WRITE(2)

NAME
write, writev - write output

SYNOPSIS
#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);

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].

For writev, 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 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.

If the file was opened with the GNO-specific flag O_TRANS, then newline translation will occur; any line feed (0x0a) character present in buf will be converted to a carridge return (0x0d) before the write is done. See also the section on BUGS, below.

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
Write and writev will fail and the file pointer will remain unchanged if:

    EBADF
        D is not a valid descriptor open for writing.

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

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

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

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

    EINVAL
        The pointer associated with d was negative.

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

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

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

    EAGAIN
        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:

    EINVAL
        Iovcnt was less than or equal to 0, or greater than UIO_MAXIOV.

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

    EINVAL
        The sum of the iov_len values in the iov array overflowed a 32-bit integer.

BUGS
If the GNO-specific flag O_TRANS was specified when the descriptor d was opened, then buf may be modified by this call; the newline translation is done in-place.

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

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

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