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 pointed 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 physi-
       cal 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
       current 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) which can be proved to occur after a write() has returned returns the new data.  Note  that
       not all file systems are POSIX conforming.

RETURN VALUE
       On  success, the number of bytes written is returned (zero indicates nothing was written).  On error, -1 is returned, and
       errno is set appropriately.

       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,  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 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.

       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.

       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 current file offset is not suitably
              aligned.

       EIO    A low-level I/O error occurred while modifying the inode.

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

       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
       A successful return from write() does not make any guarantee that data has been committed to  disk.   In  fact,  on  some
       buggy  implementations,  it does not even guarantee that space has successfully been reserved for the data.  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 writ-
       ten.

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 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-02-23                                                   WRITE(2)