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

       write - write to a file descriptor

       #include <unistd.h>

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

       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.

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

       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 will be
       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

       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.

              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.

              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

       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.

       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.

       The types size_t and ssize_t are, respectively, unsigned and signed
       integer data types specified by POSIX.1.

       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

       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.

       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, on Linux before version 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 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

       close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2),
       read(2), select(2), writev(2), fwrite(3)

       This page is part of release 5.04 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                             2019-10-10                          WRITE(2)