WRITE(2) BSD System Calls Manual WRITE(2)NAME
write, writev, pwrite, pwritev — 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
writev(int d, const struct iovec *iov, int iovcnt);
ssize_t
pwrite(int d, const void *buf, size_t nbytes, off_t offset);
ssize_t
pwritev(int d, const struct iovec *iov, int iovcnt, off_t offset);
DESCRIPTIONWrite() attempts to write nbytes of data to the object referenced by the
descriptor d from the buffer pointed to by buf. Writev() and pwritev()
perform the same action, but gather the output data from the iovcnt buf‐
fers specified by the members of the iov array: iov[0], iov[1], ...,
iov[iovcnt-1]. Pwrite() and pwritev() perform the same function, 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 {
char *iov_base; /* Base address. */
size_t iov_len; /* Length. */
};
Each iovec entry specifies the base address and length of an area in mem‐
ory 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 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 oper‐
ation 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 variable errno is
set to indicate the error.
ERRORSWrite(), 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.
[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 disk. Use disklabel(8)-W to
enable writing on the disk label area.
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] Iovcnt was less than or equal to 0, or greater than
UIO_MAXIOV.
[EINVAL] One of the iov_len values in the iov array 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 exhausted when writ‐
ing to a socket.
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 ALSOfcntl(2), lseek(2), open(2), pipe(2), select(2)STANDARDS
The write() function call is expected to conform to ISO/IEC 9945-1:1990
(“POSIX.1”). The writev() and pwrite() functions are expected to conform
to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).
HISTORY
The pwritev() function call was added in DragonFly 1.5. The pwrite()
function call appeared in AT&T System V Release 4 UNIX. The writev()
function call appeared in 4.2BSD. A write() function call appeared in
Version 6 AT&T UNIX.
BSD April 27, 2006 BSD