write
, writev
,
pwrite
, pwritev
—
write output
Standard C Library (libc, -lc)
#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);
The write
() 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. */
};
Each iovec entry specifies the base address
and length of an area in memory from which data should be written. The
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.
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.
The write
(), 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
SSIZE_MAX
(or greater than
INT_MAX
, if the sysctl
debug.iosize_max_clamp is non-zero).
- [
EINTEGRITY
]
- The backing store for fd detected corrupted data
while reading. (For example, writing a partial filesystem block may
require first reading the existing block which may trigger this
error.)
In addition, 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 is greater than
SSIZE_MAX
(or greater than
INT_MAX
, if the sysctl
debug.iosize_max_clamp is non-zero).
- [
ENOBUFS
]
- The mbuf pool has been completely exhausted when writing to a socket.
The 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.
The write
() 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”).
The pwritev
() 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 1 AT&T UNIX.
The pwrite
() system call appends the file without
changing the file offset if O_APPEND
is set, contrary
to IEEE Std 1003.1-2008 (“POSIX.1”)
where pwrite
() writes into
offset regardless of whether
O_APPEND
is set.