FOPEN(3) Library Functions Manual FOPEN(3)

fopen, fdopen, freopen — stream open functions

Standard C Library (libc, -lc)

#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);

The () 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, which may be followed by additional modifiers as indicated below:

“a”
Append; open an existing or new file for writing in append mode. The file is created if it does not exist.
“a+”
Open for reading and writing in append mode. The file is created if it does not exist.
“r”
Read; open an existing file for reading.
“r+”
Open an existing file for reading and writing.
“w”
Write; open an empty file for writing. Truncate an existing file to zero length or create a new file.
“w+”
Open an empty file for reading and writing. Truncate file to zero length or create file.

After one of those, the mode string can also include one or more of the following modifier letters:

‘b’
The letter ‘b’ may appear 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”), where it means open in “binary” mode which is identical to text mode here, so has no effect; the ‘b’ is ignored.
‘e’
The letter ‘e’ in the mode string sets the close-on-exec (O_CLOEXEC) flag of the file descriptor, which means that it will not be available after an exec(3) system call. This is a non ANSI X3.159-1989 (“ANSI C89”) extension.
‘f’
The letter ‘f’ in the mode string restricts () to regular files; if the file opened is not a regular file, fopen() will close it, and fail. This is a non ANSI X3.159-1989 (“ANSI C89”) extension.
‘l’
The letter ‘l’ in the mode string turns the don't-follow-symlinks (O_NOFOLLOW) flag of the file descriptor, which means that if the last path component is a symbolic link, it will not be followed. This is a non ANSI X3.159-1989 (“ANSI C89”) extension.
‘x’
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. In that case fopen() will fail.

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(2) value.

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 () 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 () 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 () 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 (, , or ).

Input and output against the opened stream will be fully buffered, unless it refers to an interactive terminal device, or a different kind of buffering is specified in the environment. See setvbuf(3) for additional details.

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.

The functions may fail if:

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

open(2), fclose(3), fileno(3), fseek(3), funopen(3)

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”).

An fopen() function appeared in Version 1 AT&T UNIX.

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);
	}
September 2, 2019 NetBSD 11.0