write

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



NAME
       write - write to a file descriptor

SYNOPSIS
       #include <unistd.h>

       ssize_t write(int fd, const void *buf, 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.

CONFORMING TO
       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
       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 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, 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 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.

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

COLOPHON
       This page is part of release 5.13 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2021-03-22                           WRITE(2)