libc/sys/write.2

.\"	$OpenBSD: write.2,v 1.45 2023/02/05 12:33:17 jsg 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: February 5 2023 $
.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 4.1c ,
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.