write(2) — Linux manual page


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

NAME         top

       write - write to a file descriptor

SYNOPSIS         top

       #include <unistd.h>

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

DESCRIPTION         top

       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

       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         top

       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 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 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

ERRORS         top

       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

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

CONFORMING TO         top

       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         top

       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.

BUGS         top

       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 3.14.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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)

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