NAME
write,
writev,
pwrite,
pwritev —
write output
LIBRARY
Standard C Library (libc, -lc)
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);
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 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.
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 the file. This prevents penetration of system security
by a user who “captures” a writable set-user-id file owned by the
super-user.
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.
RETURN VALUES
Upon successful completion the number of bytes which were written is returned.
Otherwise -1 is returned and the global variable
errno
is set to indicate the error.
ERRORS
write(),
writev(),
pwrite(), and
pwritev() will fail and the
file pointer will remain unchanged if:
-
-
- [
EAGAIN
]
- The file was marked for non-blocking I/O, and no data could
be written immediately.
-
-
- [
EBADF
]
- d is not a valid descriptor open for
writing.
-
-
- [
EDQUOT
]
- The user's quota of disk blocks on the file system
containing the file has been exhausted.
-
-
- [
EFAULT
]
- Part of iov or data to be written to
the file points outside the process's allocated address space.
-
-
- [
EFBIG
]
- An attempt was made to write a file that exceeds the
process's file size limit or the maximum file size.
-
-
- [
EINTR
]
- A signal was received before any data could be written to a
slow device. See
sigaction(2) for more
information on the interaction between signals and system calls.
-
-
- [
EINVAL
]
- The pointer associated with d was
negative; or the total length of the I/O is more than can be expressed by
the ssize_t return value.
-
-
- [
EIO
]
- An I/O error occurred while reading from or writing to the
file system.
-
-
- [
ENOSPC
]
- There is no free space remaining on the file system
containing the file.
-
-
- [
EPIPE
]
- An attempt is made to write to a pipe that is not open for
reading by any process; or an attempt is made to write to a socket of type
SOCK_STREAM
that is not connected to a peer
socket.
In addition,
writev() and
pwritev() may
return one of the following errors:
-
-
- [
EINVAL
]
- iovcnt was less than or equal to 0,
or greater than {
IOV_MAX
}; or one of the
iov_len values in the iov
array was negative; or the sum of the iov_len values
in the iov array overflowed a 32-bit integer.
The
pwrite() and
pwritev() calls may also
return the following errors:
-
-
- [
EINVAL
]
- The specified file offset is invalid.
-
-
- [
ESPIPE
]
- The file descriptor is associated with a pipe, socket, or
FIFO.
SEE ALSO
fcntl(2),
lseek(2),
open(2),
pipe(2),
poll(2),
select(2),
sigaction(2)
STANDARDS
The
write() function is expected to conform to
IEEE Std 1003.1-1988 (“POSIX.1”). The
writev() and
pwrite() functions conform to
X/Open Portability Guide Issue 4, Version 2
(“XPG4.2”).
HISTORY
The
pwritev() function call appeared in
NetBSD 1.4. 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)