C error: missing binary operator before token - c

Question

I'm working on building a custom version of openwrt with a build tool and keep running into a error I cant seem to fix.
heres the code block its dating back to.
#include <signal.h>
#if ! HAVE_STACK_T && ! defined stack_t
typedef struct sigaltstack stack_t;
#endif
#ifndef SIGSTKSZ
# define SIGSTKSZ 16384
#elif HAVE_LIBSIGSEGV && SIGSTKSZ < 16384
/* libsigsegv 2.6 through 2.8 have a bug where some architectures use
more than the Linux default of an 8k alternate stack when deciding
if a fault was caused by stack overflow. */
# undef SIGSTKSZ
# define SIGSTKSZ 16384
#endif
heres the out put error
In file included from /usr/include/signal.h:328,
from ./signal.h:52,
from c-stack.c:49:
c-stack.c:55:26: error: missing binary operator before token "("
55 | #elif HAVE_LIBSIGSEGV && SIGSTKSZ < 16384
| ^~~~~~~~

Answer 1

First of all please note that this is not standard C but POSIX extensions. POSIX has the nasty habit of poisoning standard libraries with non-standard, non-conforming extensions.
This means that if you compile with -std=c17 or equivalent instead of -std=gnu17 (default), gcc will strip off all non-standard, non-conforming POSIX junk. And then those things will not be found at all even if you include signal.h, which in turn can give you very confusing compiler errors.
That being said, I think you perhaps meant to do:
#elif defined(HAVE_LIBSIGSEGV) && SIGSTKSZ < 16384
As for how you can find out what SIGSTKSZ is without digging through some library header, here's a simple trick that gives you the exact macro definition:
#include <stdio.h>
#include <signal.h>
#define STR(s) #s
#define WHAT_ARE_YOU(x) puts(STR(x))
int main (void)
{
WHAT_ARE_YOU(SIGSTKSZ);
}
When compiling as GNU C on some random Linux machine, I get 8192.
You could also do gcc -E to view the preprocessor output (which may be a bit confusing to read since it expands all headers). Doing that for the above example gives puts("8192");.