What does -lm option do in g++?

Question

What does -lm option do in g++ and when is it needed?

Is there a complete description of g++ options?

Duplicate of [sqrt from math.h causes compile error](http://stackoverflow.com/questions/1711915/sqrt-from-math-h-causes-compile-error). Also related to [gcc: why the -lm flag is needed to link the math library?](http://stackoverflow.com/questions/4606301/gcc-why-the-lm-flag-is-needed-to-link-the-math-library) — Abhijit, May 04 '12 at 11:21
possible duplicate of [Why I'm Getting "undefined reference to `sqrt' " Error, Even Though I include math.h header?](http://stackoverflow.com/questions/10409032/why-im-getting-undefined-reference-to-sqrt-error-even-though-i-include-ma) — Alexey Frunze, May 04 '12 at 11:24
Why does my g++ compile without it?(at least this part is not duplicate) — a-z, May 04 '12 at 11:31

score 28 · Accepted Answer · answered May 04 '12 at 11:22

28

That's a linker option. It tells the linker to link with (-l) the m library (libm.so/dll). That's the math library. You often need it if you #include <math.h>.

answered May 04 '12 at 11:22

Mat

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i was using #include $and getting math not found. Changing it to #include resolved the issue.$ – Siba Prasad Tripathy Jun 05 '21 at 06:13

score 13 · Answer 2 · edited May 23 '17 at 12:34

The option does nothing for g++: referring to this answer https://stackoverflow.com/a/1033940/1143274 libstdc++ requires libm, so it will always be linked by g++.

However, there is also some sort of an automatic linking behaviour for gcc, investigated on this thread http://www.linuxforums.org/forum/programming-scripting/125526-c-gcc-math-h-lm.html which I can't seem to find an answer as to where that comes from and which libraries it applies to...

What does -lm option do in g++?

2 Answers2

Linked