NAME
write
, writev
,
pwrite
, pwritev
—
write output
SYNOPSIS
#include
<unistd.h>
ssize_t
write
(int
d, const void *buf,
size_t nbytes);
ssize_t
pwrite
(int
d, const void *buf,
size_t nbytes,
off_t offset);
#include <sys/uio.h>
ssize_t
writev
(int
d, const struct iovec
*iov, int
iovcnt);
#include
<sys/types.h>
#include <sys/uio.h>
ssize_t
pwritev
(int
d, const struct iovec
*iov, int iovcnt,
off_t offset);
DESCRIPTION
write
()
attempts to write nbytes of data to the object
referenced by the descriptor d from the buffer pointed
to by buf. writev
() performs
the same action, but gathers the output data from the
iovcnt buffers specified by the members of the
iov array: iov[0], iov[1], ..., iov[iovcnt-1].
pwrite
()
and pwritev
() perform the same functions, but write
to the specified position offset in the file without
modifying the file pointer.
For
writev
() and
pwritev
(),
the iovec structure is defined as:
struct iovec { void *iov_base; size_t iov_len; };
Each iovec entry specifies the
base address and length of an area in memory from which data should be
written.
writev
()
and
pwritev
()
will always write a complete area before proceeding to the next.
On objects capable of seeking, the
write
()
starts at a position given by the pointer associated with
d (see
lseek(2)). Upon return from write
(), the
pointer is incremented by the number of bytes which were written. If a file
was opened with the O_APPEND
flag (see
open(2)), calls to write
() or
writev
() will automatically set the pointer to the
end of the file before writing.
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.
If the real user is not the superuser, then
write
()
clears the set-user-ID bit on a file. This prevents penetration of system
security by a user who “captures” a writable set-user-ID file
owned by the superuser.
If
write
()
succeeds it will update the st_ctime and st_mtime fields of the file's
meta-data (see stat(2)).
When using non-blocking I/O on objects such as
sockets that are subject to flow control,
write
() and
writev
() may write fewer bytes than requested; the
return value must be noted, and the remainder of the operation should be
retried when possible.
Note that
writev
()
and
pwritev
()
will fail if the value of iovcnt exceeds the constant
IOV_MAX
.
RETURN VALUES
Upon successful completion the number of bytes which were written is returned. Otherwise, a -1 is returned and the global variable errno is set to indicate the error.
ERRORS
write
(), pwrite
(),
writev
(), and pwritev
() will
fail and the file pointer will remain unchanged if:
- [
EBADF
] - d is not a valid descriptor open for writing.
- [
EFBIG
] - An attempt was made to write a file that exceeds the process's file size limit or the maximum file size.
- [
ENOSPC
] - There is no free space remaining on the file system containing the file.
- [
EDQUOT
] - The user's quota of disk blocks on the file system containing the file has been exhausted.
- [
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.
- [
EIO
] - An I/O error occurred while reading from or writing to the file system.
- [
EFAULT
] - Part of buf points outside the process's allocated address space.
In addition, write
() and
writev
() may return the following errors:
- [
EPIPE
] - An attempt is made to write to a pipe that is not open for reading by any process.
- [
EPIPE
] - An attempt is made to write to a socket of type
SOCK_STREAM
that is not connected to a peer socket. - [
EAGAIN
] - The file was marked for non-blocking I/O, and no data could be written immediately.
- [
ENETDOWN
] - The destination address specified a network that is down.
- [
EDESTADDRREQ
] - The destination is no longer available when writing to a UNIX-domain datagram socket on which connect(2) had been used to set a destination address.
- [
EIO
] - The process is a member of a background process attempting to write to its
controlling terminal,
TOSTOP
is set on the terminal, the process isn't ignoring theSIGTTOUT
signal and the thread isn't blocking theSIGTTOUT
signal, and either the process was created with vfork(2) and hasn't successfully executed one of the exec functions or the process group is orphaned.
write
() and
pwrite
() may return the following error:
- [
EINVAL
] - nbytes was larger than
SSIZE_MAX
.
pwrite
() and
pwritev
() may return the following error:
- [
EINVAL
] - offset was negative.
- [
ESPIPE
] - d is associated with a pipe, socket, FIFO, or tty.
writev
() and
pwritev
() may return one of the following
errors:
- [
EINVAL
] - iovcnt was less than or equal to 0, or greater than
IOV_MAX
. - [
EINVAL
] - The sum of the iov_len values in the iov array overflowed an ssize_t.
- [
EFAULT
] - Part of iov points outside the process's allocated address space.
- [
ENOBUFS
] - The system lacked sufficient buffer space or a queue was full.
SEE ALSO
fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2), termios(4)
STANDARDS
The write
(),
writev
(), and pwrite
()
functions conform to IEEE Std 1003.1-2008
(“POSIX.1”).
HISTORY
The pwritev
() function call appeared in
OpenBSD 2.7. The pwrite
()
function call appeared in AT&T System V
Release 4 UNIX. The writev
() function
call appeared in 4.2BSD. The
write
() function call appeared in
Version 2 AT&T UNIX.
CAVEATS
Error checks should explicitly test for -1. Code such as
while ((nr = write(fd, buf, sizeof(buf))) > 0)
is not maximally portable, as some platforms allow for
nbytes to range between
SSIZE_MAX
and SIZE_MAX
- 2,
in which case the return value of an error-free
write
() may appear as a negative number distinct
from -1. Proper loops should use
while ((nr = write(fd, buf, sizeof(buf))) != -1 && nr != 0)