fopen(3) - NetBSD Manual Pages

FOPEN(3)                NetBSD Library Functions Manual               FOPEN(3)


NAME

     fopen, fdopen, freopen -- stream open functions


LIBRARY

     Standard C Library (libc, -lc)


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, 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 char-
           acter 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 fopen() 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'') exten-
           sion.

     `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 reposi-
     tioning 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 descrip-
     tor, 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).

     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.


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).


SEE ALSO

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


HISTORY

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


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);
             }

NetBSD 9.2                     September 2, 2019                    NetBSD 9.2