Undefined reference to sqrt (or other mathematical functions)

You may find that you have to link with the math libraries on whatever system you're using, something like:

gcc -o myprog myprog.c -L/path/to/libs -lm
                                       ^^^ - this bit here.

Including headers lets a compiler know about function declarations but it does not necessarily automatically link to the code required to perform that function.

Failing that, you'll need to show us your code, your compile command and the platform you're running on (operating system, compiler, etc).

The following code compiles and links fine:

#include <math.h>
int main (void) {
    int max = sqrt (9);
    return 0;
}

---

Just be aware that some compilation systems depend on the order in which libraries are given on the command line. By that, I mean they may process the libraries in sequence and only use them to satisfy unresolved symbols at that point in the sequence.

So, for example, given the commands:

gcc -o plugh plugh.o -lxyzzy
gcc -o plugh -lxyzzy plugh.o

and plugh.o requires something from the xyzzy library, the second may not work as you expect. At the point where you list the library, there are no unresolved symbols to satisfy.

And when the unresolved symbols from plugh.o do appear, it's too late.

I suppose you have imported math.h with #include <math.h>

So the only other reason I can see is a missing linking information. You must link your code with the -lm option.

If you're simply trying to compile one file with gcc, just add -lm to your command line, otherwise, give some informations about your building process.