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  characters  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  dif-
       ferently, 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 val-
       ues "r", "r+", "w", "w+", "a", "a+") must be compatible with the mode of the file descriptor.  The file position  indica-
       tor of the new stream is set to that belonging to fd, and the error and end-of-file indicators 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()  func-
       tion.   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 3.25 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/.



GNU                                                        2009-02-23                                                   FOPEN(3)