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WRITE(2) | System Calls Manual | WRITE(2) |
NAME¶
write
,
writev
,
pwrite
,
pwritev
—
write output
LIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include
<sys/types.h>
#include
<unistd.h>
ssize_t
write
(int
fd, const void
*buf, size_t
nbytes);
ssize_t
pwrite
(int
fd, const void
*buf, size_t
nbytes, off_t
offset);
#include
<sys/uio.h>
ssize_t
writev
(int
fd, const struct
iovec *iov, int
iovcnt);
ssize_t
pwritev
(int
fd, const struct
iovec *iov, int
iovcnt, off_t
offset);
DESCRIPTION¶
Thewrite
() system call attempts to write
nbytes of data to the object referenced by
the descriptor fd from the buffer pointed to
by buf. The
writev
() system call 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]. The pwrite
() and
pwritev
() system calls perform the same
functions, but write to the specified position in the file without modifying
the file pointer.
For writev
() and
pwritev
(), the
iovec structure is defined as:
struct iovec { void *iov_base; /* Base address. */ size_t iov_len; /* Length. */ };
writev
() system call 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
fd, see
lseek(2). Upon return from
write
(), the pointer is incremented by the
number of bytes which were written.
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 super-user, 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 super-user.
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.
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¶
Thewrite
(),
writev
(),
pwrite
() and
pwritev
() system calls will fail and the
file pointer will remain unchanged if:
- [
EBADF
] - The fd argument is not a valid descriptor open for writing.
- [
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. - [
EFBIG
] - An attempt was made to write a file that exceeds the process's file size limit or the maximum file size.
- [
EFAULT
] - Part of iov or data to be written to the file points outside the process's allocated address space.
- [
EINVAL
] - The pointer associated with fd was negative.
- [
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.
- [
EIO
] - An I/O error occurred while reading from or writing to the file system.
- [
EINTR
] - A signal interrupted the write before it could be completed.
- [
EAGAIN
] - The file was marked for non-blocking I/O, and no data could be written immediately.
- [
EROFS
] - An attempt was made to write over a disk label area at the beginning of a
slice. Use disklabel(8)
-W
to enable writing on the disk label area. - [
EINVAL
] - The value nbytes is greater than
INT_MAX
.
writev
() and
pwritev
() may return one of the following
errors:
- [
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.
- [
EINVAL
] - The iovcnt argument was less than or
equal to 0, or greater than
IOV_MAX
. - [
EINVAL
] - One of the iov_len values in the iov array was negative.
- [
EINVAL
] - The sum of the iov_len values in the iov array overflowed a 32-bit integer.
- [
ENOBUFS
] - The mbuf pool has been completely exhausted when writing to a socket.
pwrite
() and
pwritev
() system calls may also return the
following errors:
- [
EINVAL
] - The offset value was negative.
- [
ESPIPE
] - The file descriptor is associated with a pipe, socket, or FIFO.
SEE ALSO¶
fcntl(2), lseek(2), open(2), pipe(2), select(2)STANDARDS¶
Thewrite
() system call is expected to
conform to IEEE Std 1003.1-1990
(“POSIX.1”). The
writev
() and
pwrite
() system calls are expected to
conform to X/Open Portability Guide
Issue 4, Version 2 (“XPG4.2”).
HISTORY¶
Thepwritev
() system call appeared in
FreeBSD 6.0. The
pwrite
() function appeared in
AT&T System V Release 4 UNIX. The
writev
() system call appeared in
4.2BSD. The write
()
function appeared in Version 6 AT&T
UNIX.September 11, 2013 | Debian |