lkml.org 
[lkml]   [2000]   [Jul]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectClosing the FILE object.

    This is in reference to the reported seg-faults when attempting to
    fclose() an invalid file pointer.

    The following shows that my current 'C' runtime library does not
    adhere to any known standard when referencing the FILE object during
    fclose(). This is gcc 2.7.2.3 (libc 5.3.12)

    According to existing Linux documentation, this should return
    EBADF. It should NOT seg-fault.

    #include <stdio.h>
    #include <malloc.h>

    int main ()
    {
    FILE *fp;
    fp = (FILE *)malloc(sizeof(FILE));/* Make sure the pointer is valid */
    fprintf(stderr, "%p\n", fp); /* Display valid address */
    fclose(fp);
    puts("It worked??"); /* Will not occur */
    return 0;
    }


    Script started on Fri Jul 14 12:50:27 2000
    $$$ man fclose
    FCLOSE(3) Linux Programmer's Manual FCLOSE(3)

    NAME
    fclose - close a stream

    SYNOPSIS
    #include <stdio.h>

    int fclose( FILE *stream);

    DESCRIPTION
    The fclose function dissociates the named stream from its
    underlying file or set of functions. If the stream was
    being used for output, any buffered data is written first,
    using fflush(3).

    RETURN VALUES
    Upon successful completion 0 is returned. Otherwise, EOF
    is returned and the global variable errno is set to indi-
    cate the error. In either case no further access to the
    stream is possible.

    ERRORS
    line 1 EBADF The argument stream is not an open stream.
    line 4
    The fclose function may also fail and set errno for any of
    the errors specified for the routines close(2) or
    fflush(3).

    SEE ALSO
    close(2), fflush(3), fopen(3), setbuf(3)

    STANDARDS
    The fclose function conforms to ANSI C3.159-1989 (``ANSI
    C'').

    BSD MANPAGE 29 November 1993 1

    line 19/66 (END)# exit
    exit

    Script done on Fri Jul 14 12:50:47 2000


    I reported a similar problem several years ago when I way trying to
    substitute during runtime, a FILE pointer obtained from a fopen() and
    the default FILE pointer(s) of stdin, stdout, stderr. In other words,
    if the program didn't have a terminal, stdout and stderr would go to
    a file.

    Because the FILE *object didn't work as a real pointer, I could not
    do:
    FILE *fp;

    fp = fopen("filename", "w");
    fflush(stdout);
    fflush(stderr);
    stdout = fp;
    stderr = fp;

    The next write to stdout would segfault. This behavior was never seen
    on any other Unix system, including Ultrix and Sun(s). In fact, the
    interplay of the FILE object was a feature of Unix 'C' programming.

    In the example above, I could have saved the original FILE *, and
    restored it later in the program. With Linux, you can't even compile
    such a program because the assignments will result in 'invalid lvalue'
    errors. This is because Linux libc's stdout is not a FILE * at all!

    This breaks compatability rules because, for instance fprintf() takes,
    as it's first parameter, a pointer of type FILE.

    So we have the case of fprintf(stderr, ...) compiling and working because
    stderr is a FILE pointer, but I can't make a legal assignment to it like
    any other FILE pointers.

    Go figure.

    Cheers,
    Dick Johnson

    Penguin : Linux version 2.2.15 on an i686 machine (797.90 BogoMips).

    "Memory is like gasoline. You use it up when you are running. Of
    course you get it all back when you reboot..."; Actual explanation
    obtained from the Micro$oft help desk.



    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.rutgers.edu
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2005-03-22 13:57    [W:0.049 / U:0.008 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site