Compile issue

[H1N1-dev]

If i use this command:
gcc asciiFluidSimulation.c -o asciiFluidSimulation, then compiler raise error like:

/usr/bin/ld: /tmp/ccqo7MZU.o: in function «main»:
asciiFluidSimulation.c:(.text+0x367): undefined reference to «cabs»
/usr/bin/ld: asciiFluidSimulation.c:(.text+0x3c0): undefined reference to «floor»
/usr/bin/ld: asciiFluidSimulation.c:(.text+0x5e2): undefined reference to «cabs»
/usr/bin/ld: asciiFluidSimulation.c:(.text+0x63b): undefined reference to «floor»
/usr/bin/ld: asciiFluidSimulation.c:(.text+0xa4a): undefined reference to «cabs»

But, if i add -lm to command, then program compile without errors.
gcc asciiFluidSimulation.c -lm -o asciiFluidSimulation