Man Pages

write(2) - phpMan write(2) - phpMan

Command: man perldoc info search(apropos)  


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 underly-
       ing  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 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 non-blocking (O_NON-
              BLOCK), and the write would block.

       EAGAIN or EWOULDBLOCK
              The file descriptor fd refers to a socket and has been marked non-blocking (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 written.

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



Linux                             2009-02-23                          WRITE(2)