Re: strerror_r questions

[Eric Blake <eblake at redhat dot com>]

On 02/09/2011 05:29 PM, Eric Blake wrote:
> On 02/09/2011 05:17 PM, Eric Blake wrote:
>> Changes in v2: more documentation, fix the signature, change the default
>> when no feature macros are defined, don't overwrite buf if n is too
>> small, guarantee NUL termination in POSIX variant
> 
> Shoot - I missed another POSIX requirement:
> 
> "The implementation shall behave as if no function defined in this
> volume of POSIX.1-2008 calls strerror( )."
> 
> That is, strerror_r() cannot call strerror() (but we _can_ have a helper
> method that both strerror() and strerror_r() call).
> 
> The POSIX requirement means that someone can replace strerror() with
> their own link, but doing so won't affect the behavior of strerror_r().
>  Is this something that I should worry about and clean up in a followup
> patch?  Or is it too much of a corner case to bother with?

Reading further, POSIX states the reason for that requirement:

"Historically in some implementations, calls to perror( ) would
overwrite the string that the pointer returned by strerror( ) points to."

That is:

#include <stdio.h>
#include <string.h>
#include <errno.h>
int main (void) {
  char *err = strerror(1000);
  printf("%s", err);
  errno = 2000;
  peror("hi");
  printf("%s", err);
  return 0;
}

should result in the first and last lines being the same; but in the
broken case would result in the perror() corrupting the buffer.

Newlib is immune to observing that scenario (unless you provide a
non-standard _user_strerror hook), given that its implementation returns
distinct pointers for all possible errors rather than copying the result
into a common buffer and returning that buffer, and does not do any
formatting of out-of-range values.

But cygwin violates this, since it overrides newlib's strerror() to
format out-of-range numbers into a common reentrant data location, but
uses newlib's perror() which overwrites this location.

-- 
Eric Blake   eblake@redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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