FOPEN(3)

NAME
fopen, fdopen, freopen - stream open functions

SYNOPSIS
#include <stdio.h>

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);

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

fdopen(): _POSIX_C_SOURCE || _XOPEN_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 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'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, 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).

RETURN VALUE
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).

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.

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)