Using sqrtf() in C: "undefined reference to `sqrtf'"

Go to Build -> Set Build Commands then under C commands click on the empty label and it will let you specify a new label (name it Link). Type in it gcc -Wall -o "%e" "%f" -lm - where -lm will tell it to link the math library to your app. Click OK.

Then click on Build and select your newly created label - Link. This should do it for you.

In addition to the many fine answers here, the portable form of the command that supports C99 version of <math.h> is specified by POSIX as c99 -l m. That having been said, every important Linux compiler supports -lm.

You need to link with -lm to provide the math functions.