sqrt from math.h causes compile error

Question

So I've run into an interesting problem I've never seen before.

I created a small program, as follows:

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

  int main(int argc, char *argv[]) {
    int i; 
    double tmp;
    double xx;

    for(i = 1; i <= 30; i++) {
      xx = (double) i + 0.01;

      tmp = sqrt(xx);
      printf("the square root of %0.4f is %0.4f\n", xx,tmp);
      sleep(1);
      xx = 0;
    }

    return 0;
 }

When I try to compile this with the following command, I get a compiler error.

gcc -Wall calc.c -o calc

returns:

/tmp/ccavWTUB.o: In function `main':
calc.c:(.text+0x4f): undefined reference to `sqrt'
collect2: ld returned 1 exit status

If I replace the variable in the call to sqrt(xx) with a constant like sqrt(10.2), it compiles just fine. Or, if I explicitly link like the following:

gcc -Wall -lm calc.c -o calc

It also works just fine. Can anyone tell me what's causing this? I've been a C programmer for a long time (and I've written similar small programs using math.h) and I have never seen anything like this.

My version of gcc follows:

--$ gcc --version
gcc (Ubuntu 4.3.3-5ubuntu4) 4.3.3
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Answer 1

If you look at the output of the compiler in the case where you used sqrt(10.2), I'll bet you see that a call to sqrt() isn't actually made. In the case where it must link to the math library (because you're calling it with a variable), you need to link it explicitly. Some operating systems/compilers do it for you, which is why you might not have noticed in the past.