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.