FOPEN(3)

NAME
fopen, fdopen, freopen - stream open functions

LIBRARY
libc

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” Open text file for reading.

“r+”
    Open for reading and writing.

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

“w+”
    Open for reading and writing. The file is created if it does not exist, otherwise it is truncated.

“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.

The mode string can also include the letter ``b'' either as a third character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with ansiC and has no effect; the ``b'' is ignored.

Any created files will have mode \*qS_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH\*q (0666), as modified by the process' umask value (see umask(2)).

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.

Reads and writes may be intermixed on read/write streams in any order, and do not require an intermediate seek as in previous versions of stdio. This is not portable to other systems, however; 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.

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

[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 ansiC. The fdopen() function conforms to p1003.1-90.