1if tst -D_ISOC99_SOURCE -lm note{ _ISOC99_SOURCE plays nice }end link{ 2 #include <sys/types.h> 3 #include <sys/stat.h> 4 #include <stdlib.h> 5 #include <unistd.h> 6 #include <math.h> 7 int main() { return signbit(-0.0); } 8 }end { 9 #ifndef _ISOC99_SOURCE 10 #define _ISOC99_SOURCE 1 11 #endif 12 } 13endif 14