gcc - Why Am I Getting Link Errors When Calling Function in Math.h?

Question

When attempting to call functions in math.h, I'm getting link errors like the following

undefined reference to sqrt

But I'm doing a #include <math.h>
I'm using gcc and compiling as follows:

gcc -Wall -D_GNU_SOURCE blah.c -o blah

Why can't the linker find the definition for sqrt?

Answer 1

Add -lm to the command when you call gcc:
gcc -Wall -D_GNU_SOURCE blah.c -o blah -lm

This will tell the linker to link with the math library. Including math.h will tell the compiler that the math functions like sqrt() exist, but they are defined in a separate library, which the linker needs to pack with your executable.

As FreeMemory pointed out the library is called libm.a . On Unix-like systems, the rule for naming libraries is lib[blah].a . Then if you want to link them to your executable you use -l[blah] .