Bug 13152

Do you have some example code that demonstrates this?

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);
}

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.
]]

This is fixed by fdb7d390dd0d96e4a8239c46f3aa64598b90842b.  Closing bug.