These functions are gcc builtins. They are optimised away. On linux, too. (Compile with -save-temps to see what is produced, or check the disassembly with objdump -d.)

To do a proper test, use separate compilation units. 

/* test_isnanl.c */
#include <math.h>
int test_isnanl(long double x) { return isnanl(x); }

[download]
And link: cc main.o test_isnanl.o -lm && ./a.out || FAIL

However, the above is not entirely future-proof either—it's defeated with link-time optimisation (-flto).
So perhaps it would be better to 
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(int argc, char *argv[]) {
 long double x = strtold(argv[1], NULL);
 printf("%d", isnanl(x));
 printf("%Lf", erfl(x));
 return 0;
}

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Finally, if you limit your scope to the usual gcc/clang, then there is the -fno-builtin option that will make your original test work as is. This might be the best option of all.

In reply to Re^3: [C Question] Determine if gcc provides erfl() function by Anonymous Monk
in thread [C Question] Determine if gcc provides erfl() function by syphilis

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