Copyright (c) 1980 Regents of the University of California.
All rights reserved. The Berkeley software License Agreement
specifies the terms and conditions for redistribution.

@(#)write.2 6.5 (Berkeley) 05/14/86

C 4 write, writev - write output

cc = write(d, buf, nbytes)
int cc, d;
char *buf;
int nbytes;

#include <sys/types.h>
#include <sys/uio.h>

cc = writev(d, iov, iovcnt)
int cc, d;
struct iovec *iov;
int iovcnt;

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].

For writev , the iovec structure is defined as

struct iovec {
 caddr_t iov_base;
 int iov_len;
};

Each iovec entry specifies the base address and length of an area in memory from which data should be written. Writev 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 actually 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 \*(lqcaptures\*(rq 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.

Upon successful completion the number of bytes actually 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 one or more of the following are true:

15 [EBADF] D is not a valid descriptor open for writing.

15 [EPIPE] An attempt is made to write to a pipe that is not open for reading by any process.

15 [EPIPE] An attempt is made to write to a socket of type SOCK_STREAM that is not connected to a peer socket.

15 [EFBIG] An attempt was made to write a file that exceeds the process's file size limit or the maximum file size.

15 [EFAULT] Part of iov or data to be written to the file points outside the process's allocated address space.

15 [EINVAL] The pointer associated with d was negative.

15 [ENOSPC] There is no free space remaining on the file system containing the file.

15 [EDQUOT] The user's quota of disk blocks on the file system containing the file has been exhausted.

15 [EIO] An I/O error occurred while reading from or writing to the file system.

15 [EWOULDBLOCK] 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:

15 [EINVAL] Iovcnt was less than or equal to 0, or greater than 16.

15 [EINVAL] One of the iov_len values in the iov array was negative.

15 [EINVAL] 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)