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

NAME
     fopen, fdopen, freopen - stream open functions



SYNOPSIS
     #include <stdio.h>

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

     FILE *
     fdopen(int fildes, char *mode);

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

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 be-
             ginning 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 writing.  The file is created if it does not exist.  The
             stream is positioned at the end of the file.

     ``a+''  Open for reading and writing.  The file is created if it does not
             exist.  The stream is positioned at the end of the file.

     The mode string can also include the letter ``b'' either as a third char-
     acter or as a character between the characters in any of the two-charac-
     ter strings described above.  This is strictly for compatibility with AN-
     SI X3.159-1989 (``ANSI C'') and has no effect; the ``b'' is ignored.

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

     Reads and writes cannot be arbitrarily intermixed on read/write streams.
     ANSI C requires that a file positioning function intervene between output
     and input, unless an input operation encounters end-of-file.

     The fdopen() function associates a stream with the existing file descrip-
     tor fildes. The mode of the stream must be compatible with the mode of
     the file descriptor.  If fdopen() fails, the file descriptor fildes is
     not affected in any way.

     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 always closed, even if freopen() fails.
     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 stan-
     dard text stream (stderr, stdin, or stdout).

RETURN VALUES
     Upon successful completion fopen(), fdopen() and freopen() return a FILE
     pointer.  Otherwise, NULL is returned and the global variable 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).

SEE ALSO
     open(2),  fclose(3),  fseek(3),  funopen(3)

STANDARDS
     The fopen() and freopen() functions conform to ANSI X3.159-1989 (``ANSI
     C''). The fdopen() function conforms to IEEE Std1003.1-1988 (``POSIX'').

CAVEATS
     Proper code using fdopen() with error checking should close(2) fildes in
     case of failure, and fclose(3) the resulting FILE * in case of success.

             FILE *file;
             int fd;

             if ((file = fdopen(fd)) != NULL) {
                     /* perform operations on the FILE * */
                     fclose(file);
             } else {
                     /* failure, report the error */
                     close(fd);
             }


OpenBSD 2.6                      June 4, 1993                                2