Ubuntu Manpage: write - write to a file descriptor

NAME

       write - write to a file descriptor

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <unistd.h>

       ssize_t write(int fd, const void buf[.count], size_t count);

DESCRIPTION

       write()  writes  up to count bytes from the buffer starting at buf to the file referred to
       by the file descriptor fd.

       The number of bytes written may be less than count if, for example, there is  insufficient
       space on the underlying physical medium, or the RLIMIT_FSIZE resource limit is encountered
       (see setrlimit(2)), or the call was interrupted by a signal handler after  having  written
       less than count bytes.  (See also pipe(7).)

       For  a  seekable  file (i.e., one to which lseek(2) may be applied, for example, a regular
       file) writing takes place at the file offset, and the file offset is  incremented  by  the
       number  of  bytes  actually  written.   If  the file was open(2)ed with O_APPEND, the file
       offset is first set to the end of the file before writing.  The  adjustment  of  the  file
       offset and the write operation are performed as an atomic step.

       POSIX  requires  that  a  read(2) that can be proved to occur after a write() has returned
       will return the new data.  Note that not all filesystems are POSIX conforming.

       According to POSIX.1, if count is greater than SSIZE_MAX, the  result  is  implementation-
       defined; see NOTES for the upper limit on Linux.

RETURN VALUE

       On  success, the number of bytes written is returned.  On error, -1 is returned, and errno
       is set to indicate the error.

       Note that a successful write() may transfer fewer than count bytes.  Such  partial  writes
       can  occur  for  various reasons; for example, because there was insufficient space on the
       disk device to write all of the requested bytes, or because a blocked write() to a socket,
       pipe,  or  similar  was interrupted by a signal handler after it had transferred some, but
       before it had transferred all of the requested bytes.  In the event of  a  partial  write,
       the  caller can make another write() call to transfer the remaining bytes.  The subsequent
       call will either transfer further bytes or may result in an error (e.g., if  the  disk  is
       now full).

       If count is zero and fd refers to a regular file, then write() may return a failure status
       if one of the errors below is detected.  If no errors are detected, or error detection  is
       not  performed,  0  is returned without causing any other effect.  If count is zero and fd
       refers to a file other than a regular file, the results are not specified.

ERRORS

EAGAIN The file descriptor fd refers to a file other than a socket and has been marked
nonblocking (O_NONBLOCK), and the write would block. See open(2) for further
details on the O_NONBLOCK flag.

EAGAIN or EWOULDBLOCK
The file descriptor fd refers to a socket and has been marked nonblocking
(O_NONBLOCK), and the write would block. POSIX.1-2001 allows either error to be
returned for this case, and does not require these constants to have the same
value, so a portable application should check for both possibilities.

EBADF fd is not a valid file descriptor or is not open for writing.

EDESTADDRREQ
fd refers to a datagram socket for which a peer address has not been set using
connect(2).

EDQUOT The user's quota of disk blocks on the filesystem containing the file referred to
by fd has been exhausted.

EFAULT buf is outside your accessible address space.

EFBIG An attempt was made to write a file that exceeds the implementation-defined maximum
file size or the process's file size limit, or to write at a position past the
maximum allowed offset.

EINTR The call was interrupted by a signal before any data was written; see signal(7).

EINVAL fd is attached to an object which is unsuitable for writing; or the file was opened
with the O_DIRECT flag, and either the address specified in buf, the value
specified in count, or the file offset is not suitably aligned.

EIO A low-level I/O error occurred while modifying the inode. This error may relate to
the write-back of data written by an earlier write(), which may have been issued to
a different file descriptor on the same file. Since Linux 4.13, errors from write-
back come with a promise that they may be reported by subsequent. write()
requests, and will be reported by a subsequent fsync(2) (whether or not they were
also reported by write()). An alternate cause of EIO on networked filesystems is
when an advisory lock had been taken out on the file descriptor and this lock has
been lost. See the Lost locks section of fcntl(2) for further details.

ENOSPC The device containing the file referred to by fd has no room for the data.

EPERM The operation was prevented by a file seal; see fcntl(2).

EPIPE fd is connected to a pipe or socket whose reading end is closed. When this happens
the writing process will also receive a SIGPIPE signal. (Thus, the write return
value is seen only if the program catches, blocks or ignores this signal.)

Other errors may occur, depending on the object connected to fd.

STANDARDS

       POSIX.1-2008.

HISTORY

       SVr4, 4.3BSD, POSIX.1-2001.

       Under SVr4 a write may be interrupted and return EINTR at any point, not just  before  any
       data is written.

NOTES

       A  successful return from write() does not make any guarantee that data has been committed
       to disk.  On some filesystems, including NFS, it does not even guarantee  that  space  has
       successfully been reserved for the data.  In this case, some errors might be delayed until
       a future write(), fsync(2), or even close(2).  The only way to be sure is to call fsync(2)
       after you are done writing all your data.

       If  a  write()  is  interrupted by a signal handler before any bytes are written, then the
       call fails with the error EINTR; if it is interrupted after at least  one  byte  has  been
       written, the call succeeds, and returns the number of bytes written.

       On   Linux,   write()  (and  similar  system  calls)  will  transfer  at  most  0x7ffff000
       (2,147,479,552) bytes, returning the number of bytes actually transferred.  (This is  true
       on both 32-bit and 64-bit systems.)

       An  error  return value while performing write() using direct I/O does not mean the entire
       write has failed.  Partial data may be written and the data at the file  offset  on  which
       the write() was attempted should be considered inconsistent.

BUGS

       According  to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions with Regular File
       Operations"):

           All of the following functions shall be atomic with  respect  to  each  other  in  the
           effects  specified  in  POSIX.1-2008  when  they  operate on regular files or symbolic
           links: ...

       Among the APIs subsequently listed are write() and writev(2).  And among the effects  that
       should  be atomic across threads (and processes) are updates of the file offset.  However,
       before Linux 3.14, this was not the case:  if  two  processes  that  share  an  open  file
       description  (see open(2)) perform a write() (or writev(2)) at the same time, then the I/O
       operations were not atomic with respect to updating the file offset, with the result  that
       the  blocks of data output by the two processes might (incorrectly) overlap.  This problem
       was fixed in Linux 3.14.