WRITE(2)                     BSD System Calls Manual                    WRITE(2)

     write, writev, pwrite, pwritev — write output

     Standard C Library (libc, -lc)

     #include <unistd.h>

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

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

     #include <sys/uio.h>

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

     pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset);

     The write() system call attempts to write nbytes of data to the object
     referenced by the descriptor fd from the buffer pointed to by buf.  The
     writev() system call 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].  The pwrite() and pwritev() system calls
     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;  /* 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.  The writev() system call 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 fd, 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

     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.

     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.

     The write(), writev(), pwrite() and pwritev() system calls will fail and
     the file pointer will remain unchanged if:

     [EBADF]            The fd argument is not a valid descriptor open for

     [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 fd 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.

     [EINTR]            A signal interrupted the write before it could be

     [EAGAIN]           The file was marked for non-blocking I/O, and no data
                        could be written immediately.

     [EROFS]            An attempt was made to write over a disk label area at
                        the beginning of a slice.  Use disklabel(8) -W to enable
                        writing on the disk label area.

     [EINVAL]           The value nbytes is greater than SSIZE_MAX (or greater
                        than INT_MAX, if the sysctl debug.iosize_max_clamp is

     In addition, 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]           The iovcnt argument 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.

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

     The pwrite() and pwritev() system calls may also return the following

     [EINVAL]           The offset value was negative.

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

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

     The write() system call is expected to conform to ISO/IEC 9945-1:1990
     (“POSIX.1”).  The writev() and pwrite() system calls are expected to
     conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).

     The pwritev() system call appeared in FreeBSD 6.0.  The pwrite() function
     appeared in AT&T System V Release 4 UNIX.  The writev() system call
     appeared in 4.2BSD.  The write() function appeared in Version 1 AT&T UNIX.

BSD                             December 1, 2017                             BSD