FILE *fopen(const char *path, const char *mode);
FILE *fdopen(int fildes, const char *mode);
FILE *freopen(const char *path, const 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 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 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.)
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. 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() or fgetpos() 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
The fdopen() function associates a stream with the existing file descriptor, fildes. 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 fildes, 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() 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).