what does clang command option -lm mean?

Question

I found someone says that -lm means linking to the math library.
I want to figure out the meaning by using build-in help in the terminal.
So I run the $ clang --help
I don't find any stuff related to -l or -m
How can I get the meaning of -lm with the self-help terminal?

Answer 1

The -l{name} flag tells the linker to link against lib{name}. So -lm links against libm, the c math library.
This isn't a flag to the clang compiler but passed to the linker, which is why you won't find it with clang --help or man clang.
If you run clang with the --verbose flag you will see the invocation of the linker (in my case /usr/bin/ld) as the last step.
By running man ld or ld --help (or whichever linker your clang version is using) you will find the documentation.
Keep in mind that not all flags are passed to the linker in this way (-l is probably passed for compatibility with the gcc compiler as melpomene said). To make sure an option is passed, use the -Wl option of clang.