DESCRIPTION

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.