[Bug stdio/20005] New: fflush on a file opened with fmemopen resets position to 0

["gbs at panix dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=20005

            Bug ID: 20005
           Summary: fflush on a file opened with fmemopen resets position
                    to 0
           Product: glibc
           Version: 2.23
            Status: NEW
          Severity: critical
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: gbs at panix dot com
  Target Milestone: ---

As of 2.23 glibc seems to have changed its behavior on an fflush of a stream
created with fmemopen, in that it resets the file position to 0.

Here's some sample code:

    char buffer[500] = "x";
    FILE *stream;
    stream = fmemopen(buffer, 500, "r+");
    fwrite("fish",sizeof(char),5,stream);
    printf("pos-1:%ld\n",ftell(stream));
    fflush(stream);
    printf("pos-2:%ld\n",ftell(stream));


On earlier versions this would result in:

pos-1:5
pos-2:5

Now it returns:

pos-1:5
pos-2:0

I verified this using Ubuntu 15.10 for the earlier version, and 16.04 for 2.23.

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