C: How do I make a makefile that allows linking to an external library?

Like, I think I'm close... just not sure what I'm doing wrong,

cfann : main.o
    main.o -l libfann

main.o : main.c
    gcc -c main.c

clean: 
    rm -rf *o cfann
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I get this error:

main.o -l libfann
make: main.o: No such file or directory
make: *** [cfann] Error 1

Change:

cfann : main.o
    main.o -l libfann

to:

cfann : main.o
    gcc main.o -lfann -L/usr/local/lib -o cfann

This assumes that libfann.o is in /usr/local/lib - change the -L path above if it's actually somewhere else.

You need to replace:

cfann : main.o
    main.o -l libfann

with something like:

cfann : main.o
    gcc -o cfann -L/path/to/libs main.o -lfann

-L allows you to specify (multiple) paths to search for the libraries and -l lists the library names. The lib is normally prefixed for you, as are the possible extensions such as .a or .so.

What your original makefile is doing is trying to run main.o as a command, rather than the gcc that it should be running.

Try this

cfann : main.o

main.o : main.c
    gcc -c main.c -lfann

clean: 
    rm -rf *o cfann

Or, maybe without the empty cfann line

cfann : main.c
    gcc -c main.c -lfann

clean: 
    rm -rf *o cfann