NAME

fopen, fdopen, freopen —

stream open functions

LIBRARY

library “libc”

SYNOPSIS

#include
  <stdio.h>

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

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

FILE *
freopen(const char * restrict path, const char * restrict mode, FILE * restrict 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 for reading.

“r+”: Open for reading and writing.

“w”: Open for writing. Truncate file to zero length or create file.

“w+”: Open for reading and writing. Truncate file to zero length or create file.

“a”: Append; open for writing. The file is created if it does not exist.

“a+”: Append; open for reading and writing. The file is created if it does not exist.

Additionally:

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 ANSI X3.159-1989 (“ANSI C89”) and has no effect; the “b” is ignored.
The letter “f” in the mode string restricts fopen() to regular files; if the file opened is not a regular file, fopen() will fail. This is a non ANSI X3.159-1989 (“ANSI C89”) extension.
The letter “e” in the mode string sets the close-on-exec flag in the file descriptors of the newly opened file files; if the operation fails, fopen() will fail. This is a non ANSI X3.159-1989 (“ANSI C89”) extension.
The letter ‘x’ in the mode turns on exclusive open mode to the file ( O_EXCL) which means that the file will not be created if it already exists.

Opening a file with append mode causes all subsequent writes to it to be forced to the then current end of file, regardless of intervening repositioning of the stream.

The fopen() and freopen() functions initially position the stream at the start of the file unless the file is opened with append mode, in which case the stream is initially positioned at the end of the 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 stream is positioned at the file offset 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).

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

The functions may fail if:


[EFTYPE]: The file is not a regular file and the character ``f'' is specified in the mode.

[EINVAL]: The specified mode 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).

STANDARDS

The fopen() and freopen() functions conform to ANSI X3.159-1989 (“ANSI C89”). All three functions are specified in IEEE Std 1003.1-2008 (“POSIX.1”).

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