c - GCC libm not working

Question

I have a c program that calls sin, cos, and acos. When I compile I get the following errors:

/tmp/ccDfW98S.o: In function `zip_search':
main.c:(.text+0xf30): undefined reference to `sin'
main.c:(.text+0xf45): undefined reference to `sin'
main.c:(.text+0xf66): undefined reference to `cos'
main.c:(.text+0xf7b): undefined reference to `cos'
main.c:(.text+0xf9c): undefined reference to `cos'
main.c:(.text+0xfc6): undefined reference to `acos'
collect2: ld returned 1 exit status

I know this is common when you don't use the -lm gcc flag. I AM using this flag. I'm calling GCC like this:

gcc -o zipcode-server -lm main.c

When I compile on one of my computers this works fine. The only difference that I can think of is that this isn't working on x86_64 and the computer it does work on is i686. Both are Ubuntu. The file libm.a is present on the computer it isn't working on and I don't get any errors saying it can't be found. What could be causing this?

Answer 1

You should put -lm after main.c

In general, if you have multiple libraries, they should be written in order of their usage. For example, if library A uses library B, you should have -lA -lB.

In your case, the object file that is the result of compilation of main.c uses library m and therefore -lm should come after it.

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For the curious, this is mostly for efficiency reasons. With this scheme, the linker can resolve currently unknown symbols with every new library seen in the argument list, and picking up new unknown symbols from that library on the way. This means that the linker can visit the libraries one by one, and therefore match the unknown symbols against a small number of symbols provided by each library.

On the contrast, the linker could load in symbols from all libraries at once, and then start matching unknown symbols. In that case however, the linker needs to deal with a much greater number of symbols, increasing both the memory footprint and the execution time of the linker.

Since the libraries could always be declared to the linker in the proper order of their dependencies¹, there is no reason for the linker to choose the inefficient way.

¹ _{Libraries normally have a one-way relationship, in the sense that one uses the other. Circular dependencies between libraries is rare, if at all existing, but it could still be used with this model by repeating certain libraries to be reinspected.}