write(2) — Linux manual page


write(2)                   System Calls Manual                  write(2)

NAME         top

       write - write to a file descriptor

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <unistd.h>

       ssize_t write(int fd, const void buf[.count], 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 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         top

       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         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

              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

       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

       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         top


HISTORY         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

       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         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, 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.

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)

Linux man-pages (unreleased)     (date)                         write(2)

Pages that refer to this page: ps(1)pv(1)strace(1)telnet-probe(1)close(2)epoll_ctl(2)eventfd(2)fcntl(2)fsync(2)getpeername(2)getrlimit(2)io_uring_enter2(2)io_uring_enter(2)lseek(2)memfd_create(2)mmap(2)open(2)pipe(2)pread(2)read(2)readv(2)seccomp(2)select(2)select_tut(2)send(2)sendfile(2)socket(2)socketpair(2)sync(2)syscalls(2)aio_error(3)aio_return(3)aio_write(3)curs_print(3x)dbopen(3)fclose(3)fflush(3)fgetc(3)fopen(3)fread(3)gets(3)io_uring_prep_write(3)io_uring_prep_writev2(3)io_uring_prep_writev(3)libexpect(3)mkfifo(3)mpool(3)puts(3)size_t(3type)stdio(3)xdr(3)xfsctl(3)dsp56k(4)fuse(4)lirc(4)st(4)proc(5)systemd.exec(5)aio(7)cgroups(7)cpuset(7)epoll(7)fanotify(7)inode(7)inotify(7)landlock(7)pipe(7)sched(7)signal(7)signal-safety(7)socket(7)spufs(7)tcp(7)time_namespaces(7)udp(7)user_namespaces(7)vsock(7)x25(7)fsfreeze(8)netsniff-ng(8)wipefs(8)xfs_io(8)