IABSD.fr/src/lib/libutil/imsg_init.3

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• Author : schwarze
  Date : 2025-06-13 18:34:00
  Hash : 74a1e058
  Message : The mdoc(7) .Ft macro does not need quoting of its arguments, but about 10% of our manual pages using this macro employed useless quoting anyway. Remove these quotes such that they do not incite fear, uncertainty, and doubt in developers who happen to look at these pages. jmc@ and tb@ agree with the direction.

• lib/libutil/imsg_init.3

• .\" $OpenBSD: imsg_init.3,v 1.46 2025/06/13 18:34:00 schwarze Exp $
  .\"
  .\" Copyright (c) 2023 Claudio Jeker <claudio@openbsd.org>
  .\" Copyright (c) 2010 Nicholas Marriott <nicm@openbsd.org>
  .\"
  .\" Permission to use, copy, modify, and distribute this software for any
  .\" purpose with or without fee is hereby granted, provided that the above
  .\" copyright notice and this permission notice appear in all copies.
  .\"
  .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
  .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
  .\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
  .\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
  .\" WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
  .\" IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
  .\" OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  .\"
  .Dd $Mdocdate: June 13 2025 $
  .Dt IMSG_ADD 3
  .Os
  .Sh NAME
  .Nm imsg_add ,
  .Nm imsg_close ,
  .Nm imsg_compose ,
  .Nm imsg_compose_ibuf ,
  .Nm imsg_composev ,
  .Nm imsg_create ,
  .Nm imsg_forward ,
  .Nm imsg_free ,
  .Nm imsg_get_buf ,
  .Nm imsg_get_data ,
  .Nm imsg_get_fd ,
  .Nm imsg_get_ibuf ,
  .Nm imsg_get_id ,
  .Nm imsg_get_len ,
  .Nm imsg_get_pid ,
  .Nm imsg_get_strbuf ,
  .Nm imsg_get_type ,
  .Nm imsg_ibufq_pop ,
  .Nm imsg_ibufq_push ,
  .Nm imsg_set_maxsize ,
  .Nm imsgbuf_allow_fdpass ,
  .Nm imsgbuf_clear ,
  .Nm imsgbuf_flush ,
  .Nm imsgbuf_get ,
  .Nm imsgbuf_init ,
  .Nm imsgbuf_queuelen ,
  .Nm imsgbuf_read ,
  .Nm imsgbuf_set_maxsize ,
  .Nm imsgbuf_write
  .Nd IPC messaging functions
  .Sh SYNOPSIS
  .Lb libutil
  .In imsg.h
  .Fd #define IMSG_HEADER_SIZE sizeof(struct imsg_hdr)
  .Fd #define MAX_IMSGSIZE 16384
  .Ft int
  .Fn imsg_add "struct ibuf *msg" "const void *data" "size_t datalen"
  .Ft void
  .Fn imsg_close "struct imsgbuf *imsgbuf" "struct ibuf *msg"
  .Ft int
  .Fn imsg_compose "struct imsgbuf *imsgbuf" "uint32_t type" "uint32_t id" \
      "pid_t pid" "int fd" "const void *data" "size_t datalen"
  .Ft int
  .Fn imsg_compose_ibuf "struct imsgbuf *imsgbuf" "uint32_t type" \
      "uint32_t id" "pid_t pid" "struct ibuf *buf"
  .Ft struct ibuf *
  .Fn imsg_create "struct imsgbuf *imsgbuf" "uint32_t type" "uint32_t id" \
      "pid_t pid" "size_t datalen"
  .Ft int
  .Fn imsg_forward "struct imsgbuf *imsgbuf" "struct imsg *msg"
  .Ft void
  .Fn imsg_free "struct imsg *imsg"
  .Ft int
  .Fn imsg_get_buf "struct imsg *imsg" "void *data" "size_t len"
  .Ft int
  .Fn imsg_get_data "struct imsg *imsg" "void *data" "size_t len"
  .Ft int
  .Fn imsg_get_fd "struct imsg *imsg"
  .Ft int
  .Fn imsg_get_ibuf "struct imsg *imsg" "struct ibuf *ibuf"
  .Ft uint32_t
  .Fn imsg_get_id "struct imsg *imsg"
  .Ft size_t
  .Fn imsg_get_len "struct imsg *imsg"
  .Ft pid_t
  .Fn imsg_get_pid "struct imsg *imsg"
  .Ft int
  .Fn imsg_get_strbuf "struct imsg *imsg" "char *str" "size_t len"
  .Ft uint32_t
  .Fn imsg_get_type "struct imsg *imsg"
  .Ft int
  .Fn imsg_ibufq_pop "struct ibufqueue *bufq" "struct imsg *imsg"
  .Ft void
  .Fn imsg_ibufq_push "struct ibufqueue *bufq" "struct imsg *imsg"
  .Ft int
  .Fn imsg_set_maxsize "struct ibuf *msg" "size_t max"
  .Ft void
  .Fn imsgbuf_allow_fdpass "struct imsgbuf *imsgbuf"
  .Ft void
  .Fn imsgbuf_clear "struct imsgbuf *imsgbuf"
  .Ft int
  .Fn imsgbuf_flush "struct imsgbuf *imsgbuf"
  .Ft int
  .Fn imsgbuf_get "struct imsgbuf *imsgbuf" "struct imsg *imsg"
  .Ft int
  .Fn imsgbuf_init "struct imsgbuf *imsgbuf" "int fd"
  .Ft uint32_t
  .Fn imsgbuf_queuelen "struct imsgbuf *imsgbuf"
  .Ft int
  .Fn imsgbuf_read "struct imsgbuf *imsgbuf"
  .Ft int
  .Fn imsgbuf_set_maxsize "struct imsgbuf *imsgbuf" "uint32_t max"
  .Ft int
  .Fn imsgbuf_write "struct imsgbuf *imsgbuf"
  .In sys/uio.h
  .Ft int
  .Fn imsg_composev "struct imsgbuf *imsgbuf" "uint32_t type" "uint32_t id" \
      "pid_t pid" "int fd" "const struct iovec *iov" "int iovcnt"
  .Sh DESCRIPTION
  The
  .Nm imsg
  functions provide a simple mechanism for communication between local processes
  using sockets.
  Each transmitted message is guaranteed to be presented to the receiving program
  whole.
  They are commonly used in privilege separated processes, where processes with
  different rights are required to cooperate.
  .Pp
  .Fn imsgbuf_init
  initializes
  .Fa imsgbuf
  as one side of a channel associated with
  .Fa fd .
  The file descriptor is used to send and receive messages,
  but is not closed by any of the imsg functions.
  It returns 0 if successful and -1 on failure.
  .Pp
  .Fn imsgbuf_allow_fdpass
  enables file descriptor passing in both directions for this
  .Fa imsgbuf .
  .Pp
  .Fn imsgbuf_set_maxsize
  changes the default maximum payload size
  to
  .Fa max .
  It returns 0 if successful and -1 on failure.
  .Pp
  The
  .Fn imsgbuf_clear
  function frees any data allocated as part of an imsgbuf.
  This function does not close the file descriptor used for communication.
  .Pp
  The
  .Fn imsgbuf_read
  routine reads pending data with
  .Xr recvmsg 2
  and queues it as individual messages on
  .Fa imsgbuf .
  It returns 1 on success, 0 if the connection is closed, or \-1 on error
  and the global variable
  .Va errno
  is set to indicate the error.
  The errors
  .Er EINTR
  and
  .Er EAGAIN
  are treated as follows.
  .Er EINTR
  will automatically retry the read operation while the other errors are
  ignored with a 1 return.
  .Pp
  .Fn imsgbuf_write
  writes out queued messages.
  It returns 0 if it succeeds, -1 on error and the global variable
  .Va errno
  is set to indicate the error.
  The errors
  .Er EINTR ,
  .Er EAGAIN ,
  and
  .Er ENOBUFS
  are treated as follows.
  .Er EINTR
  will automatically retry the write operation while the other errors are
  ignored with a 0 return.
  .Pp
  .Fn imsgbuf_flush
  calls
  .Fn imsgbuf_write
  in a loop until all imsgs in the output buffer are sent.
  It returns 0 if it succeeds, \-1 otherwise and the global variable
  .Va errno
  is set to indicate the error.
  .Fn imsgbuf_flush
  should not be called on non-blocking sockets since it will busy loop if the
  socket is not available.
  .Pp
  .Fn imsgbuf_get
  fills in an individual imsg pending on
  .Fa imsgbuf
  into the structure pointed to by
  .Fa imsg .
  It returns 1 on success, 0 if no messages are ready, or \-1 for an error.
  Received messages are returned as a
  .Em struct imsg ,
  which must be freed by
  .Fn imsg_free
  when no longer required.
  .Pp
  .Fn imsgbuf_queuelen
  returns the number of messages ready to be sent.
  This function returns 0 if no messages are pending for transmission.
  .Pp
  .Fn imsg_create ,
  .Fn imsg_add
  and
  .Fn imsg_close
  are generic construction routines for messages that are to be sent using an
  imsgbuf.
  .Pp
  .Fn imsg_create
  creates a new message with header specified by
  .Fa type ,
  .Fa id
  and
  .Fa pid .
  A
  .Fa pid
  of zero uses the process ID returned by
  .Xr getpid 2
  when
  .Fa imsgbuf
  was initialized.
  In addition to this common imsg header,
  .Fa datalen
  bytes of space may be reserved for attaching to this imsg.
  This space is populated using
  .Fn imsg_add .
  .Fn imsg_create
  returns a pointer to a new message if it succeeds, NULL otherwise.
  .Pp
  .Fn imsg_add
  appends to
  .Fa msg
  .Fa datalen
  bytes of ancillary data pointed to by
  .Fa data .
  It returns
  .Fa datalen
  if it succeeds, otherwise
  .Fa msg
  is freed and \-1 is returned.
  .Pp
  .Fn imsg_set_maxsize
  reduces the maximum payload of
  .Fa msg
  to
  .Fa max .
  The routine returns 0 if it succeeds, \-1 otherwise.
  .Pp
  .Fn imsg_close
  completes creation of
  .Fa msg
  by adding it to
  .Fa imsgbuf
  output buffer.
  .Pp
  .Fn imsg_compose
  is used to quickly create and queue an imsg.
  It takes the same parameters as the
  .Fn imsg_create ,
  .Fn imsg_add
  and
  .Fn imsg_close
  routines,
  except that only one ancillary data buffer can be provided.
  Additionally, the file descriptor
  .Fa fd
  may be passed over the socket to the other process.
  If
  .Fa fd
  is given, it is closed in the sending program after the message is sent.
  A value of \-1 indicates no file descriptor should be passed.
  This routine returns 1 if it succeeds, \-1 otherwise.
  .Pp
  .Fn imsg_composev
  is similar to
  .Fn imsg_compose .
  It takes the same parameters, except that the ancillary data buffer is specified
  by
  .Fa iovec .
  .Pp
  .Fn imsg_compose_ibuf
  is similar to
  .Fn imsg_compose .
  It takes the same parameters, except that the ancillary data buffer is specified
  by an ibuf
  .Fa buf .
  This routine returns 1 if it succeeds, \-1 otherwise.
  In either case the buffer
  .Fa buf
  is consumed by the function.
  .Pp
  .Fn imsg_forward
  forwards a just received
  .Fa msg
  unaltered on
  .Fa imsgbuf .
  File descriptors are not forwarded by this function.
  It is possible to call
  .Fn imsg_forward
  more than once per message.
  .Pp
  The accessors
  .Fn imsg_get_type ,
  .Fn imsg_get_pid ,
  .Fn imsg_get_id ,
  and
  .Fn imsg_get_len ,
  return the
  .Fa type ,
  .Fa pid ,
  .Fa id ,
  and payload length used in
  .Fn imsg_create
  to build the
  .Fa imsg .
  If there is no payload
  .Fn imsg_get_len
  returns 0.
  .Pp
  .Fn imsg_get_fd
  returns the file descriptor and passes the responsibility to track the
  descriptor back to the program.
  Unclaimed file descriptors are closed by
  .Fn imsg_free .
  .Pp
  .Fn imsg_get_data
  and
  .Fn imsg_get_ibuf
  are used to extract the payload of an
  .Fa imsg .
  .Fn imsg_get_data
  can be used if the structure of the payload is known and can be extracted
  in one go.
  0 is returned on success and \-1 on failure.
  .Fn imsg_get_ibuf
  initializes the passed
  .Fa ibuf
  to hold the payload which can be read using
  .Xr ibuf_get 3 .
  The
  .Fa ibuf
  remains valid until
  .Fn imsg_free
  is called and there is no need to call
  .Fn ibuf_free
  on this stack based buffer.
  The function returns 0 on success, \-1 otherwise.
  .Pp
  .Fn imsg_get_buf
  and
  .Fn imsg_get_strbuf
  read
  .Fa len
  bytes from the
  .Fa imsg
  and copy them into
  .Fa buf
  or
  .Fa str ,
  respectively.
  .Fn imsg_get_strbuf
  ensures that
  .Fa str
  is NUL-terminated.
  The functions return 0 on success, \-1 otherwise.
  .Pp
  .Fn imsg_set_maxsize
  reduces the maximum payload of
  .Fa msg
  to
  .Fa max .
  The routine returns 0 if it succeeds, \-1 otherwise.
  .Pp
  .Fn imsg_ibufq_pop
  and
  .Fn imsg_ibufq_push
  allow to requeue an imsg onto the ibufqueue
  .Fa bufq .
  .Fn imsg_ibufq_pop
  returns 1 on success, 0 if no messages are ready, or \-1 for an error.
  See
  .Xr ibufq_new 3 for more info.
  .Pp
  MAX_IMSGSIZE is defined as the maximum size of a single imsg, currently
  16384 bytes.
  .Sh EXAMPLES
  In a typical program, a channel between two processes is created with
  .Xr socketpair 2 ,
  and an
  .Em imsgbuf
  created around one file descriptor in each process:
  .Bd -literal -offset indent
  struct imsgbuf	parent_ibuf, child_ibuf;
  int		imsg_fds[2];
  
  if (socketpair(AF_UNIX, SOCK_STREAM, PF_UNSPEC, imsg_fds) == -1)
  	err(1, "socketpair");
  
  switch (fork()) {
  case -1:
  	err(1, "fork");
  case 0:
  	/* child */
  	close(imsg_fds[0]);
  	if (imsgbuf_init(&child_ibuf, imsg_fds[1]) == -1)
  		err(1, NULL);
  	exit(child_main(&child_ibuf));
  }
  
  /* parent */
  close(imsg_fds[1]);
  if (imsgbuf_init(&parent_ibuf, imsg_fds[0]) == -1)
  	err(1, NULL);
  exit(parent_main(&parent_ibuf));
  .Ed
  .Pp
  Messages may then be composed and queued on the
  .Em imsgbuf ,
  for example using the
  .Fn imsg_compose
  function:
  .Bd -literal -offset indent
  enum imsg_type {
  	IMSG_A_MESSAGE,
  	IMSG_MESSAGE2
  };
  
  int
  child_main(struct imsgbuf *imsgbuf)
  {
  	int	idata;
  	...
  	idata = 42;
  	imsg_compose(imsgbuf, IMSG_A_MESSAGE,
  	    0, 0, -1, &idata, sizeof idata);
  	...
  }
  .Ed
  .Pp
  A mechanism such as
  .Xr poll 2
  or the
  .Xr event 3
  library is used to monitor the socket file descriptor.
  When the socket is ready for writing, queued messages are transmitted with
  .Fn imsgbuf_write :
  .Bd -literal -offset indent
  	if (imsgbuf_write(imsgbuf) == -1) {
  		if (errno == EPIPE)
  			/* handle closed connection */
  		else
  			/* handle write failure */
  	}
  .Ed
  .Pp
  And when ready for reading, messages are first received using
  .Fn imsgbuf_read
  and then extracted with
  .Fn imsgbuf_get :
  .Bd -literal -offset indent
  void
  dispatch_imsg(struct imsgbuf *imsgbuf)
  {
  	struct imsg	imsg;
  	int		n, idata;
  
  	switch (imsgbuf_read(imsgbuf)) {
  	case -1:
  		/* handle read error */
  		break;
  	case 0:
  		/* handle closed connection */
  		break;
  	}
  
  	for (;;) {
  		if ((n = imsgbuf_get(imsgbuf, &imsg)) == -1) {
  			/* handle read error */
  			break;
  		}
  		if (n == 0)	/* no more messages */
  			return;
  
  		switch (imsg_get_type(&imsg)) {
  		case IMSG_A_MESSAGE:
  			if (imsg_get_data(&imsg, &idata,
  			    sizeof(idata)) == -1) {
  				/* handle corrupt message */
  			}
  			/* handle message received */
  			break;
  		...
  		}
  
  		imsg_free(&imsg);
  	}
  }
  .Ed
  .Sh SEE ALSO
  .Xr socketpair 2 ,
  .Xr ibuf_add 3 ,
  .Xr unix 4