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FOPEN(3)                   Linux Programmer's Manual                  FOPEN(3)



NAME
       fopen, fdopen, freopen - stream open functions

SYNOPSIS
       #include <stdio.h>

       FILE *fopen(const char *path, const char *mode);

       FILE *fdopen(int fd, const char *mode);

       FILE *freopen(const char *path, const char *mode, FILE *stream);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       fdopen(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

DESCRIPTION
       The  fopen()  function  opens the file whose name is the string pointed to by path and associates a stream with
       it.

       The argument mode points to a string beginning with one of the following sequences (Additional  characters  may
       follow these sequences.):

       r      Open text file for reading.  The stream is positioned at the beginning of the file.

       r+     Open for reading and writing.  The stream is positioned at the beginning of the file.

       w      Truncate file to zero length or create text file for writing.  The stream is positioned at the beginning
              of the file.

       w+     Open for reading and writing.  The file is created if it does not exist, otherwise it is truncated.  The
              stream is positioned at the beginning of the file.

       a      Open  for  appending (writing at end of file).  The file is created if it does not exist.  The stream is
              positioned at the end of the file.

       a+     Open for reading and appending (writing at end of file).  The file is created if it does not exist.  The
              initial  file position for reading is at the beginning of the file, but output is always appended to the
              end of the file.

       The mode string can also include the letter 'b' either as a last character or as a character between the  char-
       acters  in  any  of the two-character strings described above.  This is strictly for compatibility with C89 and
       has no effect; the 'b' is ignored on all POSIX conforming systems, including Linux.  (Other systems  may  treat
       text  files  and binary files differently, and adding the 'b' may be a good idea if you do I/O to a binary file
       and expect that your program may be ported to non-Unix environments.)

       See NOTES below for details of glibc extensions for mode.

       Any created files will have mode S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH (0666), as  modified
       by the process's umask value (see umask(2)).

       Reads  and  writes may be intermixed on read/write streams in any order.  Note that ANSI C requires that a file
       positioning function intervene between output and input, unless an input operation encounters end-of-file.  (If
       this  condition  is not met, then a read is allowed to return the result of writes other than the most recent.)
       Therefore it is good practice (and indeed sometimes necessary under Linux) to put  an  fseek(3)  or  fgetpos(3)
       operation  between  write and read operations on such a stream.  This operation may be an apparent no-op (as in
       fseek(..., 0L, SEEK_CUR) called for its synchronizing side effect.

       Opening a file in append mode (a as the first character of mode) causes all subsequent write operations to this
       stream to occur at end-of-file, as if preceded by an

           fseek(stream,0,SEEK_END);

       call.

       The  fdopen()  function associates a stream with the existing file descriptor, fd.  The mode of the stream (one
       of the values "r", "r+", "w", "w+", "a", "a+") must be compatible with the mode of the  file  descriptor.   The
       file position indicator of the new stream is set to that belonging to fd, and the error and end-of-file indica-
       tors are cleared.  Modes "w" or "w+" do not cause truncation of the file.  The file descriptor is  not  dup'ed,
       and  will be closed when the stream created by fdopen() is closed.  The result of applying fdopen() to a shared
       memory object is undefined.

       The freopen() function opens the file whose name is the string pointed to by path  and  associates  the  stream
       pointed to by stream with it.  The original stream (if it exists) is closed.  The mode argument is used just as
       in the fopen() function.  The primary use of the freopen() function is to change the  file  associated  with  a
       standard text stream (stderr, stdin, or stdout).

RETURN VALUE
       Upon  successful completion fopen(), fdopen() and freopen() return a FILE pointer.  Otherwise, NULL is returned
       and errno is set to indicate the error.

ERRORS
       EINVAL The mode provided to fopen(), fdopen(), or freopen() was invalid.

       The fopen(), fdopen() and freopen() functions may also fail and set errno for any of the errors  specified  for
       the routine malloc(3).

       The fopen() function may also fail and set errno for any of the errors specified for the routine open(2).

       The fdopen() function may also fail and set errno for any of the errors specified for the routine fcntl(2).

       The  freopen()  function  may also fail and set errno for any of the errors specified for the routines open(2),
       fclose(3) and fflush(3).

CONFORMING TO
       The fopen() and freopen() functions conform to C89.  The fdopen() function conforms to POSIX.1-1990.

NOTES
   Glibc Notes
       The GNU C library allows the following extensions for the string specified in mode:

       c (since glibc 2.3.3)
              Do not make the open operation, or subsequent read and write operations, thread cancellation points.

       e (since glibc 2.7)
              Open the file with the O_CLOEXEC flag.  See open(2) for more information.

       m (since glibc 2.3)
              Attempt to access the file using mmap(2), rather than I/O system calls (read(2), write(2)).   Currently,
              use of mmap(2) is only attempted for a file opened for reading.

       x      Open the file exclusively (like the O_EXCL flag of open(2)).  If the file already exists, fopen() fails,
              and sets errno to EEXIST.  This flag is ignored for fdopen().

SEE ALSO
       open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(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/.



GNU                               2009-02-23                          FOPEN(3)