write, writev, pwrite, pwritev - write output
Standard C Library (libc, -lc)
#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);
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() 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-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 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.
write(), writev(), pwrite(), 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.
[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 d was negative.
[EINVAL] The total length of the I/O is more than can be
expressed by the ssize_t return value.
[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, 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] 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.
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.
fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2)
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 4.2 (``XPG4.2'').
The pwritev() function call appeared in NetBSD 1.4. 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 6 AT&T
UNIX.
BSD October 16, 2001 BSD
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