write(2)

NAME

write, writev, pwrite - write output

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/types.h>
#include <sys/uio.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);
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

The write() system call attempts to write nbytes of data to

the object
referenced by the descriptor d 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 posi

tion 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 secu

rity 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 vari

able errno is
set to indicate the error.

ERRORS

The write(), writev(), pwrite() and pwritev() system calls

will fail and
the file pointer will remain unchanged if:

[EBADF] The d 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 d was nega

tive.

[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 com

pleted.

[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 diskla

bel(8) -W to
enable writing on the disk label area.

[EINVAL] The value nbytes is greater than INT_MAX.

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 ar

ray was nega

tive.

[EINVAL] The sum of the iov_len values in the iov

array over

flowed a 32-bit integer.

[ENOBUFS] The mbuf pool has been completely ex

hausted when writ

ing 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.

SEE ALSO

fcntl(2), lseek(2), open(2), pipe(2), select(2)

STANDARDS

The write() system call is expected to conform to ISO/IEC

9945-1:1990
(``POSIX.1''). The writev() and pwrite() system calls are

expected to
conform to X/Open Portability Guide Issue 4.2 (``XPG4.2'').

HISTORY

The pwritev() system call appeared in FreeBSD 6.0. The

pwrite() function
appeared in AT&T System V.4 UNIX. The writev() system call

appeared in
4.2BSD. The write() function appeared in Version 6 AT&T

UNIX.

BSD October 16, 2004

BSD