[Bug stdio/13151] fmemopen streams fail to read eof

["mtk.manpages at gmail dot com" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13151

Michael Kerrisk <mtk.manpages at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |mtk.manpages at gmail dot
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--- Comment #1 from Michael Kerrisk <mtk.manpages at gmail dot com> 2012-04-27 20:15:20 UTC ---
I think your example isn't quite right, but there is a real problem here.

See the fopen(3) man page:

       a+     Open for reading and appending (writing at end  of
              file).   The file is created if it does not exist.
              The initial file position for reading  is  at  the
              beginning  of  the  file,  but  output  is  always
              appended to the end of the file.

So, "a+" should set the file pointer to byte 0.

It looks like the problem is that the file position is not correctly reset to 0
when mode is "a+". Without a command-line argument, the program below does an
fmemopen(), and then tries reading characters, and we see the problem:

$ ./a.out
Initial ftell(): -1
Segmentation fault (core dumped)

With a command-line argument, the program does an initial fseek() to explicitly
place the file pointer at byte 0, and then all is well:

./a.out x
Initial ftell(): -1
Resetting file position
New ftell(): 0
 0: h 104; feof()=0; ftell() = 1
 1: e 101; feof()=0; ftell() = 2
 2: l 108; feof()=0; ftell() = 3
 3: l 108; feof()=0; ftell() = 4
 4: o 111; feof()=0; ftell() = 5
 5: � -1; feof()=1; ftell() = 5

So, it seems that the fix would be that when fmemopen() mode is "a+", the glibc
implementation should set the file position to 0.

=========
#define _XOPEN_SOURCE 700
#include <stdio.h>
#include <string.h>
#include <assert.h>
#include <stdlib.h>

#define errExit(msg)     do { perror(msg); exit(EXIT_FAILURE); \
            } while (0)

int
main(int argc, char **argv)
{
    FILE *fp = fmemopen((char[20]){"hello, world"}, 5, "a+");
    char c;
    int j;

    printf("Initial ftell(): %ld\n", ftell(fp));

    if (argc > 1) {
    printf("Resetting file position\n");
        fseek(fp, 0, SEEK_SET);
        printf("New ftell(): %ld\n", ftell(fp));
    }

    for (j = 0; j < 20 && !feof(fp); j++) {
        c = fgetc(fp);
    printf("%2d: %c %d; feof()=%d; ftell() = %ld\n",
        j, c, c, feof(fp), ftell(fp));
    }
}

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