IABSD.fr/src/lib/libc/sys/write.2

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• Author : jan
  Date : 2021-11-21 23:44:55
  Hash : 0215088d
  Message : improve legibility of structs in several manpages General uses tabs for general indentation and 4 spaces on tight spots. Also uses extra space to align pointers and non-pointers as we do this on certain places in our source. with improvements from schwarze@ OK schwarze@

• lib/libc/sys/write.2

• .\"	$OpenBSD: write.2,v 1.43 2021/11/21 23:44:55 jan Exp $
  .\"	$NetBSD: write.2,v 1.6 1995/02/27 12:39:43 cgd Exp $
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  .\"     @(#)write.2	8.5 (Berkeley) 4/2/94
  .\"
  .Dd $Mdocdate: November 21 2021 $
  .Dt WRITE 2
  .Os
  .Sh NAME
  .Nm write ,
  .Nm writev ,
  .Nm pwrite ,
  .Nm pwritev
  .Nd write output
  .Sh SYNOPSIS
  .In unistd.h
  .Ft ssize_t
  .Fn write "int d" "const void *buf" "size_t nbytes"
  .Ft ssize_t
  .Fn pwrite "int d" "const void *buf" "size_t nbytes" "off_t offset"
  .Pp
  .In sys/uio.h
  .Ft ssize_t
  .Fn writev "int d" "const struct iovec *iov" "int iovcnt"
  .In sys/types.h
  .In sys/uio.h
  .Ft ssize_t
  .Fn pwritev "int d" "const struct iovec *iov" "int iovcnt" "off_t offset"
  .Sh DESCRIPTION
  .Fn write
  attempts to write
  .Fa nbytes
  of data to the object referenced by the descriptor
  .Fa d
  from the buffer pointed to by
  .Fa buf .
  .Fn writev
  performs the same action, but gathers the output data from the
  .Fa iovcnt
  buffers specified by the members of the
  .Fa iov
  array: iov[0], iov[1], ..., iov[iovcnt-1].
  .Fn pwrite
  and
  .Fn pwritev
  perform the same functions, but write to the specified position
  .Fa offset
  in the file without modifying the file pointer.
  .Pp
  For
  .Fn writev
  and
  .Fn pwritev ,
  the
  .Vt iovec
  structure is defined as:
  .Bd -literal -offset indent
  struct iovec {
  	void	*iov_base;
  	size_t	 iov_len;
  };
  .Ed
  .Pp
  Each
  .Vt iovec
  entry specifies the base address and length of an area
  in memory from which data should be written.
  .Fn writev
  and
  .Fn pwritev
  will always write a complete area before proceeding to the next.
  .Pp
  On objects capable of seeking, the
  .Fn write
  starts at a position given by the pointer associated with
  .Fa d
  (see
  .Xr lseek 2 ) .
  Upon return from
  .Fn write ,
  the pointer is incremented by the number of bytes which were written.
  If a file was opened with the
  .Dv O_APPEND
  flag (see
  .Xr open 2 ) ,
  calls to
  .Fn write
  or
  .Fn writev
  will automatically set the pointer to the end of the file before writing.
  .Pp
  Objects that are not capable of seeking always write from the current
  position.
  The value of the pointer associated with such an object is undefined.
  .Pp
  If the real user is not the superuser, then
  .Fn write
  clears the set-user-ID bit on a file.
  This prevents penetration of system security by a user who
  .Dq captures
  a writable set-user-ID file owned by the superuser.
  .Pp
  If
  .Fn write
  succeeds it will update the st_ctime and st_mtime fields of the file's
  meta-data (see
  .Xr stat 2 ) .
  .Pp
  When using non-blocking I/O on objects such as sockets that are subject
  to flow control,
  .Fn write
  and
  .Fn writev
  may write fewer bytes than requested; the return value must be noted,
  and the remainder of the operation should be retried when possible.
  .Pp
  Note that
  .Fn writev
  and
  .Fn pwritev
  will fail if the value of
  .Fa iovcnt
  exceeds the constant
  .Dv IOV_MAX .
  .Sh RETURN VALUES
  Upon successful completion the number of bytes which were written
  is returned.
  Otherwise, a \-1 is returned and the global variable
  .Va errno
  is set to indicate the error.
  .Sh EXAMPLES
  A typical loop allowing partial writes looks like this:
  .Bd -literal
  const char *buf;
  size_t bsz, off;
  ssize_t nw;
  int d;
  
  for (off = 0; off < bsz; off += nw)
  	if ((nw = write(d, buf + off, bsz - off)) == 0 || nw == -1)
  		err(1, "write");
  .Ed
  .Sh ERRORS
  .Fn write ,
  .Fn pwrite ,
  .Fn writev ,
  and
  .Fn pwritev
  will fail and the file pointer will remain unchanged if:
  .Bl -tag -width Er
  .It Bq Er EBADF
  .Fa d
  is not a valid descriptor open for writing.
  .It Bq Er EFBIG
  An attempt was made to write a file that exceeds the process's
  file size limit or the maximum file size.
  .It Bq Er ENOSPC
  There is no free space remaining on the file system containing the file.
  .It Bq Er EDQUOT
  The user's quota of disk blocks on the file system containing the file
  has been exhausted.
  .It Bq Er EINTR
  A write to a slow device
  (i.e. one that might block for an arbitrary amount of time)
  was interrupted by the delivery of a signal
  before any data could be written.
  .It Bq Er EIO
  An I/O error occurred while reading from or writing to the file system.
  .It Bq Er EFAULT
  Part of
  .Fa buf
  points outside the process's allocated address space.
  .El
  .Pp
  In addition,
  .Fn write
  and
  .Fn writev
  may return the following errors:
  .Bl -tag -width Er
  .It Bq Er EPIPE
  An attempt is made to write to a pipe that is not open
  for reading by any process.
  .It Bq Er EPIPE
  An attempt is made to write to a socket of type
  .Dv SOCK_STREAM
  that is not connected to a peer socket.
  .It Bq Er EAGAIN
  The file was marked for non-blocking I/O, and no data could be
  written immediately.
  .It Bq Er ENETDOWN
  The destination address specified a network that is down.
  .It Bq Er EDESTADDRREQ
  The destination is no longer available when writing to a
  .Ux Ns -domain
  datagram socket on which
  .Xr connect 2
  had been used to set a destination address.
  .It Bq Er EIO
  The process is a member of a background process attempting to write
  to its controlling terminal,
  .Dv TOSTOP
  is set on the terminal,
  the process isn't ignoring the
  .Dv SIGTTOUT
  signal and the thread isn't blocking the
  .Dv SIGTTOUT
  signal,
  and either the process was created with
  .Xr vfork 2
  and hasn't successfully executed one of the exec functions or
  the process group is orphaned.
  .El
  .Pp
  .Fn write
  and
  .Fn pwrite
  may return the following error:
  .Bl -tag -width Er
  .It Bq Er EINVAL
  .Fa nbytes
  was larger than
  .Dv SSIZE_MAX .
  .El
  .Pp
  .Fn pwrite
  and
  .Fn pwritev
  may return the following error:
  .Bl -tag -width Er
  .It Bq Er EINVAL
  .Fa offset
  was negative.
  .It Bq Er ESPIPE
  .Fa d
  is associated with a pipe, socket, FIFO, or tty.
  .El
  .Pp
  .Fn writev
  and
  .Fn pwritev
  may return one of the following errors:
  .Bl -tag -width Er
  .It Bq Er EINVAL
  .Fa iovcnt
  was less than or equal to 0, or greater than
  .Dv IOV_MAX .
  .It Bq Er EINVAL
  The sum of the
  .Fa iov_len
  values in the
  .Fa iov
  array overflowed an
  .Vt ssize_t .
  .It Bq Er EFAULT
  Part of
  .Fa iov
  points outside the process's allocated address space.
  .It Bq Er ENOBUFS
  The system lacked sufficient buffer space or a queue was full.
  .El
  .Sh SEE ALSO
  .Xr fcntl 2 ,
  .Xr lseek 2 ,
  .Xr open 2 ,
  .Xr pipe 2 ,
  .Xr poll 2 ,
  .Xr select 2 ,
  .Xr termios 4
  .Sh STANDARDS
  The
  .Fn write ,
  .Fn writev ,
  and
  .Fn pwrite
  functions conform to
  .St -p1003.1-2008 .
  .Sh HISTORY
  The
  .Fn write
  function call appeared in
  .At v1 ,
  .Fn pwrite
  in
  .At V.4 ,
  .Fn writev
  in
  .Bx 4.2 ,
  and
  .Fn pwritev
  in
  .Ox 2.7 .
  .Sh CAVEATS
  Error checks should explicitly test for \-1.
  On some platforms, if
  .Fa nbytes
  is larger than
  .Dv SSIZE_MAX
  but smaller than
  .Dv SIZE_MAX
  \- 2, the return value of an error-free call
  may appear as a negative number distinct from \-1.