Compiling C/C++ With the Math Library

Quite often I see, or get asked why certain math functions don’t compile properly when all the necessary includes are correctly applied. For example:

#include <stdio.h>
#include <math.h>
 
int main()
{
  double value = 9.0;
  double ans = sqrt(value);
 
  char szBuffer[256];
  sprintf(szBuffer,"%f",ans);
  printf("%s",szBuffer);
 
  return 0;
}

Compiling the above code with the command

gcc main.c

gives an error indicating that sqrt is undefined.

$ gcc main.c
/tmp/ccHazXRB.o: In function `main':
main.c:(.text+0x48): undefined reference to `sqrt'
collect2: ld returned 1 exit status

The simple problem here is that the code isn’t linked against the math library so the math functions cannot be resolved. The correct way to compile is using the -lm option which instructs the linker to use the math library.

$ gcc main.c -lm

One thing to remember is the location of the -lm option. This usually has to be placed after the filename that is being compiled or it does not get picked up when using gcc as the compiler. When using g++, the location of the -lm option doesn’t matter.