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WRITE(2) System Calls Manual WRITE(2)

write, writev, pwrite, pwritev
write output

library “libc”

#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() 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.

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.

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:
[]
The file was marked for non-blocking I/O, and no data could be written immediately.
[]
d is not a valid descriptor open for writing.
[]
The user's quota of disk blocks on the file system containing the file has been exhausted.
[]
Part of iov or data to be written to the file points outside the process's allocated address space.
[]
An attempt was made to write a file that exceeds the process's file size limit or the maximum file size.
[]
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.
[]
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.
[]
An I/O error occurred while reading from or writing to the file system.
[]
There is no free space remaining on the file system containing the file.
[]
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:

[]
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:

[]
The specified file offset is invalid.
[]
The file descriptor is associated with a pipe, socket, or FIFO.

fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2), sigaction(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, Version 2 (“XPG4.2”).

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.

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)
April 3, 2010 NetBSD-7.0.1