[Bug libc/13152] New: fmemopen does not honor append mode

[Bug libc/13152] New: fmemopen does not honor append mode

[bugdal at aerifal dot cx]

http://sourceware.org/bugzilla/show_bug.cgi?id=13152

             Bug #: 13152
           Summary: fmemopen does not honor append mode
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper.fsp@gmail.com
        ReportedBy: bugdal@aerifal.cx
    Classification: Unclassified


fmemopen sets the initial offset correctly for append mode, but otherwise seems
to treat append mode just like "r+" (read/update) mode. Writes are not forced
to start at the "current size" of the stream, as specified by POSIX:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/fmemopen.html

In other words, opening a stream for append, then seeking to the beginning and
attempting a write overwrites the beginning of the buffer rather than
appending.

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[Bug stdio/13152] fmemopen does not honor append mode

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=13152

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|libc                        |stdio

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[Bug stdio/13152] fmemopen does not honor append mode

[mtk.manpages at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13152

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mtk.manpages at gmail dot
                   |                            |com

--- Comment #1 from Michael Kerrisk <mtk.manpages at gmail dot com> 2012-04-27 20:18:01 UTC ---
Do you have some example code that demonstrates this?

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[Bug stdio/13152] fmemopen does not honor append mode

[mtk.manpages at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13152

--- Comment #2 from Michael Kerrisk <mtk.manpages at gmail dot com> 2012-04-27 20:48:37 UTC ---
Okay -- I created an example. See below.

In the below, the "X", should appear at the end of "hello, world", rather than
overwriting the first byte.

$ ./a.out x
Initial ftell(): 12
Resetting file position
New ftell(): 0
Xello, world
=====
#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)
{
    char buf[20] = {"hello, world"};
    FILE *fp = fmemopen(buf, 20, "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));
    }

    fflush(fp);
    fprintf(fp, "X");
    fflush(fp);
    printf("%s\n", buf);
}

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[Bug stdio/13152] fmemopen does not honor append mode

[mtk.manpages at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13152

--- Comment #3 from Michael Kerrisk <mtk.manpages at gmail dot com> 2012-04-28 03:10:35 UTC ---
I've added the following text to the fmemopen(3) man page, under bugs:

[[
Specifying append mode ("a" or "a+") for
.BR fmemopen ()
sets the initial file position to the first null byte,
but (if the file offset is reset to a location other than
the end of the stream) 
does not force subsequent to append at the end of the stream.
]]

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[Bug stdio/13152] fmemopen does not honor append mode

[schwab@linux-m68k.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=13152

Andreas Schwab <schwab@linux-m68k.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|drepper.fsp at gmail dot    |unassigned at sourceware
                   |com                         |dot org

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[Bug stdio/13152] fmemopen does not honor append mode

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=13152

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-

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[Bug stdio/13152] fmemopen does not honor append mode

[adhemerval.zanella at linaro dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=13152

Adhemerval Zanella <adhemerval.zanella at linaro dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |adhemerval.zanella at linaro dot o
                   |                            |rg
         Resolution|---                         |FIXED

--- Comment #4 from Adhemerval Zanella <adhemerval.zanella at linaro dot org> ---
This is fixed by fdb7d390dd0d96e4a8239c46f3aa64598b90842b.  Closing bug.

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