erf(x) and math.h

Question

According to this site the error function erf(x) comes from math.h. But actually looking in math.h, it isn't there, and gcc cannot compile the following test program while g++ can:

#include <math.h>
#include <stdio.h>

int main(int argc, char* argv[]) {
  double x;
  double erfX;
  x = 1.0;
  erfX = erf(x);

  printf("erf(%f) = %f", x, erfX);
}

$ gcc mathHTest.c
/tmp/ccWfNox5.o: In function `main':
mathHTest.c:(.text+0x28): undefined reference to `erf'
collect2: ld returned 1 exit status
$ g++ mathHTest.c

What does g++ pull in that gcc doesn't? Looking in /usr/include, the only place I could find erf(x) was in tgmath.h, which I don't include. So g++ must be grabbing different headers than gcc, but which ones?

EDIT: I wasn't linking in libm with gcc, hence the link error. However, I still don't understand why erf() is not in math.h. Where is it coming from?

Answer 1

You need to link the math library (libm) too:

$ gcc mathHTest.c -lm

All of the normal math library functions are actually there, and not in the standard C library (libc).

According to my tests, g++ does include libm automatically, but gcc doesn't.

Answer 2

'erf' is actually declared in bits/mathcalls.h, which is #included by math.h. The actual declaration is heavily obscured by macro magic to make it do the right thing for both C and C++

Answer 3

I had the same problem using gcc from cygwin on a x86 processor. The "-lm" library include parameter (after the file list!) worked perfectly.