DESCRIPTION
The write system call attempts to write nbytes of data to the object referenced by the descriptor d from the buffer pointed to by buf. The writev system call 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]. The pwrite and pwritev system calls 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; /* Base address. */
size_t iov_len; /* Length. */
};
Each iovec entry specifies the base address and length of an area in memory from which data should be written. The writev system call 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.
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.
RETURN VALUES
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.
ERRORS
The write, writev, pwrite and pwritev system calls will fail and the file pointer will remain unchanged if:
