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WRITE(2) System Calls Manual WRITE(2)

write, writev — write output

#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);

() 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].

For (), 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. () will always write a complete area before proceeding to the next.

On objects capable of seeking, the () 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 () 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, () 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() and writev() will fail and the file pointer will remain unchanged if:

[]
D is not a valid descriptor open for writing.
[]
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 that is not connected to a peer socket.
[]
An attempt was made to write a file that exceeds the process's file size limit or the maximum file size.
[]
Part of iov or data to be written to the file points outside the process's allocated address space.
[]
The pointer associated with d was negative.
[]
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.
[]
An I/O error occurred while reading from or writing to the file system.
[]
The file was marked for non-blocking I/O, and no data could be written immediately.

In addition, writev() may return one of the following errors:

[]
Iovcnt was less than or equal to 0, or greater than UIO_MAXIOV.
[]
One of the iov_len values in the iov array was negative.
[]
The sum of the iov_len values in the iov array overflowed a 32-bit integer.

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

Write() is expected to conform to IEEE Std 1003.1-1988 (“POSIX”).

The writev() function call appeared in 4.2BSD. A write function call appeared in Version 6 AT&T UNIX.

BSD 4 April 2, 1994 WRITE(2)