fopen, fdopen, freopen - stream open functions

#include <stdio.h>
FILE *fopen(const char *restrict pathname, const char *restrict mode);
FILE *fdopen(int fd, const char *mode);
FILE *freopen(const char *restrict pathname, const char *restrict mode,
 FILE *restrict stream);

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

    _POSIX_C_SOURCE

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

The argument mode points to a string beginning with one of the following sequences (possibly followed by additional characters, as described below):

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. Output is always appended to the end of the file. POSIX is silent on what the initial read position is when using this mode. For glibc, the initial file position for reading is at the beginning of the file, but for Android/BSD/MacOS, the initial file position for reading is at 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 ISO C 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 file will have the 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 fsetpos(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 the call:

fseek(stream, 0, SEEK_END);

The file descriptor associated with the stream is opened as if by a call to open(2) with the following flags:

fopen() mode	open() flags
r	O_RDONLY
w	O_WRONLY | O_CREAT | O_TRUNC
a	O_WRONLY | O_CREAT | O_APPEND
r+	O_RDWR
w+	O_RDWR | O_CREAT | O_TRUNC
a+	O_RDWR | O_CREAT | O_APPEND

fdopen()

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 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.

freopen()

The freopen() function opens the file whose name is the string pointed to by pathname 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.

If the pathname argument is a null pointer, freopen() changes the mode of the stream to that specified in mode; that is, freopen() reopens the pathname that is associated with the stream. The specification for this behavior was added in the C99 standard, which says:

In this case, the file descriptor associated with the stream need not be closed if the call to freopen() succeeds. It is implementation-defined which changes of mode are permitted (if any), and under what circumstances.

The primary use of the freopen() function is to change the file associated with a standard text stream (stderr, stdin, or stdout).

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

open(2), fclose(3), fileno(3), fmemopen(3), fopencookie(3), open_memstream(3)