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FOPEN(3)                 BSD Library Functions Manual                 FOPEN(3)

NAME
     fdopen, fopen, freopen -- stream open functions

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

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

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

     FILE *
     freopen(const char *restrict filename, const char *restrict mode, FILE *restrict stream);

DESCRIPTION
     The fopen() function opens the file whose name is the string pointed to by filename and associates a
     stream with it.

     The argument mode points to a string beginning with one of the following sequences (Additional charac-ters characters
     ters 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 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 trun-cated. truncated.
             cated.  The stream is positioned at the beginning of the file.

     ``a''   Open for writing.  The file is created if it does not exist.  The stream is positioned at the
             end of the file.  Subsequent writes to the file will always end up at the then current end of
             file, irrespective of any intervening fseek(3) or similar.

     ``a+''  Open for reading and writing.  The file is created if it does not exist.  The stream is posi-tioned positioned
             tioned at the end of the file.  Subsequent writes to the file will always end up at the then
             current end of file, irrespective of any intervening fseek(3) or similar.

     The mode string can also include the letter ``b'' either as last character or as a character between
     the characters in any of the two-character strings described above.  This is strictly for compatibility
     with ISO/IEC 9899:1990 (``ISO C90'') and has no effect; the ``b'' is ignored.

     Finally, as an extension to the standards (and thus may not be portable), mode string may end with the
     letter ``x'', which insists on creating a new file when used with ``w'' or ``a''.  If path exists, then
     an error is returned (this is the equivalent of specifying O_EXCL with open(2)).

     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 may be intermixed on read/write streams in any order, and do not require an intermedi-ate intermediate
     ate 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.  When the stream is closed via
     fclose(3), fildes is closed also.

     The freopen() function opens the file whose name is the string pointed to by filename and associates
     the stream pointed to by stream with it.  The original stream (if it exists) is closed.  The mode argu-ment argument
     ment is used just as in the fopen() function.

     If the filename argument is NULL, freopen() attempts to re-open the file associated with stream with a
     new mode.  The new mode must be compatible with the mode that the stream was originally opened with:

           •   Streams originally opened with mode ``r'' can only be reopened with that same mode.

           •   Streams originally opened with mode ``a'' can be reopened with the same mode, or mode ``w''.

           •   Streams originally opened with mode ``w'' can be reopened with the same mode, or mode ``a''.

           •   Streams originally opened with mode ``r+'', ``w+'', or ``a+'' can be reopened with any mode.

     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 argument to fopen(), fdopen(), or freopen() was invalid.

     The fopen(), fdopen() and freopen() functions may also fail and set errno for any of the errors speci-fied specified
     fied 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), fileno(3), fseek(3), funopen(3)

STANDARDS
     The fopen() and freopen() functions conform to ISO/IEC 9899:1990 (``ISO C90'').  The fdopen() function
     conforms to IEEE Std 1003.1-1988 (``POSIX.1'').

BSD                            January 26, 2003                            BSD