WRITE(2)

NAME
write, writev, pwrite - write output

LIBRARY
libc

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);
ssize_t
pwrite(int d, const void *buf, size_t nbytes, off_t offset);

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() performs the same function, but writes to the specified position in the file without modifying the file pointer.

For writev(), the iovec structure is defined as:

    struct iovec {
    	char   *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. 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.

IMPLEMENTATION NOTES

In the non-threaded library write() is implemented as the write syscall.

In the threaded library, the write syscall is assembled to _thread_sys_write() and write() is implemented as a function which locks d for read and write, then calls _thread_sys_write(). If the call to _thread_sys_write() would block, a context switch is performed. Before returning, write() unlocks d.

In the non-threaded library writev() is implemented as the writev syscall.

In the threaded library, the writev syscall is assembled to _thread_sys_writev() and writev() is implemented as a function which locks d for read and write, then calls _thread_sys_writev(). If the call to _thread_sys_writev() would block, a context switch is performed. Before returning, writev() unlocks d.

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(), writev(), and pwrite() 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:

[EDESTADDRREQ]
    The destination is no longer available when writing to a UNIX domain datagram socket on which connect(2) had been used to set a destination address.

[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.

[ENOBUFS]
    The mbuf pool has been completely exhausted when writing to a socket.

The pwrite() call may also return the following errors:

[EINVAL]
    The specified file offset is invalid.

[ESPIPE]
    The file descriptor is associated with a pipe, socket, or FIFO.

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

STANDARDS
The write() function call is expected to conform to p1003.1-90. The writev() and pwrite() functions are expected to conform to xpg4.2.

HISTORY
The pwrite() function call appeared in AT&T UNIX. The writev() function call appeared in 4.2BSD. A write() function call appeared in AT&T UNIX.