[Bug libc/18241] New: failed fseek on memstream does not set errno and can fail when it shouldnt

["nszabolcs at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18241

            Bug ID: 18241
           Summary: failed fseek on memstream does not set errno and can
                    fail when it shouldnt
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: nszabolcs at gmail dot com
                CC: drepper.fsp at gmail dot com

it seems fseek does not work as it should on memstreams opened by
open_memstream and fmemopen:

fseek outside of the bounds fails but does not set errno to EINVAL.

fseek(f,-1,SEEK_CUR) fails.


the code below prints:
errno == EINVAL failed
  errno = Success
fseek(f, -1, SEEK_CUR) >= 0 failed
ftell(f) == 0 failed
strcmp(s, "b") == 0 failed
  s = "ab"


#define _POSIX_C_SOURCE 200809L
#include <errno.h>
#include <string.h>
#include <stdio.h>

#define A(x) ((x) || (printf(#x " failed\n"),0))

int main(void)
{
FILE *f;
char *s;
size_t l;

f = open_memstream(&s, &l);
A(f);
A(putc('a', f) == 'a');
A(fflush(f) == 0);
A(fseek(f, -2, SEEK_CUR) == -1);
A(errno == EINVAL) || printf("  errno = %s\n", strerror(errno));
A(ftell(f) == 1);
A(ferror(f) == 0);

A(fseek(f, -1, SEEK_CUR) >= 0) || printf("  errno = %s\n", strerror(errno));
A(ftell(f) == 0);
A(ferror(f) == 0);
A(putc('b', f) == 'b');
A(fflush(f) == 0);
A(strcmp(s, "b") == 0) || printf("  s = \"%s\"\n", s);
}

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