Re: assertion "ptr != MAP_FAILED" failed while using mmap

["Alex Vinokur" <alexvn@connect.to>]

"Igor Pechtchanski" <pechtcha@cs.nyu.edu> wrote in message news:Pine.GSO.4.44.0306251849140.22307-100000@slinky.cs.nyu.edu...
[snip]
>
> Umm, Elfyn, the semantics of "assert()" is that you assert some predicate
> to be true.  If the predicate is indeed true, the program continues
> normally.  If the predicate is false, the program fails.
> The predicate in this case is "ptr != MAP_FAILED".  Thus, the predicate
> was false when the assertion failed, and ptr == MAP_FAILED.
>
> To the OP: this means that mmap() did fail for some reason.  It should
> have set errno to indicate why.  You should check that instead of
> asserting -- mmap does fail occasionally.  Also FYI, once you assert, the
> following test for "ptr != MAP_FAILED" is redundant -- the program will
> not get there otherwise.
> Igor
[snip]


Here is updated function.
--------------------------------------
void read_file (char* filename_i)
{
int fd = open(filename_i, O_RDONLY);
  assert (fd > 2);

off_t sz = lseek(fd, 0, SEEK_END);
char* ptr = (char*)mmap(0, sz, PROT_READ, 0, fd, 0);

   errno = 0;
  if (ptr != MAP_FAILED)
  {
    string str(ptr, ptr+sz);
    munmap(ptr, sz);
  }
  else
  {
    assert (ptr == MAP_FAILED);
    printf ("=== Error : %u %s ===\n", errno, strerror (errno));
  }
  assert (ptr != MAP_FAILED);  // Here assertion failed

  close(fd);
}
--------------------------------------

The program prints:
=== Error : 0 No error ===

Regards,
   =====================================
   Alex Vinokur
     mailto:alexvn@connect.to
     http://mathforum.org/library/view/10978.html
   =====================================






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