I am attempting to use the libtommath library. I'm using the NetBeans IDE for my project on Ubuntu linux. I have downloaded and built the library, I have done a 'make install' to put the resulting .a file into /usr/lib/ and the .h files into /usr/include It appears to be finding the files appropriately (since I no longer get those errors, which I did before installing into the /usr directories). However, when I create a simple main making a call to mp_init (which is in the library), I get the following error when I attempt to make my project: mkdir -p build/Debug/GNU-Linux-x86 rm -f build/Debug/GNU-Linux-x86/main.o.d gcc -c -g -MMD -MP -MF build/Debug/GNU-Linux-x86/main.o.d -o build/Debug/GNU-Linux-x86/main.o main.c mkdir -p dist/Debug/GNU-Linux-x86 gcc -o dist/Debug/GNU-Linux-x86/cproj1 build/Debug/GNU-Linux-x86/main.o build/Debug/GNU-Linux-x86/main.o: In function 'main': /home/[[myusername]]/NetBeansProjects/CProj1/main.c:18: undefined reference to `mp_init' collect2: ld returned 1 exit status make[2]: *** [dist/Debug/GNU-Linux-x86/cproj1] Error 1 So, it looks like the linker can't find the function within the library, however it IS there, so I just don't know what could be causing this. Any help would be appreciated. I get the same error if I type the gcc command directly and skip the makefile, I also made sure the static library got compiled with gcc as well. Edited to Add: I get these same errors if I do the compile directly and add the library with -l or -L: $ gcc -l /usr/lib/libtommath.a main.c /usr/bin/ld: cannot find -l/usr/lib/libtommath.a collect2: ld returned 1 exit status $ gcc -llibtommath.a main.c /usr/bin/ld: cannot find -llibtommath.a collect2: ld returned 1 exit status $ gcc -Llibtommath.a main.c /tmp/ccOxzclw.o: In function `main': main.c:(.text+0x18): undefined reference to `mp_init' collect2: ld returned 1 exit status $ gcc -Llibtommath.a main.c /tmp/ccOxzclw.o: In function `main': main.c:(.text+0x18): undefined reference to `mp_init' collect2: ld returned 1 exit status I am very rusty on this stuff, so I'm not sure I'm using the right command here, in the -L examples are the libraries being found? If the library isn't being found how on earth do I get it to find the library? It's in /usr/lib, I've tried it with the .a file in the current directory, etc. Is there an environment variable I need to set? If so, how, etc. Thanks so much for the help. I've tried a completely different library (GMP) and had the EXACT same problem. This has got to be some kind of Ubuntu environment issue? Anyone have any idea how to fix this?