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] .

Answer 2

You need to link the math library explicitly. Add -lm to the flags you're passing to gcc so that the linker knows to link libm.a

Answer 3

Append -lm to the end of the gcc command to link the math library:

gcc -Wall -D_GNU_SOURCE blah.c -o blah -lm

For things to be linked properly, the order of the compiler flags matters! Specifically, the -lm should be placed at the end of the line.

If you're wondering why the math.h library needs to be included at all when compiling in C, check out this explanation here.