FOPEN(3) | Library Functions Manual | 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);

# 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`” or “`rb`”
  Open file for reading.
“`r+`” or “`rb+`” or “`r+b`”
  Open for reading and writing.
“`w`” or “`wb`”
  Open for writing. The file is created if it does not exist, otherwise it is truncated.
“`w+`” or “`wb+`” or “`w+b`”
  Open for reading and writing. The file is created if it does not exist, otherwise it is truncated.
“`a`” or “`ab`”
  Open for writing. The file is created if it does not exist.
“`a+`” or “`ab+`” or “`a+b`”
  Open for reading and writing. The file is created if it does not exist.

The letter “b” in the mode strings above is strictly for compatibility with ANSI X3.159-1989 (“ANSI C89”) and has no effect; the “b” is ignored.

After any of the above prefixes, the mode string can also include zero or more of the following:

“`e`”
  The close-on-exec flag is set on the underlying file descriptor of the new FILE.
“`x`”
  If the mode string starts with “w” or “a” then the function shall fail if the file specified by path already exists, as if the `O_EXCL` flag was passed to the open(2) function. It has no effect if used with `fdopen`() or the mode string begins with “r”.

The `fopen`() and `freopen`() functions initially position the stream at the start of the file unless the file is opened in append mode (‘a’ or ‘a+’), in which case the stream is initially positioned at the end of the file.

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

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 cannot be arbitrarily intermixed on read/write streams. ANSI C requires that a file positioning function intervene between output and input, unless an input operation encounters end-of-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. If `fdopen`() fails, the file descriptor fildes is not affected in any way.

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 always closed, even if `freopen`() fails. 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

[`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).

# SEE ALSO

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

# STANDARDS

The `fopen`() and `freopen`() functions conform to ANSI X3.159-1989 (“ANSI C89”). The `fdopen`() function conforms to IEEE Std 1003.1-1988 (“POSIX.1”).

# HISTORY

The `fopen`() function first appeared in Version 1 AT&T UNIX. The `fdopen`() and `freopen`() functions first appeared in Version 7 AT&T UNIX.

Support for the “e” and “x” mode letters appeared in OpenBSD 5.7.

# 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);
    	}

September 28, 2023 | Debian