Problems with detection of strerror_r about lua-compat-5.3

[daurnimator]

Looks like https://github.com/keplerproject/lua-compat-5.3/blob/9cb6834fe07297ba79e7453978cf7e7d8d8c339a/c-api/compat-5.3.c#L36 should be change to just !defined(_GNU_SOURCE)

[siffiejoe]

First, it should be !defined(_GNU_SOURCE). At least that's what the official headers use apparently (even though the manpage claims (_POSIX_C_SOURCE >= 200112L) && ! _GNU_SOURCE), probably to avoid this exact issue.

Second, we have another problem: It seems that we don't have a reliable way to detect whether we have the GNU version or the POSIX version of strerror_r. Checking the same preprocessor symbols might not do it, because those can change between inclusion of string.h and our tests.

At the moment I'm considering something like the following when a glibc is used:

char buffer[255] = { 0 };  /* zero initialized */
int myerrno = errno;
strerror_r(myerrno, buffer, sizeof(buffer));  /* use either version of strerror_r */
if (buffer[0] != '\0')
  return buffer; /* strerror_r wrote into our buffer */
else
  /* buffer is unchanged, so we probably have GNU strerror_r which returned a
   * static constant string. Chances are that strerror will return the same static
   * constant string and therefore be thread-safe. */
  return strerror(myerrno);

When I find time I'll take a look at the glibc sources to make sure that the last assumption at least holds for the current glibc version.

I'm open to other suggestions.

[hishamhm]

That sounds reasonable (except for returning a local stack buffer of course, but I mean the general idea), and more reliable than the myriad of preprocessor directives.