int sel_rc;

  FD_ZERO(&readfds);
  FD_SET(smtp->sock, &readfds);
  timeout.tv_sec  = smtp->timeout_sec;
  timeout.tv_usec = 0;
  sel_rc = select(smtp->sock + 1, &readfds, NULL, NULL, &timeout);
  if(sel_rc < 1){
    return smtp_status_code_set(smtp, SMTP_STATUS_RECV);
  }
  return smtp->status_code;
}

/**
 * This function gets called by the @ref smtp_str_getdelimfd interface when it
 * needs to read in more data.
 *
 * It reads using either the plain socket connection if encryption not
 * enabled, or it reads using OpenSSL if it has an active TLS connection.
 *
 * @param[in]  gdfd  See @ref str_getdelimfd.
 * @param[out] buf   Pointer to buffer for storing bytes read.
 * @param[in]  count Maximum number of bytes to try reading.
 * @retval >=0 Number of bytes read.
 * @retval -1  Failed to read from the socket.
 */
static ssize_t
smtp_str_getdelimfd_read(struct str_getdelimfd *const gdfd,
                         void *buf,
                         size_t count){
  struct smtp *smtp;
  ssize_t bytes_read;

  smtp = gdfd->user_data;

  if(smtp_str_getdelimfd_read_timeout(smtp) != SMTP_STATUS_OK){
    return -1;
  }

  bytes_read = 0;
  if(smtp->tls_on){
#ifdef SMTP_OPENSSL
    do{
      bytes_read = SSL_read(smtp->tls, buf, count);
    } while(bytes_read <= 0 && BIO_should_retry(smtp->tls_bio));
#endif /* SMTP_OPENSSL */
  }
  else{
    bytes_read = recv(smtp->sock, buf, count, 0);
  }
  return bytes_read;
}

/**
 * Find and return the location of the delimiter character in the
 * search buffer.
 *
 * This function gets used by the main socket parsing function which
 * continually reads from the socket and expands the buffer until it
 * encounters the expected delimiter. This function provides the logic
 * to check for the delimiter character in order to simplify the code
 * in the main parse function.
 *
 * @param[in]  buf       Search buffer used to find the delimiter.
 * @param[in]  buf_len   Number of bytes to search for in buf.
 * @param[in]  delim     The delimiter to search for in buf.
 * @param[out] delim_pos If delimiter found in buf, return the delimiter
 *                       position in this parameter.
 * @retval 1 If the delimiter character found.
 * @retval 0 If the delimiter character not found.
 */
static int
smtp_str_getdelimfd_search_delim(const char *const buf,
                                 size_t buf_len,
                                 int delim,
                                 size_t *const delim_pos){
  size_t i;

  *delim_pos = 0;
  for(i = 0; i < buf_len; i++){
    if(buf[i] == delim){
      *delim_pos = i;
      return 1;
    }
  }
  return 0;
}

/**
 * Set the internal line buffer to the number of bytes specified.
 *
 * @param[in] gdfd     See @ref str_getdelimfd.
 * @param[in] copy_len Number of bytes to copy to the internal line buffer.
 * @retval  0 Successfully allocated and copied data over to the new
 *            line buffer.
 * @retval -1 Failed to allocate memory for the new line buffer.
 */
static int
smtp_str_getdelimfd_set_line_and_buf(struct str_getdelimfd *const gdfd,
                                     size_t copy_len){
  if(gdfd->line){
    free(gdfd->line);
  }
  if((gdfd->line = calloc(1, copy_len + 1)) == NULL){
    return -1;
  }
  memcpy(gdfd->line, gdfd->_buf, copy_len);
  gdfd->line_len = copy_len;
  memmove(gdfd->_buf, gdfd->_buf + copy_len + 1, gdfd->_buf_len - copy_len);
  if(gdfd->_buf_len != 0){
    gdfd->_buf_len -= copy_len + 1;
  }
  return 0;
}

/**
 * Free memory in the @ref str_getdelimfd data structure.
 *
 * @param[in] gdfd Frees memory stored in this socket parsing structure.
 */
SMTP_LINKAGE void
smtp_str_getdelimfd_free(struct str_getdelimfd *const gdfd){
  free(gdfd->_buf);
  free(gdfd->line);
  gdfd->_buf = NULL;
  gdfd->_bufsz = 0;
  gdfd->_buf_len = 0;
  gdfd->line = NULL;
  gdfd->line_len = 0;
}

/**
 * Read and parse a delimited string using a custom socket read function.
 *
 * This interface handles all of the logic for expanding the buffer,
 * parsing the delimiter in the buffer, and returning each "line"
 * to the caller for handling.
 *
 * @param[in] gdfd See @ref str_getdelimfd.
 * @return @ref str_getdelim_retcode.
 */
SMTP_LINKAGE enum str_getdelim_retcode
smtp_str_getdelimfd(struct str_getdelimfd *const gdfd){
  size_t delim_pos;
  ssize_t bytes_read;
  void *read_buf_ptr;
  char *buf_new;
  size_t buf_sz_remaining;
  size_t buf_sz_new;

  if(gdfd->getdelimfd_read == NULL){
    return STRING_GETDELIMFD_ERROR;
  }

  bytes_read = -1;

  while(1){
    if(smtp_str_getdelimfd_search_delim(gdfd->_buf,
                                        gdfd->_buf_len,
                                        gdfd->delim,
                                        &delim_pos)){
      if(smtp_str_getdelimfd_set_line_and_buf(gdfd, delim_pos) < 0){
        smtp_str_getdelimfd_free(gdfd);
        return STRING_GETDELIMFD_ERROR;
      }
      return STRING_GETDELIMFD_NEXT;
    }else if(bytes_read == 0){
      if(smtp_str_getdelimfd_set_line_and_buf(gdfd, gdfd->_buf_len) < 0){
        smtp_str_getdelimfd_free(gdfd);
        return STRING_GETDELIMFD_ERROR;
      }
      return STRING_GETDELIMFD_DONE;
    }

    buf_sz_remaining = gdfd->_bufsz - gdfd->_buf_len;
    if(buf_sz_remaining < SMTP_GETDELIM_READ_SZ){
      buf_sz_new = buf_sz_remaining + SMTP_GETDELIM_READ_SZ;
      buf_new = realloc(gdfd->_buf, buf_sz_new);
      if(buf_new == NULL){
        smtp_str_getdelimfd_free(gdfd);
        return STRING_GETDELIMFD_ERROR;
      }
      gdfd->_buf = buf_new;
      gdfd->_bufsz = buf_sz_new;
    }
    read_buf_ptr = gdfd->_buf + gdfd->_buf_len;
    bytes_read = (*gdfd->getdelimfd_read)(gdfd,
                                          read_buf_ptr,
                                          SMTP_GETDELIM_READ_SZ);
    if(bytes_read < 0){
      smtp_str_getdelimfd_free(gdfd);
      return STRING_GETDELIMFD_ERROR;
    }
    gdfd->_buf_len += bytes_read;
  }
}

/**
 * Copy a string into a new dynamically allocated buffer.
 *
 * Returns a dynamically allocated string, with the same contents as the
 * input string. The caller must free the returned string when finished.
 *
 * @param[in] s String to duplicate.
 * @retval char* Pointer to a new dynamically allocated string duplicated
 *               from @p s.
 * @retval NULL  Failed to allocate memory for the new duplicate string.
 */
SMTP_LINKAGE char *
smtp_strdup(const char *s){
  char *dup;
  size_t dup_len;

  dup_len = strlen(s) + 1;
  if((dup = malloc(dup_len)) != NULL){
    memcpy(dup, s, dup_len);
  }
  return dup;
}

/**
 * Search for all substrings in a string and replace each instance with a
 * replacement string.
 *
 * @param[in] search  Substring to search for in @p s.
 * @param[in] replace Replace each instance of the search string with this.
 * @param[in] s       Null-terminated string to search and replace.
 * @retval char* A dynamically allocated string with the replaced instances
 *               as described above. The caller must free the allocated
 *               memory when finished.
 * @retval NULL  Memory allocation failure.
 */
SMTP_LINKAGE char *
smtp_str_replace(const char *const search,
                 const char *const replace,
                 const char *const s){
  size_t search_len;
  size_t replace_len;
  size_t slen;
  size_t s_idx;
  int found_matches;
  char *snew;
  size_t snew_len;
  size_t snew_sz;
  char *stmp;

  search_len    = strlen(search);
  replace_len   = strlen(replace);
  slen          = strlen(s);
  s_idx         = 0;
  found_matches = 0;
  snew          = NULL;
  snew_len      = 0;
  snew_sz       = 0;

  if(s[0] == '\0'){
    return smtp_strdup("");
  }
  else if(search_len < 1){
    return smtp_strdup(s);
  }

  while(s[s_idx]){
    if(strncmp(&s[s_idx], search, search_len) == 0){
      if(snew_len + replace_len + 1 >= snew_sz){
        snew_sz += snew_sz + slen + replace_len + 1;
        if((stmp = realloc(snew, snew_sz)) == NULL){
          free(snew);
          return NULL;
        }
        snew = stmp;
      }
      memcpy(&snew[snew_len], replace, replace_len);
      snew_len += replace_len;
      s_idx += search_len;
      found_matches += 1;
    }
    else{
      if(snew_len + 1 >= snew_sz){
        snew_sz += snew_sz + slen + snew_len + 1;
        if((stmp = realloc(snew, snew_sz)) == NULL){
          free(snew);
          return NULL;
        }
        snew = stmp;
      }
      snew[snew_len] = s[s_idx];
      s_idx += 1;
      snew_len += 1;
    }
  }
  snew[snew_len] = '\0';

  return snew;
}

/**
 * Lookup table used to encode data into base64.
 *
 * Base64 encoding takes six bits of data and encodes those bits using this
 * table. Since 2^6 = 64, this array has 64 entries which maps directly from
 * the 6 bit value into the corresponding array value.
 */
static unsigned char g_base64_encode_table[] = {
  'A','B','C','D','E','F','G','H','I','J',
  'K','L','M','N','O','P','Q','R','S','T',
  'U','V','W','X','Y','Z',
  'a','b','c','d','e','f','g','h','i','j',
  'k','l','m','n','o','p','q','r','s','t',
  'u','v','w','x','y','z',
  '0','1','2','3','4','5','6','7','8','9',
  '+','/'
};

/**
 * Encode a single block of binary data into base64.
 *
 * @param[in]  buf          Buffer with data to encode.
 * @param[in]  buf_block_sz Number of bytes in buf to encode (min 1, max 3).
 * @param[out] b64          Pointer to buffer with at least 4 bytes for
 *                          storing the base64 encoded result.
 */
static void
smtp_base64_encode_block(const char *const buf,
                         size_t buf_block_sz,
                         char *const b64){
  char inb[3] = {0};
  unsigned char in_idx[4] = {0};
  char outb[5] = {'=', '=', '=', '=', '\0'};
  size_t i;

  memcpy(inb, buf, buf_block_sz);

  in_idx[0] = ((inb[0] >> 2))                         & 0x3F;
  in_idx[1] = ((inb[0] << 4) | ((inb[1] >> 4) & 0xF)) & 0x3F;
  in_idx[2] = ((inb[1] << 2) | ((inb[2] >> 6) & 0x3)) & 0x3F;
  in_idx[3] = ((inb[2]     ))                         & 0x3F;
  for(i = 0; i < 4; i++){
    if(i < buf_block_sz + 1){
      outb[i] = g_base64_encode_table[in_idx[i]];
    }
    b64[i] = outb[i];
  }
}

/**
 * Encode binary data into a base64 string.
 *
 * @param[in] buf    Binary data to encode in base64.
 * @param[in] buflen Number of bytes in the @p buf parameter.
 * @retval char* Dynamically allocated base64 encoded string. The caller
 *               must free this string when finished.
 * @retval NULL  Memory allocation failure.
 */
SMTP_LINKAGE char *
smtp_base64_encode(const char *const buf,
                   ssize_t buflen){
  char *b64;
  size_t b64_sz;
  size_t buf_i;
  size_t b64_i;
  size_t remaining_block_sz;
  size_t buf_block_sz;

  if(buflen < 0){
    buflen = strlen(buf);
  }

  /*
   * base64 size expands by 33%
   * +1 to round integer division up
   * +2 for '=' padding
   * +1 null terminator
   */
  b64_sz = (4 * buflen / 3) + 1 + 2 + 1;
  if((b64 = calloc(1, b64_sz)) == NULL){
    return NULL;
  }

  if(buflen == 0){
    return b64;
  }

  buf_i = 0;
  b64_i = 0;
  remaining_block_sz = buflen;
  while(remaining_block_sz > 0){
    if(remaining_block_sz >= 3){
      buf_block_sz = 3;
    }
    else{
      buf_block_sz = remaining_block_sz;
    }

    smtp_base64_encode_block(&buf[buf_i], buf_block_sz, &b64[b64_i]);
    if(buf_block_sz < 3){
      break;
    }

    buf_i += 3;
    b64_i += 4;
    remaining_block_sz -= 3;
  }

  return b64;
}

#ifdef SMTP_OPENSSL
/**
 * Lookup table used to decode base64 data.
 *
 * For base64 encoding, every six bits have been encoded using only the ASCII
 * characters from @ref g_base64_encode_table. This table has entries which
 * allow the reversal of that process. It has 128 entries which map over to
 * the index value from the encoding table. If an indexing result ends up
 * with -1 during the decoding process, then that indicates an invalid base64
 * character in the encoded data.
 */
static char g_base64_decode_table[] = {
  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
  -1, -1, -1,
  62,                                     /*   +   */
  -1, -1, -1,
  63,                                     /*   /   */
  52, 53, 54, 55, 56, 57, 58, 59, 60, 61, /* 0 - 9 */
  -1, -1, -1, -1, -1, -1, -1,
   0,  1,  2,  3,  4,  5,  6,  7,  8,  9, /* A - J */
  10, 11, 12, 13, 14, 15, 16, 17, 18, 19, /* K - T */
  20, 21, 22, 23, 24, 25,                 /* U - Z */
  -1, -1, -1, -1, -1, -1,
  26, 27, 28, 29, 30, 31, 32, 33, 34, 35, /* a - j */
  36, 37, 38, 39, 40, 41, 42, 43, 44, 45, /* k - t */
  46, 47, 48, 49, 50, 51,                 /* u - z */
  -1, -1, -1, -1, -1
};

/**
 * Decodes a base64 block of up to four bytes at a time.
 *
 * @param[in]  buf    Buffer containing bytes to decode.
 * @param[out] decode Buffer for storing base64 decoded bytes.
 * @retval >0 Length of the decoded block.
 * @retval -1 If the block contains invalid base64 data.
 */
static int
smtp_base64_decode_block(const unsigned char *const buf,
                         unsigned char *const decode){
  int decode_block_len;
  size_t i;
  char decode_table[4];
  unsigned char outb[3];

  decode_block_len = 0;
  for(i = 0; i < 4; i++){
    if(buf[i] == '='){
      decode_table[i] = 0;
      continue;
    }
    decode_table[i] = g_base64_decode_table[(unsigned char)buf[i]];
    if(decode_table[i] < 0){
      return -1;
    }
  }

  outb[0] = ((decode_table[0] << 2) & 0xFC) | ((decode_table[1] >> 4) & 0x03);
  outb[1] = ((decode_table[1] << 4) & 0xF0) | ((decode_table[2] >> 2) & 0x0F);
  outb[2] = ((decode_table[2] << 6) & 0xC0) | ((decode_table[3]     ) & 0x3F);

  decode[0] = outb[0];
  decode_block_len += 1;

  if(buf[2] == '='){
    decode[1] = '\0';
  }
  else{
    decode[1] = outb[1];
    decode_block_len += 1;
  }

  if(buf[3] == '='){
    decode[2] = '\0';
  }
  else{
    decode[2] = outb[2];
    decode_block_len += 1;
  }

  return decode_block_len;
}

/**
 * Decode a base64 string.
 *
 * The decode parameter will get dynamically allocated by this function
 * if it successfully completes. Therefore, the caller must free the decode
 * parameter after use.
 *
 * @param[in]  buf    Null-terminated base64 string.
 * @param[out] decode Pointer to buffer which will get dynamically allocated
 *                    and will contain the decoded binary data. This parameter
 *                    will get set to NULL if the memory allocation fails.
 * @retval >=0 Length of the data stored in the decode parameter.
 * @retval -1  Memory allocation failure or invalid base64 byte sequences.
 */
SMTP_LINKAGE ssize_t
smtp_base64_decode(const char *const buf,
                   unsigned char **decode){
  size_t buf_len;
  size_t buf_i;
  unsigned char *b64_decode;
  ssize_t decode_len;
  int decode_block_len;

  *decode = NULL;

  buf_len = strlen(buf);
  if(buf_len % 4 != 0){
    return -1;
  }

  if((b64_decode = calloc(1, buf_len + 1)) == NULL){
    return -1;
  }

  decode_len = 0;
  for(buf_i = 0; buf_i < buf_len; buf_i += 4){
    if((decode_block_len = smtp_base64_decode_block(
                             (const unsigned char*)&buf[buf_i],
                             &b64_decode[decode_len])) < 0){
      free(b64_decode);
      return -1;
    }
    decode_len += decode_block_len;
  }
  *decode = b64_decode;
  return decode_len;
}

/**
 * Convert binary data to lowercase hexadecimal representation.
 *
 * @param[in] s    Buffer containing binary data to convert.
 * @param[in] slen Number of bytes in @p s.
 * @retval char* Dynamically allocated string consisting of a hexadecimal
 *               representation of binary data in @p s. The caller must free
 *               this memory when finished.
 * @retval NULL  Memory allocation or encoding error.
 */
SMTP_LINKAGE char *
smtp_bin2hex(const unsigned char *const s,
             size_t slen){
  char *snew;
  size_t alloc_sz;
  size_t i;
  size_t j;
  unsigned hex;
  int rc;

  alloc_sz = slen * 2 + 1;
  if((snew = malloc(alloc_sz)) == NULL){
    return NULL;
  }

  for(i = 0, j = 0; i < slen; i++, j += 2){
    hex = s[i];
    rc = sprintf(&snew[j], "%02x", hex);
    if(rc < 0 || (size_t)rc >= 3){
      free(snew);
      return NULL;
    }
  }
  snew[j] = '\0';

  return snew;
}
#endif /* SMTP_OPENSSL */

/**
 * Get the length in bytes of the current UTF-8 character.
 *
 * This consists of a very simple check and assumes the user provides a valid
 * UTF-8 byte sequence. It gets the length from the first byte in the sequence
 * and does not validate any other bytes in the character sequence or any other
 * bits in the first byte of the character sequence.
 *
 * @param[in] c The first byte in a valid UTF-8 character sequence.
 * @retval >0 Number of bytes for the current UTF-8 character sequence.
 * @retval  0 Invalid byte sequence.
 */
SMTP_LINKAGE int
smtp_utf8_charlen(unsigned char c){
  if((c & 0x80) == 0){         /* 0XXXXXXX */
    return 1;
  }
  else if((c & 0xE0) == 0xC0){ /* 110XXXXX */
    return 2;
  }
  else if((c & 0xF0) == 0xE0){ /* 1110XXXX */
    return 3;
  }
  else if((c & 0xF8) == 0xF0){ /* 11110XXX */
    return 4;
  }
  else{                        /* invalid  */
    return -1;
  }
}

/**
 * Check if a string contains non-ASCII UTF-8 characters.
 *
 * Uses the simple algorithm from @ref smtp_utf8_charlen to check for
 * UTF-8 characters.
 *
 * @param[in] s UTF-8 string.
 * @retval 1 String contains non-ASCII UTF-8 characters.
 * @retval 0 String contains only ASCII characters.
 */
SMTP_LINKAGE int
smtp_str_has_nonascii_utf8(const char *const s){
  int i;
  int charlen;

  for(i = 0; s[i]; i++){
    charlen = smtp_utf8_charlen(s[i]);
    if(charlen != 1){
      return 1;
    }
  }
  return 0;
}

/**
 * Get the number of bytes in a UTF-8 string, or a shorter count if
 * the string exceeds a maximum specified length.
 *
 * See @p maxlen for more information on multi-byte parsing.
 *
 * @param[in] s      Null-terminated UTF-8 string.
 * @param[in] maxlen Do not check more than @p maxlen bytes of string @p s
 *                   except if in the middle of a multi-byte character.
 * @retval strlen(s) If length of s has less bytes than maxlen or the same
 *                   number of bytes as maxlen. See @p maxlen for more details.
 * @retval maxlen    If length of s has more bytes than maxlen.
 * @retval -1        If @p s contains an invalid UTF-8 byte sequence.
 */
SMTP_LINKAGE ssize_t
smtp_strnlen_utf8(const char *s,
                  size_t maxlen){
  size_t i;
  int utf8_i;
  int utf8_len;

  for(i = 0; *s && i < maxlen; i += utf8_len){
    utf8_len = smtp_utf8_charlen(*s);
    if(utf8_len < 0){
      return -1;
    }

    for(utf8_i = 0; utf8_i < utf8_len; utf8_i++){
      if(!*s){
        return -1;
      }
      s += 1;
    }
  }
  return i;
}

/**
 * Splits a string into smaller chunks separated by a terminating string.
 *
 * @param[in] s        The string to chunk.
 * @param[in] chunklen Number of bytes for each chunk in the string.
 * @param[in] end      Terminating string placed at the end of each chunk.
 * @retval char* Pointer to an allocated string with the contents split into
 *               separate chunks. The caller must free this memory when done.
 * @retval NULL  Memory allocation failure.
 */
SMTP_LINKAGE char *
smtp_chunk_split(const char *const s,
                 int chunklen,
                 const char *const end){
  char *snew;
  size_t bodylen;
  size_t endlen;
  size_t snewlen;
  size_t chunk_i;
  size_t snew_i;
  size_t body_i;
  ssize_t body_copy_len;

  if(chunklen < 1){
    errno = EINVAL;
    return NULL;
  }

  bodylen = strlen(s);
  endlen = strlen(end);

  if(bodylen < 1){
    return smtp_strdup(end);
  }

  snewlen = bodylen + (endlen + 1) * (bodylen / chunklen + 1) + 1;

  if((snew = calloc(1, snewlen)) == NULL){
    return NULL;
  }

  body_i = 0;
  snew_i = 0;
  for(chunk_i = 0; chunk_i < bodylen / chunklen + 1; chunk_i++){
    body_copy_len = smtp_strnlen_utf8(&s[body_i], chunklen);
    if(body_copy_len < 0){
      free(snew);
      errno = EINVAL;
      return NULL;
    }
    memcpy(&snew[snew_i], &s[body_i], body_copy_len);
    snew_i += body_copy_len;
    if(s[body_i] == '\0'){
      snew_i += 1;
    }
    body_i += body_copy_len;

    if(endlen > 0){
      memcpy(&snew[snew_i], end, endlen);
    }
    snew_i += endlen;
  }

  return snew;
}

/**
 * Read the entire contents of a file stream and store the data into a
 * dynamically allocated buffer.
 *
 * @param[in]  stream     File stream already opened by the caller.
 * @param[out] bytes_read Number of bytes stored in the return buffer.
 * @retval char* A dynamically allocated buffer which contains the entire
 *               contents of @p stream. The caller must free this memory
 *               when done.
 * @retval NULL Memory allocation or file read error.
 */
SMTP_LINKAGE char *
smtp_ffile_get_contents(FILE *stream,
                        size_t *bytes_read){
  char *read_buf;
  size_t bufsz;
  char *new_buf;
  const size_t BUFSZ_INCREMENT = 512;

  read_buf = NULL;
  bufsz = 0;

  if(bytes_read){
    *bytes_read = 0;
  }

  do{
    size_t bytes_read_loop;
    if((new_buf = realloc(read_buf, bufsz + BUFSZ_INCREMENT)) == NULL){
      free(read_buf);
      return NULL;
    }
    read_buf = new_buf;
    bufsz += BUFSZ_INCREMENT;

    bytes_read_loop = fread(&read_buf[bufsz - BUFSZ_INCREMENT],
                            sizeof(char),
                            BUFSZ_INCREMENT,
                            stream);
    if(bytes_read){
      *bytes_read += bytes_read_loop;
    }
    if(ferror(stream)){
      free(read_buf);
      return NULL;
    }
  } while(!feof(stream));

  return read_buf;
}

/**
 * Read the entire contents of a file from a given path, and store the data
 * into a dynamically allocated buffer.
 *
 * @param[in]  filename   Path of file to open and read from.
 * @param[out] bytes_read Number of bytes stored in the return buffer.
 * @retval char* A dynamically allocated buffer which has the contents of
 *         the file at @p filename. The caller must free this memory when
 *         done.
 * @retval NULL Memory allocation or file read error.
 */
SMTP_LINKAGE char *
smtp_file_get_contents(const char *const filename,
                       size_t *bytes_read){
  FILE *fp;
  char *read_buf;

  if((fp = fopen(filename, "r")) == NULL){
    return NULL;
  }

  read_buf = smtp_ffile_get_contents(fp, bytes_read);

  if(fclose(fp) == EOF){
    free(read_buf);
    read_buf = NULL;
  }

  return read_buf;
}

/**
 * Parse a server response line into the @ref smtp_command data structure.
 *
 * @param[in]  line Server response string.
 * @param[out] cmd  Structure containing the server response data broken up
 *                  into its separate components.
 * @return See @ref smtp_result_code.
 */
SMTP_LINKAGE int
smtp_parse_cmd_line(char *const line,
                    struct smtp_command *const cmd){
  char *ep;
  char code_str[4];
  size_t line_len;

  line_len = strlen(line);
  if(line_len < 5){
    cmd->code = SMTP_INTERNAL_ERROR;
    cmd->more = 0;
    cmd->text = line;
    return cmd->code;
  }

  cmd->text = &line[4];

  memcpy(code_str, line, 3);
  code_str[3] = '\0';
  cmd->code = strtoul(code_str, &ep, 10);
  if(*ep != '\0'){
    cmd->code = SMTP_INTERNAL_ERROR;
  }
  cmd->more = line[3] == '-' ? 1 : 0;
  return cmd->code;
}

/**
 * Prints communication between the client and server to stderr only if
 * the debug flag has been set.
 *
 * @param[in] smtp   SMTP client context.
 * @param[in] prefix Print this prefix before the main debug line text.
 * @param[in] str    Debug text to print out.
 */
static void
smtp_puts_dbg(struct smtp *const smtp,
              const char *const prefix,
              const char *const str){
  char *sdup;
  size_t i;

  if(smtp->flags & SMTP_DEBUG){
    if((sdup = smtp_strdup(str)) == NULL){
      return;
    }

    /* Remove carriage return and newline when printing to stderr. */
    for(i = 0; sdup[i]; i++){
      if(sdup[i] == '\r' || sdup[i] == '\n'){
        sdup[i] = ' ';
      }
    }

    if(fprintf(stderr, "[smtp %s]: %s\n", prefix, sdup) < 0){
      /* Do not care if this fails. */
    }
    free(sdup);
  }
}

/**
 * Read a server response line.
 *
 * @param[in] smtp SMTP client context.
 * @return @ref str_getdelim_retcode.
 */
static enum str_getdelim_retcode
smtp_getline(struct smtp *const smtp){
  enum str_getdelim_retcode rc;

  errno = 0;
  rc = smtp_str_getdelimfd(&smtp->gdfd);
  if(errno == ENOMEM){
    smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
    return rc;
  }
  else if(rc == STRING_GETDELIMFD_ERROR){
    smtp_status_code_set(smtp, SMTP_STATUS_RECV);
    return STRING_GETDELIMFD_ERROR;
  }

  if(smtp->gdfd.line_len > 0){
    /* Remove the carriage-return character ('\r'). */
    smtp->gdfd.line[smtp->gdfd.line_len - 1] = '\0';
    smtp_puts_dbg(smtp, "Server", smtp->gdfd.line);
  }
  return rc;
}

/**
 * Loop through all of the server response lines until the last line, and
 * then return the status code from the last response line.
 *
 * @param[in] smtp SMTP client context.
 * @return @ref smtp_result_code.
 */
static int
smtp_read_and_parse_code(struct smtp *const smtp){
  struct smtp_command cmd;
  enum str_getdelim_retcode rc;

  do{
    rc = smtp_getline(smtp);
    if(rc == STRING_GETDELIMFD_ERROR){
      return SMTP_INTERNAL_ERROR;
    }

    smtp_parse_cmd_line(smtp->gdfd.line, &cmd);
  }while (rc != STRING_GETDELIMFD_DONE && cmd.more);

  return cmd.code;
}

/**
 * Send data to the SMTP server.
 *
 * Writes a buffer of length len into either the unencrypted TCP socket or
 * the TLS encrypted socket, depending on the current underlying mode of
 * the socket.
 *
 * @param[in] smtp SMTP client context.
 * @param[in] buf Data to send to the SMTP server.
 * @param[in] len Number of bytes in buf.
 * @return @ref smtp_status_code.
 */
static int
smtp_write(struct smtp *const smtp,
           const char *const buf,
           size_t len){
  smtp_puts_dbg(smtp, "Client", buf);

  if(smtp->tls_on){
#ifdef SMTP_OPENSSL
    if(SSL_write(smtp->tls, buf, len) <= 0){
      return smtp_status_code_set(smtp, SMTP_STATUS_SEND);
    }
#endif /* SMTP_OPENSSL */
  }
  else{
    if(send(smtp->sock, buf, len, 0) < 0){
      return smtp_status_code_set(smtp, SMTP_STATUS_SEND);
    }
  }

  return smtp->status_code;
}

/**
 * Send a null-terminated string to the SMTP server.
 *
 * @param[in] smtp SMTP client context.
 * @param[in] s    Null-terminated string to send to the SMTP server.
 * @return See @ref smtp_status_code and @ref smtp_write.
 */
static int
smtp_puts(struct smtp *const smtp,
          const char *const s){
  return smtp_write(smtp, s, strlen(s));
}

/**
 * Same as @ref smtp_puts except this function also appends the line
 * terminating carriage return and newline bytes at the end of the string.
 *
 * @param[in] smtp SMTP client context.
 * @param[in] s    Null-terminated string to send to the SMTP server.
 * @return See @ref smtp_status_code and @ref smtp_puts.
 */
static int
smtp_puts_terminate(struct smtp *const smtp,
                    const char *const s){
  int rc;
  char *line;

  if((line = malloc(strlen(s) + 3)) == NULL){
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }
  sprintf(line, "%s\r\n", s);
  rc = smtp_puts(smtp, line);
  free(line);
  return rc;
}

/**
 * Connect to the server using a standard TCP socket.
 *
 * This function handles the server name lookup to get an IP address
 * for the server, and then to connect to that IP using a normal TCP
 * connection.
 *
 * @param[in] smtp   SMTP client context.
 * @param[in] server Mail server name or IP address.
 * @param[in] port   Mail server port number.
 * @retval  0 Successfully connected to server.
 * @retval -1 Failed to connect to server.
 */
static int
smtp_connect(struct smtp *const smtp,
             const char *const server,
             const char *const port){
  struct addrinfo hints;
  struct addrinfo *res0;
  struct addrinfo *res;

  /*
   * Windows requires initializing the socket library before we call any
   * socket functions.
   */
#ifdef SMTP_IS_WINDOWS
  /* Windows global network socket data structure. */
  WSADATA wsa_data;
  if(WSAStartup(MAKEWORD(2, 2), &wsa_data) != 0){
    return -1;
  }
#endif /* SMTP_IS_WINDOWS */

  memset(&hints, 0, sizeof(hints));
  hints.ai_family   = AF_UNSPEC;
  hints.ai_socktype = SOCK_STREAM;
  hints.ai_flags    = 0;
  hints.ai_protocol = IPPROTO_TCP;

  if(getaddrinfo(server, port, &hints, &res0) != 0){
    return -1;
  }

  for(res = res0; res; res = res->ai_next){
    smtp->sock = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
    if(smtp->sock < 0){
      continue;
    }

    if(connect(smtp->sock, res->ai_addr, res->ai_addrlen) < 0){
      close(smtp->sock);
      smtp->sock = -1;
      continue;
    }
    break;
  }

  freeaddrinfo(res0);
  if(smtp->sock < 0){
    return -1;
  }

  return 0;
}

#ifdef SMTP_OPENSSL
/**
 * Initialize the TLS library and establish a TLS handshake with the server
 * on the existing socket connection.
 *
 * @param[in] smtp   SMTP client context.
 * @param[in] server Server name or IP address.
 * @retval  0 Successfully established a TLS connection with the server.
 * @retval -1 Failed to establish a TLS connection with the server.
 */
static int
smtp_tls_init(struct smtp *const smtp,
              const char *const server){
  X509 *X509_cert_peer;

  /* Do not need to check the return value since this always returns 1. */
  SSL_library_init();

  SSL_load_error_strings();
  ERR_load_BIO_strings();
  OpenSSL_add_all_algorithms();

  if((smtp->tls_ctx = SSL_CTX_new(SSLv23_client_method())) == NULL){
    return -1;
  }

  /* Disable SSLv2, SSLv3, and TLSv1.0. */
  SSL_CTX_set_options(smtp->tls_ctx,
                      SSL_OP_NO_SSLv2 |
                      SSL_OP_NO_SSLv3 |
                      SSL_OP_NO_TLSv1);

  SSL_CTX_set_mode(smtp->tls_ctx, SSL_MODE_AUTO_RETRY);
  if((smtp->flags & SMTP_NO_CERT_VERIFY) == 0){
    SSL_CTX_set_verify(smtp->tls_ctx, SSL_VERIFY_PEER, NULL);
  }

  /*
   * Set the path to the user-provided CA file or use the default cert paths
   * if not provided.
   */
  if(smtp->cafile){
    if(SSL_CTX_load_verify_locations(smtp->tls_ctx, smtp->cafile, NULL) != 1){
      SSL_CTX_free(smtp->tls_ctx);
      return -1;
    }
  }
  else{
    X509_STORE_set_default_paths(SSL_CTX_get_cert_store(smtp->tls_ctx));
    if(ERR_peek_error() != 0){
      SSL_CTX_free(smtp->tls_ctx);
      return -1;
    }
  }

  if((smtp->tls = SSL_new(smtp->tls_ctx)) == NULL){
    SSL_CTX_free(smtp->tls_ctx);
    return -1;
  }

  if((smtp->tls_bio = BIO_new_socket(smtp->sock, 0)) == NULL){
    SSL_CTX_free(smtp->tls_ctx);
    SSL_free(smtp->tls);
    return -1;
  }

  SSL_set_bio(smtp->tls, smtp->tls_bio, smtp->tls_bio);
  SSL_set_connect_state(smtp->tls);
  if(SSL_connect(smtp->tls) != 1){
    SSL_CTX_free(smtp->tls_ctx);
    SSL_free(smtp->tls);
    return -1;
  }

  if(SSL_do_handshake(smtp->tls) != 1){
    SSL_CTX_free(smtp->tls_ctx);
    SSL_free(smtp->tls);
    return -1;
  }

  /* Verify matching subject in certificate. */
  if((smtp->flags & SMTP_NO_CERT_VERIFY) == 0){
    if((X509_cert_peer = SSL_get_peer_certificate(smtp->tls)) == NULL){
      SSL_CTX_free(smtp->tls_ctx);
      SSL_free(smtp->tls);
      return -1;
    }
    if(X509_check_host(X509_cert_peer, server, 0, 0, NULL) != 1){
      SSL_CTX_free(smtp->tls_ctx);
      SSL_free(smtp->tls);
      return -1;
    }
    X509_free(X509_cert_peer);
  }

  smtp->tls_on = 1;
  return 0;
}
#endif /* SMTP_OPENSSL */

/**
 * Send the EHLO command and parse through the responses.
 *
 * Ignores all of the server extensions that get returned. If a server
 * doesn't support an extension we need, then we should receive an error
 * later on when we try to use that extension.
 *
 * @param[in] smtp SMTP client context.
 * @return @ref smtp_status_code.
 */
static int
smtp_ehlo(struct smtp *const smtp){
  if(smtp_puts(smtp, "EHLO smtp\r\n") != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  smtp_read_and_parse_code(smtp);
  return smtp->status_code;
}

/**
 * Authenticate using the PLAIN method.
 *
 *   1. Set the text to the following format: "\0<user>\0<password>",
 *      or as shown in the format string: "\0%s\0%s", email, password.
 *   2. Base64 encode the text from (1).
 *   3. Send the constructed auth text from (2) to the server:
 *      "AUTH PLAIN <b64>\r\n".
 *
 * @param[in] smtp SMTP client context.
 * @param[in] user SMTP account user name.
 * @param[in] pass SMTP account password.
 * @retval  0 Successfully authenticated.
 * @retval -1 Failed to authenticate.
 */
static int
smtp_auth_plain(struct smtp *const smtp,
                const char *const user,
                const char *const pass){
  size_t user_len;
  size_t pass_len;
  char *login_str;
  int login_len;
  char *login_b64;
  char *login_send;

  /* (1) */
  user_len = strlen(user);
  pass_len = strlen(pass);
  login_len = 1 + user_len + 1 + pass_len;
  if((login_str = malloc(login_len)) == NULL){
    return -1;
  }
  login_str[0] = '\0';
  memcpy(&login_str[1], user, user_len);
  login_str[1 + user_len] = '\0';
  memcpy(&login_str[1 + user_len + 1], pass, pass_len);

  /* (2) */
  login_b64 = smtp_base64_encode(login_str, login_len);
  free(login_str);
  if(login_b64 == NULL){
    return -1;
  }

  /* (3) */
  if((login_send = malloc(strlen(login_b64) + 14)) == NULL){
    free(login_b64);
    return -1;
  }
  sprintf(login_send, "AUTH PLAIN %s\r\n", login_b64);
  free(login_b64);
  smtp_puts(smtp, login_send);
  free(login_send);
  if(smtp->status_code != SMTP_STATUS_OK){
    return -1;
  }

  if(smtp_read_and_parse_code(smtp) != SMTP_AUTH_SUCCESS){
    return -1;
  }
  return 0;
}

/**
 * Authenticate using the LOGIN method.
 *
 *   1. Base64 encode the user name.
 *   2. Send string from (1) as part of the login:
 *      "AUTH LOGIN <b64_username>\r\n".
 *   3. Base64 encode the password and send that by itself on a separate
 *      line: "<b64_password>\r\n".
 *
 * @param[in] smtp SMTP client context.
 * @param[in] user SMTP account user name.
 * @param[in] pass SMTP account password.
 * @retval  0 Successfully authenticated.
 * @retval -1 Failed to authenticate.
 */
static int
smtp_auth_login(struct smtp *const smtp,
                const char *const user,
                const char *const pass){
  char *b64_user;
  char *b64_pass;
  char *login_str;

  /* (1) */
  if((b64_user = smtp_base64_encode(user, -1)) == NULL){
    return -1;
  }

  /* (2) */
  if((login_str = malloc(strlen(b64_user) + 14)) == NULL){
    free(b64_user);
    return -1;
  }
  sprintf(login_str, "AUTH LOGIN %s\r\n", b64_user);
  free(b64_user);
  smtp_puts(smtp, login_str);
  free(login_str);
  if(smtp->status_code != SMTP_STATUS_OK){
     return -1;
   }

  if(smtp_read_and_parse_code(smtp) != SMTP_AUTH_CONTINUE){
    return -1;
  }

  /* (3) */
  if((b64_pass = smtp_base64_encode(pass, -1)) == NULL){
    return -1;
  }
  smtp_puts_terminate(smtp, b64_pass);
  free(b64_pass);
  if(smtp->status_code != SMTP_STATUS_OK){
    return -1;
  }

  if(smtp_read_and_parse_code(smtp) != SMTP_AUTH_SUCCESS){
    return -1;
  }
  return 0;
}

#ifdef SMTP_OPENSSL
/**
 * Authenticate using the CRAM-MD5 method.
 *
 *   1. Send "AUTH CRAM-MD5\r\n" to the server.
 *   2. Decode the base64 challenge response from the server.
 *   3. Do an MD5 HMAC on (2) using the account password as the key.
 *   4. Convert the binary data in (3) to lowercase hex characters.
 *   5. Construct the string: "<user> <(4)>".
 *   6. Encode (5) into base64 format.
 *   7. Send the final string from (6) to the server and check the response.
 *
 * @param[in] smtp SMTP client context.
 * @param[in] user SMTP account user name.
 * @param[in] pass SMTP account password.
 * @retval  0 Successfully authenticated.
 * @retval -1 Failed to authenticate.
 */
static int
smtp_auth_cram_md5(struct smtp *const smtp,
                   const char *const user,
                   const char *const pass){
  struct smtp_command cmd;
  unsigned char *challenge_decoded;
  ssize_t challenge_decoded_len;
  const char *key;
  int key_len;
  unsigned char hmac[EVP_MAX_MD_SIZE];
  unsigned int hmac_len;
  unsigned char *hmac_ret;
  char *challenge_hex;
  char *auth_concat;
  size_t auth_concat_len;
  char *b64_auth;

  /* (1) */
  if(smtp_puts(smtp, "AUTH CRAM-MD5\r\n") != SMTP_STATUS_OK){
    return -1;
  }
  if(smtp_getline(smtp) == STRING_GETDELIMFD_ERROR){
    return -1;
  }
  if(smtp_parse_cmd_line(smtp->gdfd.line, &cmd) != SMTP_AUTH_CONTINUE){
    return -1;
  }

  /* (2) */
  challenge_decoded_len = smtp_base64_decode(cmd.text, &challenge_decoded);
  if(challenge_decoded_len < 0){
    return -1;
  }

  /* (3) */
  key = pass;
  key_len = strlen(pass);
  hmac_ret = HMAC(EVP_md5(),
                  key, key_len,
                  challenge_decoded, challenge_decoded_len,
                  hmac, &hmac_len);
  free(challenge_decoded);
  if(hmac_ret == NULL){
    return -1;
  }

  /* (4) */
  challenge_hex = smtp_bin2hex(hmac, hmac_len);
  if(challenge_hex == NULL){
    return -1;
  }

  /* (5) */
  auth_concat_len = strlen(user) + 1 + strlen(challenge_hex) + 1;
  if((auth_concat = malloc(auth_concat_len)) == NULL){
    free(challenge_hex);
    return -1;
  }
  sprintf(auth_concat, "%s %s", user, challenge_hex);
  free(challenge_hex);

  /* (6) */
  b64_auth = smtp_base64_encode(auth_concat, auth_concat_len - 1);
  free(auth_concat);
  if(b64_auth == NULL){
    return -1;
  }

  /* (7) */
  smtp_puts_terminate(smtp, b64_auth);
  free(b64_auth);
  if(smtp->status_code != SMTP_STATUS_OK){
    return -1;
  }

  if(smtp_read_and_parse_code(smtp) != SMTP_AUTH_SUCCESS){
    return -1;
  }
  return 0;
}
#endif /* SMTP_OPENSSL */

/**
 * Set the timeout for the next socket read operation.
 *
 * @param[in] smtp    SMTP client context.
 * @param[in] seconds Timeout in seconds.
 */
static void
smtp_set_read_timeout(struct smtp *const smtp,
                 long seconds){
  smtp->timeout_sec = seconds;
}

/**
 * Perform a handshake with the SMTP server which includes optionally
 * setting up TLS and sending the EHLO greeting.
 *
 * At this point, the client has already connected to the SMTP server
 * through its socket connection. In this function, the client will:
 *   1. Optionally convert the connection to TLS (SMTP_SECURITY_TLS).
 *   2. Read the initial server greeting.
 *   3. Send an EHLO to the server.
 *   4. Optionally initiate STARTTLS and resend the EHLO
 *      (SMTP_SECURITY_STARTTLS).
 *
 * @param[in] smtp                SMTP client context.
 * @param[in] server              Server name or IP address.
 * @param[in] connection_security See @ref smtp_connection_security.
 * @return @ref smtp_status_code.
 */
static int
smtp_initiate_handshake(struct smtp *const smtp,
                        const char *const server,
                        enum smtp_connection_security connection_security){
  /* Eliminate unused warnings if not using SMTP_OPENSSL. */
  (void)server;
  (void)connection_security;

  /* (1) */
#ifdef SMTP_OPENSSL
  if(connection_security == SMTP_SECURITY_TLS){
    if(smtp_tls_init(smtp, server) < 0){
      return smtp_status_code_set(smtp, SMTP_STATUS_HANDSHAKE);
    }
  }
#endif /* SMTP_OPENSSL */

  /* (2) */
  /* Get initial 220 message - 5 minute timeout. */
  smtp_set_read_timeout(smtp, 60 * 5);
  if(smtp_getline(smtp) == STRING_GETDELIMFD_ERROR){
    return smtp->status_code;
  }

  /* (3) */
  if(smtp_ehlo(smtp) != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  /* (4) */
#ifdef SMTP_OPENSSL
  if(connection_security == SMTP_SECURITY_STARTTLS){
    if(smtp_puts(smtp, "STARTTLS\r\n") != SMTP_STATUS_OK){
      return smtp->status_code;
    }
    if(smtp_read_and_parse_code(smtp) != SMTP_READY){
      return smtp_status_code_set(smtp, SMTP_STATUS_HANDSHAKE);
    }
    if(smtp_tls_init(smtp, server) < 0){
      return smtp_status_code_set(smtp, SMTP_STATUS_HANDSHAKE);
    }
    if(smtp_ehlo(smtp) != SMTP_STATUS_OK){
      return smtp->status_code;
    }
  }
#endif /* SMTP_OPENSSL */

  return smtp->status_code;
}

/**
   Maximum size of an RFC 2822 date string.

   @verbatim
   Thu, 21 May 1998 05:33:29 -0700
   12345678901234567890123456789012
           10        20        30 32 (bytes)
   @endverbatim
 */
#define SMTP_DATE_MAX_SZ 32

/**
 * Convert the time into an RFC 2822 formatted string.
 *
 * Example date format:
 * Thu, 21 May 1998 05:33:29 -0700
 *
 * @param[out] date Buffer that has at least SMTP_DATE_MAX_SZ bytes.
 * @retval  0 Successfully copied the current date to the buffer.
 * @retval -1 Failed to establish the current date or an output
 *            format error occurred.
 */
SMTP_LINKAGE int
smtp_date_rfc_2822(char *const date){
  time_t t;
  time_t t_local;
  time_t t_utc;
  struct tm tm_local;
  struct tm tm_utc;
  long offset_utc;
  int rc;

  const char weekday_abbreviation[7][4] = {
    "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
  };

  const char month_abbreviation[12][4] = {
    "Jan", "Feb", "Mar", "Apr", "May", "Jun",
    "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
  };

  if((t = time(NULL)) == (time_t)(-1)){
    return -1;
  }

#ifdef SMTP_IS_WINDOWS
  if(localtime_s(&tm_local, &t) ||
     gmtime_s(&tm_utc, &t)){
    return -1;
  }
#else /* POSIX */
  /* Not defined if system does not have localtime_r or gmtime_r. */
# ifdef SMTP_TIME_NO_REENTRANT
  struct tm *tm;

  /* localtime() not thread-safe. */
  if((tm = localtime(&t)) == NULL){
    return -1;
  }
  memcpy(&tm_local, tm, sizeof(tm_local));

  /* gmtime() not thread-safe. */
  if((tm = gmtime(&t)) == NULL){
    return -1;
  }
  memcpy(&tm_utc, tm, sizeof(tm_utc));
# else /* Reentrant versions: localtime_r() and gmtime_r(). */
  if(localtime_r(&t, &tm_local) == NULL ||
     gmtime_r(&t, &tm_utc) == NULL){
    return -1;
  }
# endif /* SMTP_TIME_NO_REENTRANT */
#endif /* SMTP_IS_WINDOWS */

  if((t_local = mktime(&tm_local)) == (time_t)(-1)){
    return -1;
  }

  if((t_utc = mktime(&tm_utc)) == (time_t)(-1)){
    return -1;
  }

  offset_utc = difftime(t_local, t_utc);
  offset_utc = offset_utc / 60 / 60 * 100;

  rc = sprintf(date,
               "%s, %02d %s %d %02d:%02d:%02d %0+5ld",
               weekday_abbreviation[tm_local.tm_wday],
               tm_local.tm_mday,
               month_abbreviation[tm_local.tm_mon],
               tm_local.tm_year + 1900,
               tm_local.tm_hour,
               tm_local.tm_min,
               tm_local.tm_sec, /* 0 - 60 (leap second) */
               offset_utc);
  if(rc + 1 != SMTP_DATE_MAX_SZ){
    return -1;
  }

  return 0;
}

/**
 * Minimum length of buffer required to hold the MIME boundary test:
 * mimeXXXXXXXXXX
 * 123456789012345
 * 1       10   15 bytes
 */
#define SMTP_MIME_BOUNDARY_LEN 15

/**
 * Generate the MIME boundary text field and store it in a user-supplied
 * buffer.
 *
 * For example:
 * mimeXXXXXXXXXX
 * where each X gets set to a pseudo-random uppercase ASCII character.
 *
 * This uses a simple pseudo-random number generator since we only care
 * about preventing accidental boundary collisions.
 *
 * @param[out] boundary Buffer that has at least SMTP_MIME_BOUNDARY_LEN bytes.
 */
static void
smtp_gen_mime_boundary(char *const boundary){
  size_t i;

  strcpy(boundary, "mime");
  srand(time(NULL));
  for(i = 4; i < SMTP_MIME_BOUNDARY_LEN - 1; i++){
    /* Modulo bias okay since we only need to prevent accidental collision. */
    boundary[i] = rand() % 26 + 'A';
  }
  boundary[SMTP_MIME_BOUNDARY_LEN - 1] = '\0';
}

/**
 * Print the MIME header and the MIME section containing the email body.
 *
 * @param[in] smtp     SMTP client context.
 * @param[in] boundary MIME boundary text.
 * @param[in] body     Email body text.
 * @return @ref smtp_status_code.
 */
static int
smtp_print_mime_header_and_body(struct smtp *const smtp,
                                const char *const boundary,
                                const char *const body){
  char *data_double_dot;
  size_t bufsz;
  char *data_header_and_body;

  /*
   * Insert an extra dot for each line that begins with a dot. This will
   * prevent data in the body parameter from prematurely ending the DATA
   * segment.
   */
  if((data_double_dot = smtp_str_replace("\r\n.", "\r\n..", body)) == NULL){
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }

  bufsz = strlen(data_double_dot) + 1000;
  if((data_header_and_body = malloc(bufsz)) == NULL){
    free(data_double_dot);
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }

  sprintf(data_header_and_body,
          "MIME-Version: 1.0\r\n"
          "Content-Type: multipart/mixed; boundary="
          "%s" /* boundary */
          "\r\n"
          "\r\n"
          "Multipart MIME message.\r\n"
          "--"
          "%s" /* boundary */
          "\r\n"
          "Content-Type: text/plain; charset=\"UTF-8\"\r\n"
          "\r\n"
          "%s" /* data_double_dot */
          "\r\n"
          "\r\n",
          boundary,
          boundary,
          data_double_dot);
  free(data_double_dot);
  smtp_puts(smtp, data_header_and_body);
  free(data_header_and_body);
  return smtp->status_code;
}

/**
 * Print a MIME section containing an attachment.
 *
 * @param[in] smtp       SMTP client context.
 * @param[in] boundary   MIME boundary text.
 * @param[in] attachment @ref smtp_attachment.
 * @return @ref smtp_status_code.
 */
static int
smtp_print_mime_attachment(struct smtp *const smtp,
                           const char *const boundary,
                           const struct smtp_attachment *const attachment){
  size_t bufsz;
  char *mime_attach_text;

  bufsz = SMTP_MIME_BOUNDARY_LEN       +
          strlen(attachment->name)     +
          strlen(attachment->b64_data) +
          1000;
  if((mime_attach_text = malloc(bufsz)) == NULL){
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }

  sprintf(mime_attach_text,
          "--"
          "%s" /* boundary */
          "\r\n"
          "Content-Type: application/octet-stream\r\n"
          "Content-Disposition: attachment; filename=\""
          "%s" /* attachment->name */
          "\"\r\n"
          "Content-Transfer-Encoding: base64\r\n"
          "\r\n"
          "%s" /* attachment->b64_data */
          "\r\n"
          "\r\n",
          boundary,
          attachment->name,
          attachment->b64_data);
  smtp_puts(smtp, mime_attach_text);
  free(mime_attach_text);
  return smtp->status_code;
}

/**
 * Prints double hyphen on both sides of the MIME boundary which indicates
 * the end of the MIME sections.
 *
 * @param[in] smtp     SMTP client context.
 * @param[in] boundary MIME boundary text.
 * @return See @ref smtp_status_code and @ref smtp_puts.
 */
static int
smtp_print_mime_end(struct smtp *const smtp,
                    const char *const boundary){
  char mime_end[2 + SMTP_MIME_BOUNDARY_LEN + 4 + 1];

  sprintf(mime_end, "--%s--\r\n", boundary);
  return smtp_puts(smtp, mime_end);
}

/**
 * Send the main email body to the SMTP server.
 *
 * This includes the MIME sections for the email body and attachments.
 *
 * @param[in] smtp SMTP client context.
 * @param[in] body Null-terminated string to send in the email body.
 * @return @ref smtp_status_code.
 */
static int
smtp_print_mime_email(struct smtp *const smtp,
                      const char *const body){
  char boundary[SMTP_MIME_BOUNDARY_LEN];
  size_t i;
  struct smtp_attachment *attachment;

  smtp_gen_mime_boundary(boundary);

  if(smtp_print_mime_header_and_body(smtp, boundary, body) != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  for(i = 0; i < smtp->num_attachment; i++){
    attachment = &smtp->attachment_list[i];
    if(smtp_print_mime_attachment(smtp,
                                  boundary,
                                  attachment) != SMTP_STATUS_OK){
      return smtp->status_code;
    }
  }

  return smtp_print_mime_end(smtp, boundary);
}

/**
 * Email header lines should have no more than 78 characters.
 */
#define SMTP_HEADER_CHUNKLEN 76

/**
 * Convert a header into an RFC 5322 formatted string and send it to the
 * SMTP server.
 *
 * This will adding proper line wrapping and indentation for long
 * header lines.
 *
 * @param[in] smtp   SMTP client context.
 * @param[in] header @ref smtp_header.
 * @return @ref smtp_status_code.
 */
static int
smtp_print_header(struct smtp *const smtp,
                  const struct smtp_header *const header){
  size_t concat_len;
  char *header_concat;
  char *header_fmt;

  if(header->value == NULL){
    return smtp->status_code;
  }

  concat_len = strlen(header->key) + 2 + strlen(header->value) + 1;
  if((header_concat = malloc(concat_len)) == NULL){
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }
  sprintf(header_concat, "%s: %s", header->key, header->value);

  header_fmt = smtp_chunk_split(header_concat,
                                SMTP_HEADER_CHUNKLEN,
                                "\r\n  ");
  free(header_concat);
  if(header_fmt == NULL){
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }

  /*
   * Delete two space characters at the end of the last chunk because
   * the next line should not get indented.
   */
  header_fmt[strlen(header_fmt) - 2] = '\0';

  smtp_puts(smtp, header_fmt);
  free(header_fmt);
  return smtp->status_code;
}

/**
 * Take a FROM, TO, and CC address and add it into the email header list.
 *
 * The following example shows what the final header might look like when
 * the client sends an email to two CC addresses:
 * Cc: mail1@example.com, mail2@example.com
 *
 * @param[in] smtp         SMTP client context.
 * @param[in] address_type @ref smtp_address_type.
 * @param[in] key          Header key value, for example, To From Cc.
 * @return @ref smtp_status_code.
 */
static int
smtp_append_address_to_header(struct smtp *const smtp,
                              enum smtp_address_type address_type,
                              const char *const key){
  size_t i;
  size_t num_address_in_header;
  size_t header_value_sz;
  struct smtp_address *address;
  char *header_value;
  char *header_value_new;

  num_address_in_header = 0;
  header_value_sz = 0;
  header_value = NULL;
  for(i = 0; i < smtp->num_address; i++){
    address = &smtp->address_list[i];
    if(address->type == address_type){
      /* + 3 -> ', ' */
      header_value_sz += strlen(address->email) + 3;
      if((header_value_new = realloc(header_value,
                                     header_value_sz)) == NULL){
        free(header_value);
        return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
      }
      header_value = header_value_new;
      if(num_address_in_header > 0){
        strcat(header_value, ", ");
        strcat(header_value, address->email);
      }
      else{
        strcpy(header_value, address->email);
      }
      num_address_in_header += 1;
    }
  }

  if(header_value){
    smtp_header_add(smtp, key, header_value);
    free(header_value);
  }
  return smtp->status_code;
}

/**
 * Send envelope MAIL FROM or RCPT TO header address.
 *
 * Examples:
 * MAIL FROM:<mail@example.com>
 * RCPT TO:<mail@example.com>
 *
 * @param[in] smtp    SMTP client context.
 * @param[in] header  Either "MAIL FROM" or "RCPT TO".
 * @param[in] address @ref smtp_address -> email field.
 * @return @ref smtp_status_code.
 */
static int
smtp_mail_envelope_header(struct smtp *const smtp,
                          const char *const header,
                          const struct smtp_address *const address){
  const char *const SMTPUTF8 = " SMTPUTF8";
  size_t bufsz;
  char *envelope_address;
  const char *smtputf8_opt;

  bufsz = 14 + strlen(address->email) + strlen(SMTPUTF8) + 1;
  if((envelope_address = malloc(bufsz)) == NULL){
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }

  smtputf8_opt = "";
  if(smtp_str_has_nonascii_utf8(address->email)){
    smtputf8_opt = SMTPUTF8;
  }

  sprintf(envelope_address,
          "%s:<%s>%s\r\n",
          header,
          address->email,
          smtputf8_opt);
  smtp_puts(smtp, envelope_address);
  free(envelope_address);

  if(smtp->status_code != SMTP_STATUS_OK){
    return smtp->status_code;
  }
  smtp_read_and_parse_code(smtp);
  return smtp->status_code;
}

/**
 * Comparison function for qsort which sorts headers alphabetically based
 * on the key.
 *
 * @param[in] v1 The first @ref smtp_header to compare.
 * @param[in] v2 The second @ref smtp_header to compare.
 * @retval  0 If the keys match.
 * @retval !0 If the keys do not match.
 */
static int
smtp_header_cmp(const void *v1,
                const void *v2){
  const struct smtp_header *header1;
  const struct smtp_header *header2;

  header1 = v1;
  header2 = v2;
  return strcmp(header1->key, header2->key);
}

/**
 * Search function used by bsearch which allows the caller to check for
 * headers with existing keys.
 *
 * @param v1 String to search for in the list.
 * @param v2 The @ref smtp_header to compare.
 * @retval  0 If the keys match.
 * @retval !0 If the keys do not match.
 */
static int
smtp_header_cmp_key(const void *const v1,
                    const void *const v2){
  const char *key;
  const struct smtp_header *header2;

  key = v1;
  header2 = v2;
  return strcmp(key, header2->key);
}

/**
 * Determine if the header key has already been defined in this context.
 *
 * @param[in] smtp SMTP client context.
 * @param[in] key  Header key value to search for.
 * @retval 1 If the header already exists in this context.
 * @retval 0 If the header does not exist in this context.
 */
static int
smtp_header_exists(const struct smtp *const smtp,
                   const char *const key){
  if(bsearch(key,
             smtp->header_list,
             smtp->num_headers,
             sizeof(*smtp->address_list),
             smtp_header_cmp_key) == NULL){
    return 0;
  }
  return 1;
}

/**
 * Must consist only of printable US-ASCII characters except colon.
 *
 * @param[in] key Header key to validate.
 * @retval  0 Successful validation.
 * @retval -1 Failed to validate.
 */
SMTP_LINKAGE int
smtp_header_key_validate(const char *const key){
  unsigned c;
  size_t i;
  size_t keylen;

  keylen = strlen(key);
  if(keylen < 1){
    return -1;
  }

  for(i = 0; i < keylen; i++){
    c = key[i];
    if(c <= ' ' || c > 126 || c == ':'){
      return -1;
    }
  }

  return 0;
}

/**
 * Must consist only of printable character, space, or horizontal tab.
 *
 * @param[in] value Header value to validate.
 * @retval  0 Successful validation.
 * @retval -1 Failed to validate.
 */
SMTP_LINKAGE int
smtp_header_value_validate(const char *const value){
  size_t i;
  unsigned char c;

  for(i = 0; value[i]; i++){
    c = value[i];
    if((c < ' ' || c > 126) &&
        c != '\t' &&
        c < 0x80){ /* Allow UTF-8 byte sequence. */
      return -1;
    }
  }
  return 0;
}

/**
 *  The email address must consist only of printable characters excluding
 *  the angle brackets (<) and (>).
 *
 * @param[in] email The email address of the party.
 * @retval  0 Successful validation.
 * @retval -1 Failed to validate.
 */
SMTP_LINKAGE int
smtp_address_validate_email(const char *const email){
  size_t i;
  unsigned char c;

  for(i = 0; email[i]; i++){
    c = email[i];
    if(c <= ' ' || c == 127 ||
       c == '<' || c == '>'){
      return -1;
    }
  }
  return 0;
}

/**
 * Email user name must consist only of printable characters, excluding the
 * double quote character.
 *
 * @param[in] name Email name to validate.
 * @retval  0 Successful validation.
 * @retval -1 Failed to validate.
 */
SMTP_LINKAGE int
smtp_address_validate_name(const char *const name){
  size_t i;
  unsigned char c;

  for(i = 0; name[i]; i++){
    c = name[i];
    if(c < ' ' || c == 127 || c == '\"'){
      return -1;
    }
  }
  return 0;
}

/**
 * Must consist only of printable characters or the space character ( ), and
 * excluding the quote characters (') and (").
 *
 * @param[in] name Filename of the attachment shown to recipients.
 * @retval  0 Successful validation.
 * @retval -1 Failed to validate.
 */
SMTP_LINKAGE int
smtp_attachment_validate_name(const char *const name){
  size_t i;
  unsigned c;

  for(i = 0; name[i]; i++){
    c = name[i];
    if(c < ' ' || c == 127 ||
       c == '\'' || c == '\"'){
      return -1;
    }
  }
  return 0;
}

/**
 * Special flag value for the SMTP context used to determine if the initial
 * memory allocation failed to create the context.
 */
#define SMTP_FLAG_INVALID_MEMORY (enum smtp_flag)(0xFFFFFFFF)

/**
 * Open a connection to an SMTP server and return the context.
 *
 * After successfully connecting and performing a handshake with the
 * SMTP server, this function will return an SMTP client context which
 * the calling program can use in the other API function calls. The
 * caller must always use this function prior to any other smtp-client
 * library function.
 *
 * This function always returns a valid SMTP client context even if
 * the server connection or memory allocation fails. However, the error
 * status will continue to propagate to any further function calls for
 * the SMTP context while in this failure mode.
 *
 * @param[in]  server              Server name or IP address.
 * @param[in]  port                Server port number.
 * @param[in]  connection_security See @ref smtp_connection_security.
 * @param[in]  flags               See @ref smtp_flag.
 * @param[in]  cafile              Path to certificate file, or NULL to use
 *                                 certificates in the default path.
 * @param[out] smtp                Pointer to a new SMTP context which will
 *                                 always have a valid state even if memory
 *                                 allocation fails. When finished, the caller
 *                                 must free this context using
 *                                 @ref smtp_close.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_open(const char *const server,
          const char *const port,
          enum smtp_connection_security connection_security,
          enum smtp_flag flags,
          const char *const cafile,
          struct smtp **smtp){
  struct smtp *snew;

  /*
   * This error structure used for the single error case where we cannot
   * initially allocate memory. This makes it easier to propagate any
   * error codes when calling the other header functions because the
   * caller will always get a valid SMTP structure returned.
   */
  static struct smtp smtp_error = {0};

  if((snew = calloc(1, sizeof(**smtp))) == NULL){
    smtp_error.flags = SMTP_FLAG_INVALID_MEMORY;
    *smtp = &smtp_error;
    return smtp_status_code_set(*smtp, SMTP_STATUS_NOMEM);
  }
  *smtp = snew;

  snew->flags = flags;
  snew->cafile = cafile;

  if(smtp_connect(snew, server, port) < 0){
    return smtp_status_code_set(*smtp, SMTP_STATUS_CONNECT);
  }

  /* All other gdfd fields already set to NULL from the snew calloc. */
  snew->gdfd.delim           = '\n';
  snew->gdfd.getdelimfd_read = smtp_str_getdelimfd_read;
  snew->gdfd.user_data       = snew;

  if(smtp_initiate_handshake(snew,
                             server,
                             connection_security) != SMTP_STATUS_OK){
    return smtp_status_code_set(*smtp, SMTP_STATUS_HANDSHAKE);
  }

  return snew->status_code;
}

/**
 * Authenticate the user using one of the methods listed in
 * @ref smtp_authentication_method.
 *
 * @param[in] smtp        SMTP client context.
 * @param[in] auth_method See @ref smtp_authentication_method.
 * @param[in] user        Server authentication user name.
 * @param[in] pass        Server authentication user password.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_auth(struct smtp *const smtp,
          enum smtp_authentication_method auth_method,
          const char *const user,
          const char *const pass){
  int auth_rc;

  if(smtp->status_code != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  switch(auth_method){
    case SMTP_AUTH_PLAIN:
      auth_rc = smtp_auth_plain(smtp, user, pass);
      break;
    case SMTP_AUTH_LOGIN:
      auth_rc = smtp_auth_login(smtp, user, pass);
      break;

#ifdef SMTP_OPENSSL
    case SMTP_AUTH_CRAM_MD5:
      auth_rc = smtp_auth_cram_md5(smtp, user, pass);
      break;
#endif /* SMTP_OPENSSL */

    case SMTP_AUTH_NONE:
      auth_rc = 0;
      break;
    default:
      return smtp_status_code_set(smtp, SMTP_STATUS_PARAM);
  }

  if(auth_rc < 0){
    return smtp_status_code_set(smtp, SMTP_STATUS_AUTH);
  }

  return smtp->status_code;
}

/**
 * Sends an email using the addresses, attachments, and headers defined
 * in the current SMTP context.
 *
 * The caller must use the smtp_open function prior to this function. The
 * 'Date' header also gets generated here if it hasn't already been provided.
 *
 * @param[in] smtp SMTP client context.
 * @param[in] body Null-terminated string to send in the email body.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_mail(struct smtp *const smtp,
          const char *const body){
  size_t i;
  int has_mail_from;
  struct smtp_address *address;
  char date[SMTP_DATE_MAX_SZ];

  if(smtp->status_code != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  /* MAIL timeout 5 minutes. */
  smtp_set_read_timeout(smtp, 60 * 5);
  has_mail_from = 0;
  for(i = 0; i < smtp->num_address; i++){
    address = &smtp->address_list[i];
    if(address->type == SMTP_ADDRESS_FROM){
      if(smtp_mail_envelope_header(smtp,
                                   "MAIL FROM",
                                   address) != SMTP_STATUS_OK){
        return smtp->status_code;
      }
      has_mail_from = 1;
      break;
    }
  }

  if(!has_mail_from){
    return smtp_status_code_set(smtp, SMTP_STATUS_PARAM);
  }

  /* RCPT timeout 5 minutes. */
  smtp_set_read_timeout(smtp, 60 * 5);

  for(i = 0; i < smtp->num_address; i++){
    address = &smtp->address_list[i];
    if(address->type != SMTP_ADDRESS_FROM){
      if(smtp_mail_envelope_header(smtp,
                                   "RCPT TO",
                                   address) != SMTP_STATUS_OK){
        return smtp->status_code;
      }
    }
  }

  /* DATA timeout 2 minutes. */
  smtp_set_read_timeout(smtp, 60 * 2);

  if(smtp_puts(smtp, "DATA\r\n") != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  /* 354 response to DATA must get returned before we can send the message. */
  if(smtp_read_and_parse_code(smtp) != SMTP_BEGIN_MAIL){
    return smtp_status_code_set(smtp, SMTP_STATUS_SERVER_RESPONSE);
  }

  if(!smtp_header_exists(smtp, "Date")){
    if(smtp_date_rfc_2822(date) < 0){
      return smtp_status_code_set(smtp, SMTP_STATUS_DATE);
    }
    if(smtp_header_add(smtp, "Date", date) != SMTP_STATUS_OK){
      return smtp->status_code;
    }
  }

  /* DATA block timeout 3 minutes. */
  smtp_set_read_timeout(smtp, 60 * 3);

  if(smtp_append_address_to_header(smtp,
                                   SMTP_ADDRESS_FROM,
                                   "From") != SMTP_STATUS_OK ||
     smtp_append_address_to_header(smtp,
                                   SMTP_ADDRESS_TO,
                                   "To") != SMTP_STATUS_OK ||
     smtp_append_address_to_header(smtp,
                                   SMTP_ADDRESS_CC,
                                   "Cc") != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  for(i = 0; i < smtp->num_headers; i++){
    if(smtp_print_header(smtp, &smtp->header_list[i]) != SMTP_STATUS_OK){
      return smtp->status_code;
    }
  }

  if(smtp_print_mime_email(smtp, body) != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  /* End of DATA segment. */
  if(smtp_puts(smtp, ".\r\n") != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  /* DATA termination timeout 250 return code - 10 minutes. */
  smtp_set_read_timeout(smtp, 60 * 10);
  if(smtp_read_and_parse_code(smtp) != SMTP_DONE){
    return smtp_status_code_set(smtp, SMTP_STATUS_SERVER_RESPONSE);
  }

  return smtp->status_code;
}

/**
 * Close the SMTP connection and frees all resources held by the
 * SMTP context.
 *
 * @param[in] smtp SMTP client context.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_close(struct smtp *smtp){
  enum smtp_status_code status_code;

  status_code = smtp->status_code;

  if(smtp->flags == SMTP_FLAG_INVALID_MEMORY){
    return status_code;
  }

  if(smtp->sock){
    /*
     * Do not error out if this fails because we still need to free the
     * SMTP client resources.
     */
    smtp->status_code = SMTP_STATUS_OK;
    smtp_puts(smtp, "QUIT\r\n");

#ifdef SMTP_OPENSSL
    if(smtp->tls_on){
      SSL_free(smtp->tls);
      SSL_CTX_free(smtp->tls_ctx);
    }
#endif /* SMTP_OPENSSL */

#ifdef SMTP_IS_WINDOWS
    closesocket(smtp->sock);
    WSACleanup();
#else /* POSIX */
    if(close(smtp->sock) < 0){
      if(smtp->status_code == SMTP_STATUS_OK){
        smtp_status_code_set(smtp, SMTP_STATUS_CLOSE);
      }
    }
#endif /* SMTP_IS_WINDOWS */
  }

  smtp_str_getdelimfd_free(&smtp->gdfd);
  smtp_header_clear_all(smtp);
  smtp_address_clear_all(smtp);
  smtp_attachment_clear_all(smtp);
  if(status_code == SMTP_STATUS_OK){
    status_code = smtp->status_code;
  }
  free(smtp);

  return status_code;
}

/**
 * Get the current status/error code described in @ref smtp_status_code.
 *
 * @param[in] smtp SMTP client context.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_status_code_get(const struct smtp *const smtp){
  return smtp->status_code;
}

/**
 * Set the error status of the SMTP client context and return the same code.
 *
 * This function allows the caller to clear an error status to SMTP_STATUS_OK
 * so that previous errors will stop propagating. However, this will only
 * work correctly for clearing the SMTP_STATUS_PARAM and SMTP_STATUS_FILE
 * errors. Clearing the status from any other error code will invoke
 * undefined behavior and will almost never work correctly.
 *
 * @param[in] smtp        SMTP client context.
 * @param[in] status_code See @ref smtp_status_code.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_status_code_set(struct smtp *const smtp,
                     enum smtp_status_code status_code){
  if((unsigned)status_code >= SMTP_STATUS__LAST){
    return smtp_status_code_set(smtp, SMTP_STATUS_PARAM);
  }
  smtp->status_code = status_code;
  return status_code;
}

/**
 * Convert a standard smtp-client return code to a description.
 *
 * @param[in] status_code Status code returned from one of the other
 *                        smtp-client library functions.
 * @return String containing a description of the @p status_code. The caller
 *         must not free or modify this string.
 */
const char *
smtp_status_code_errstr(enum smtp_status_code status_code){
  const char *const status_code_err_str[] = {
    /* SMTP_STATUS_OK */
    "Success",
    /* SMTP_STATUS_NOMEM */
    "Memory allocation failed",
    /* SMTP_STATUS_CONNECT */
    "Failed to connect to the mail server",
    /* SMTP_STATUS_HANDSHAKE */
    "Failed to handshake or negotiate a TLS connection with the server",
    /* SMTP_STATUS_AUTH */
    "Failed to authenticate with the given credentials",
    /* SMTP_STATUS_SEND */
    "Failed to send bytes to the server",
    /* SMTP_STATUS_RECV */
    "Failed to receive bytes from the server",
    /* SMTP_STATUS_CLOSE */
    "Failed to properly close a connection",
    /* SMTP_STATUS_SERVER_RESPONSE */
    "SMTP server sent back an unexpected status code",
    /* SMTP_STATUS_PARAM */
    "Invalid parameter",
    /* SMTP_STATUS_FILE */
    "Failed to read or open a local file",
    /* SMTP_STATUS_DATE */
    "Failed to get the local date and time",
    /* SMTP_STATUS__LAST */
    "Unknown error"
  };

  if((unsigned)status_code > SMTP_STATUS__LAST){
    status_code = SMTP_STATUS__LAST;
  }
  return status_code_err_str[status_code];
}

/**
 * Add a key/value header to the header list in the SMTP context.
 *
 * This will insert instead of replacing an existing header with the same key.
 *
 * @param[in] smtp  SMTP client context.
 * @param[in] key   Key name for new header. It must consist only of
 *                  printable US-ASCII characters except colon.
 * @param[in] value Value for new header. It must consist only of printable
 *                  US-ASCII, space, or horizontal tab. If set to NULL,
 *                  this will prevent the header from printing out.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_header_add(struct smtp *const smtp,
                const char *const key,
                const char *const value){
  struct smtp_header *new_header_list;
  struct smtp_header *new_header;
  size_t new_realloc_sz;

  if(smtp->status_code != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  if(smtp_header_key_validate(key) < 0){
    return smtp_status_code_set(smtp, SMTP_STATUS_PARAM);
  }

  if(value && smtp_header_value_validate(value) < 0){
    return smtp_status_code_set(smtp, SMTP_STATUS_PARAM);
  }

  new_realloc_sz = (smtp->num_headers + 1) * sizeof(*smtp->header_list);
  if((new_header_list = realloc(smtp->header_list, new_realloc_sz)) == NULL){
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }
  smtp->header_list = new_header_list;
  new_header = &smtp->header_list[smtp->num_headers];

  new_header->key = smtp_strdup(key);
  new_header->value = value ? smtp_strdup(value) : NULL;
  if(new_header->key == NULL ||
     (new_header->value == NULL && value)){
    free(new_header->key);
    free(new_header->value);
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }

  smtp->num_headers += 1;

  qsort(smtp->header_list,
        smtp->num_headers,
        sizeof(*smtp->header_list),
        smtp_header_cmp);

  return smtp->status_code;
}

/**
 * Free all memory related to email headers.
 *
 * @param[in] smtp SMTP client context.
 */
void smtp_header_clear_all(struct smtp *const smtp){
  size_t i;
  struct smtp_header *header;

  for(i = 0; i < smtp->num_headers; i++){
    header = &smtp->header_list[i];
    free(header->key);
    free(header->value);
  }
  free(smtp->header_list);
  smtp->header_list = NULL;
  smtp->num_headers = 0;
}

/**
 * Add a FROM, TO, CC, or BCC address destination to this SMTP context.
 *
 * @note Some SMTP servers may reject over 100 recipients.
 *
 * @param[in] smtp  SMTP client context.
 * @param[in] type  See @ref smtp_address_type.
 * @param[in] email The email address of the party. Must consist only of
 *                  printable characters excluding the angle brackets
 *                  (<) and (>).
 * @param[in] name  Name or description of the party. Must consist only of
 *                  printable characters, excluding the quote characters. If
 *                  set to NULL, no name will get associated with this email.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_address_add(struct smtp *const smtp,
                 enum smtp_address_type type,
                 const char *const email,
                 const char *const name){
  size_t new_size;
  struct smtp_address *new_address_list;
  struct smtp_address *new_address;

  if(smtp->status_code != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  if(smtp_address_validate_email(email) < 0){
    return smtp_status_code_set(smtp, SMTP_STATUS_PARAM);
  }

  if(name && smtp_address_validate_name(name) < 0){
    return smtp_status_code_set(smtp, SMTP_STATUS_PARAM);
  }

  new_size = (smtp->num_address + 1) * sizeof(*new_address_list);
  new_address_list = realloc(smtp->address_list, new_size);
  if(new_address_list == NULL){
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }
  new_address = &new_address_list[smtp->num_address];

  smtp->address_list = new_address_list;

  new_address->type = type;
  new_address->email = smtp_strdup(email);
  new_address->name = name ? smtp_strdup(name) : NULL;
  if(new_address->email == NULL ||
     (new_address->name == NULL && name)){
    free(new_address->email);
    free(new_address->name);
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }
  smtp->num_address += 1;

  return smtp->status_code;
}

/**
 * Free all memory related to the address list.
 *
 * @param[in] smtp SMTP client context.
 */
void smtp_address_clear_all(struct smtp *const smtp){
  size_t i;
  struct smtp_address *address;

  for(i = 0; i < smtp->num_address; i++){
    address = &smtp->address_list[i];
    free(address->email);
    free(address->name);
  }
  free(smtp->address_list);
  smtp->address_list = NULL;
  smtp->num_address = 0;
}

/**
 * Add a file attachment from a path.
 *
 * See @ref smtp_attachment_add_mem for more details.
 *
 * @param[in] smtp SMTP client context.
 * @param[in] name Filename of the attachment shown to recipients.
 * @param[in] path Path of file location to read from.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_attachment_add_path(struct smtp *const smtp,
                         const char *const name,
                         const char *const path){
  char *data;
  size_t bytes_read;

  if(smtp->status_code != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  errno = 0;
  if((data = smtp_file_get_contents(path, &bytes_read)) == NULL){
    if(errno == ENOMEM){
      return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
    }
    return smtp_status_code_set(smtp, SMTP_STATUS_FILE);
  }
  smtp_attachment_add_mem(smtp, name, data, bytes_read);
  free(data);
  return smtp->status_code;
}

/**
 * Add an attachment using a file pointer.
 *
 * See @ref smtp_attachment_add_mem for more details.
 *
 * @param[in] smtp SMTP client context.
 * @param[in] name Filename of the attachment shown to recipients.
 * @param[in] fp   File pointer already opened by the caller.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_attachment_add_fp(struct smtp *const smtp,
                       const char *const name,
                       FILE *fp){
  char *data;
  size_t bytes_read;

  if(smtp->status_code != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  errno = 0;
  if((data = smtp_ffile_get_contents(fp, &bytes_read)) == NULL){
    if(errno == ENOMEM){
      return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
    }
    return smtp_status_code_set(smtp, SMTP_STATUS_FILE);
  }
  smtp_attachment_add_mem(smtp, name, data, bytes_read);
  free(data);
  return smtp->status_code;
}

/**
 * Add a MIME attachment to this SMTP context with the data retrieved
 * from memory.
 *
 * The attachment data will get base64 encoded before sending to the server.
 *
 * @param[in] smtp   SMTP client context.
 * @param[in] name   Filename of the attachment shown to recipients. Must
 *                   consist only of printable characters excluding the
 *                   quote characters (') and ("), or the space character
 *                   ( ).
 * @param[in] data   Raw attachment data stored in memory.
 * @param[in] datasz Number of bytes in @p data, or -1 if data
 *                   null-terminated.
 * @return @ref smtp_status_code.
 */
enum smtp_status_code
smtp_attachment_add_mem(struct smtp *const smtp,
                        const char *const name,
                        const void *const data,
                        ssize_t datasz){
  size_t new_size;
  struct smtp_attachment *new_attachment_list;
  struct smtp_attachment *new_attachment;

  if(smtp->status_code != SMTP_STATUS_OK){
    return smtp->status_code;
  }

  if(smtp_attachment_validate_name(name) < 0){
    return smtp_status_code_set(smtp, SMTP_STATUS_PARAM);
  }

  if(datasz < 0){
    datasz = strlen(data);
  }

  new_size = (smtp->num_attachment + 1) * sizeof(*new_attachment_list);
  if((new_attachment_list = realloc(smtp->attachment_list,
                                    new_size)) == NULL){
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }
  smtp->attachment_list = new_attachment_list;
  new_attachment = &new_attachment_list[smtp->num_attachment];

  new_attachment->name = smtp_strdup(name);
  new_attachment->b64_data = smtp_base64_encode(data, datasz);
  if(new_attachment->name == NULL || new_attachment->b64_data == NULL){
    free(new_attachment->name);
    free(new_attachment->b64_data);
    return smtp_status_code_set(smtp, SMTP_STATUS_NOMEM);
  }

  smtp->num_attachment += 1;
  return smtp->status_code;
}

/**
 * Remove all attachments from the SMTP client context.
 *
 * @param[in] smtp SMTP client context.
 */
void smtp_attachment_clear_all(struct smtp *const smtp){
  size_t i;
  struct smtp_attachment *attachment;

  for(i = 0; i < smtp->num_attachment; i++){
    attachment = &smtp->attachment_list[i];
    free(attachment->name);
    free(attachment->b64_data);
  }
  free(smtp->attachment_list);
  smtp->attachment_list = NULL;
  smtp->num_attachment = 0;
}
kc3-lang/smtp/src/smtp.c

Commit

kc3-lang/smtp /src/smtp.c
