Properly Linking a static library with the C math library

Question

I have a library which uses the log function from math.h. When I compile and package this library, I get no compilation errors, which is normal (I think).

Now when I try to use the library in an application, gcc gives me linker errors:

Compiling mytestlist using "mytestlist.o":
gcc mytestlist.o -I/student/cmpt332/pthreads -I. -std=c99 -Wall -pedantic -L. -L/student/cmpt332/pthreads/lib/linuxx86_64/  -llist -o mytestlist 
./liblist.a(list_adders.o): In function `NodeCreate':
list_adders.c:(.text+0x343): undefined reference to `log'
./liblist.a(list_adders.o): In function `ListCreate':
list_adders.c:(.text+0x62f): undefined reference to `log'
./liblist.a(list_adders.o): In function `ListFree':
list_adders.c:(.text+0xdcc): undefined reference to `log'
list_adders.c:(.text+0xe55): undefined reference to `log'
list_adders.c:(.text+0xefb): undefined reference to `log'
./liblist.a(list_adders.o):list_adders.c:(.text+0xf96): more undefined references to `log' follow
collect2: error: ld returned 1 exit status
Makefile:47: recipe for target 'mytestlist' failed
make: *** [mytestlist] Error 1

Why is this happening? The only solution that works is that I have to supply the -lm option to gcc when I compile the program that uses the library (even though the program itself makes no use of math.h), however I find this cumbersome to do.

I've also tried supplying the -lm option when compiling the library, but when the application is compiled using the library, I get the same linker errors.

Is there a way to compile the library with math.h without having to supply -lm to other programs that make use of the library?

In case you're wondering, I compile each object that makes up the library using:

gcc -std=c99 -Wall -pedantic -static -I. -c list_adders.c -o list_something.o -lm

And the library is packaged using:

ar cvfr liblist.a list_something.o ...

A static library is not linked. There is no way to link it with the math library because there is no way to link it at all. You link an application or a shared library, but not a static library. Nor there is a way to mark or record dependecies of your static library in the library itself. — n. 1.8e9-where's-my-share m.

Unknown Unknown · Accepted Answer · 2016-10-03T18:30:14

In your gcc -c command, the -lm isn't doing anything. It's a linker option, and -c means "don't link".

The proper place to put -lm is in fact after the -llist whenever you use it. That's how static library dependencies are done. Put it in the documentation for liblist.

If you want something fancier, there's pkg-config. With the appropriate configuration files, pkg-config --static --libs liblist will output -llist -lm.

Properly Linking a static library with the C math library

1 Answers