write(2) - NetBSD Manual Pages

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WRITE(2)                  NetBSD Programmer's Manual                  WRITE(2)


NAME

     write, writev, pwrite, pwritev - write output


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <unistd.h>

     ssize_t
     write(int d, const void *buf, size_t nbytes);

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

     #include <sys/uio.h>

     ssize_t
     writev(int d, const struct iovec *iov, int iovcnt);

     ssize_t
     pwritev(int d, const struct iovec *iov, int iovcnt, 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 speci-
     fied 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 spec-
     ified 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 mem-
     ory from which data should be written.  writev() will always write a com-
     plete 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 po-
     sition.  The value of the pointer associated with such an object is unde-
     fined.

     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 request-
     ed; 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 re-
     turned.  Otherwise a -1 is returned and the global variable errno is set
     to indicate the error.


ERRORS

     write(), writev(), pwrite(), and pwritev() 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 pro-
                   cess's file size limit or the maximum file size.

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

     [EINVAL]      The pointer associated with d was negative.

     [EINVAL]      The total length of the I/O is more than can be expressed
                   by the ssize_t return value.

     [ENOSPC]      There is no free space remaining on the file system con-
                   taining the file.

     [EDQUOT]      The user's quota of disk blocks on the file system contain-
                   ing 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() and pwritev() may return one of the following er-
     rors:

     [EINVAL]      iovcnt was less than or equal to 0, or greater than
                   {IOV_MAX}.

     [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.

     pwrite() and pwritev() calls 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), poll(2), select(2)


STANDARDS

     The write() function is expected to conform to IEEE Std 1003.1-1988
     (``POSIX'').  The writev() and pwrite() functions conform to X/Open
     Portability Guide Issue 4, Version 2 (``XPG4.2'').


HISTORY

     The pwritev() function call appeared in NetBSD 1.4.  The pwrite() func-
     tion call appeared in AT&T System V.4 UNIX.  The writev() function call
     appeared in 4.2BSD.  The write() function call appeared in Version 6 AT&T
     UNIX.

NetBSD 1.6.1                   October 16, 2001                              2