FILE *fopen( char *path, char *mode);
FILE *fdopen( int fildes, char *mode);
FILE *freopen( char *path, char *mode, FILE *stream);
The argument mode points to a string beginning with one of the following sequences (Additional characters may follow these sequences.):
The mode string can also include the letter ``b'' either as a third character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with ANSI C3.159-1989 (``ANSI C'') and has no effect; the ``b'' is ignored. Linux may not behave this way.
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 may be intermixed on read/write streams in any order, and do not require an intermediate seek as in previous versions of stdio. This is not portable to other systems, however, and may not work under Linux (someone should find out and fix this manpage); 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 descriptor, fildes. The mode of the stream must be compatible with the mode of the file descriptor.
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).
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).