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WRITE(2) System Calls Manual WRITE(2)

write, writev, pwrite, pwritev —
write output

#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);
#include <sys/types.h>
#include <sys/uio.h>
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 offset 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. If a file was opened with the O_APPEND flag (see open(2)), calls to write() or writev() will automatically set the pointer to the end of the file before writing.
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-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 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:
 
 
[]
d is not a valid descriptor open for writing.
 
 
[]
An attempt was made to write a file that exceeds the process's file size limit or the maximum file size.
 
 
[]
There is no free space remaining on the file system containing the file.
 
 
[]
The user's quota of disk blocks on the file system containing the file has been exhausted.
 
 
[]
A write to a slow device (i.e. one that might block for an arbitrary amount of time) was interrupted by the delivery of a signal before any data could be written.
 
 
[]
An I/O error occurred while reading from or writing to the file system.
 
 
[]
Part of buf points outside the process's allocated address space.
In addition, write() and writev() may return the following errors:
 
 
[]
An attempt is made to write to a pipe that is not open for reading by any process.
 
 
[]
An attempt is made to write to a socket of type SOCK_STREAM that is not connected to a peer socket.
 
 
[]
The file was marked for non-blocking I/O, and no data could be written immediately.
 
 
[]
The destination address specified a network that is down.
 
 
[]
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.
 
 
[]
The process is a member of a background process attempting to write to its controlling terminal, TOSTOP is set on the terminal, the process isn't ignoring the SIGTTOUT signal and the thread isn't blocking the SIGTTOUT signal, and either the process was created with vfork(2) and hasn't successfully executed one of the exec functions or the process group is orphaned.
write() and pwrite() may return the following error:
 
 
[]
nbytes was larger than SSIZE_MAX.
pwrite() and pwritev() may return the following error:
 
 
[]
offset was negative.
 
 
[]
d is associated with a pipe, socket, FIFO, or tty.
writev() and pwritev() may return one of the following errors:
 
 
[]
iovcnt was less than or equal to 0, or greater than IOV_MAX.
 
 
[]
The sum of the iov_len values in the iov array overflowed an ssize_t.
 
 
[]
Part of iov points outside the process's allocated address space.
 
 
[]
The system lacked sufficient buffer space or a queue was full.

fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2), termios(4)

The write(), writev(), and pwrite() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

The pwritev() function call appeared in OpenBSD 2.7. 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)
February 5, 2015 OpenBSD-5.9