NAME

       write, writev, pwrite, pwritev — write output

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <unistd.h>

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

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

       #include <sys/uio.h>

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

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

DESCRIPTION

       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 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.

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

       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 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 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 completed.

       [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 non-zero).

       [EINTEGRITY]       The backing store for fd detected corrupted data while reading.  (For example, writing
                          a partial filesystem block may require first reading  the  existing  block  which  may
                          trigger this error.)

       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 errors:

       [EINVAL]           The offset value was negative.

       [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() 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”).

HISTORY

       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.

Debian                                           March 30, 2020                                         WRITE(2)