MirOS Manual: pwrite(2), pwritev(2), write(2), writev(2)

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

NAME

     write, writev, pwrite, pwritev - write output

SYNOPSIS

     #include <sys/types.h>
     #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/types.h>
     #include <sys/uio.h>
     #include <unistd.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
     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;
                   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() and pwritev() will al-
     ways 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 po-
     sition. The value of the pointer associated with such an object is unde-
     fined.

     If the real user is not the superuser, 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 superuser.

     If write() succeeds it will update the st_ctime and st_mtime fields of
     the file's meta-data (see stat(2)).

     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.

     Note that writev() and pwritev() will fail if the value of iovcnt exceeds
     the constant IOV_MAX.

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(), pwrite(), writev(), 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
                   process's file size limit or the maximum file size.

     [EINVAL]      The pointer associated with d was negative.

     [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, write() and pwrite() may return the following error:

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

     [EINVAL]      nbytes was larger than SSIZE_MAX.

     Also, writev() and pwritev() 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
                   IOV_MAX.

     [EINVAL]      The sum of the iov_len values in the iov array overflowed
                   an ssize_t.

SEE ALSO

     fcntl(2), lseek(2), open(2), pipe(2), poll(2), select(2)

STANDARDS

     The write() function conforms to IEEE Std 1003.1-1990 ("POSIX"). The wri-
     tev() and pwrite() functions conform to X/Open Portability Guide Issue
     4.2 ("XPG4.2").

HISTORY

     The pwritev() function call appeared in OpenBSD 2.7. The pwrite() func-
     tion call appeared in AT&TNon-Null System V.4 UNIX. The writev() function
     call appeared in 4.2BSD. The write() function call appeared in Version 2
     AT&T UNIX.

CAVEATS

     Error checks should explicitly test for -1. Code such as

           while ((nr = write(fd, buf, sizeof(buf))) > 0)

     is not maximally portable, as some platforms allow for nbytes to range
     between SSIZE_MAX and SIZE_MAX - 2, in which case the return value of an
     error-free write() may appear as a negative number distinct from -1.
     Proper loops should use

           while ((nr = write(fd, buf, sizeof(buf))) != -1 && nr != 0)

MirOS BSD #10-current           July 28, 1998                                2

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