write, writev, pwrite, pwritev - write output
#include <sys/types.h>
#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/types.h>
#include <sys/uio.h>
#include <unistd.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);
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 superuser, then write() clears
the set-userID
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.
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.
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.
[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.
[EINVAL] The pointer associated with d 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.
[EAGAIN] The file was marked for non-blocking I/O, and
no data could
be written immediately.
In addition, write() and pwrite() may return the following
error:
[EFAULT] Part of iov or data to be written to the file
points outside
the process's allocated address space.
[EINVAL] nbytes was larger than SSIZE_MAX.
Also, 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] 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.
fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2)
The write() function conforms to IEEE Std 1003.1-1990
(``POSIX''). The
writev() and pwrite() functions conform to X/Open Portability Guide Issue
4.2 (``XPG4.2'').
The pwritev() function call appeared in OpenBSD 2.7. The
pwrite() function
call appeared in AT&T System V.4 UNIX. The writev()
function call
appeared in 4.2BSD. The write() function call appeared in
Version 2 AT&T
UNIX.
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)
OpenBSD 3.6 July 28, 1998
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