Hi, I have written some useful functions that I call from the programs that I code. The functions are compile into a .o object file with the followin:
gcc -c func_useful.c

I'd then normally compile it into my programs if I need them with the following:
gcc -o executableName executableSrc.c func_useful.o -lc -lpthread

However, what's the difference between doing it this way versus me using "ar" to compile func_useful.c into func_useful.a?

Both ways require the code to be statically linked in to the executable before run time.

A *.a library can (usually does) contain multiple *.o objects; when you use it in linkage, only the necessary objects will be added to the executable from the library.

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Thanks. Essentially I guess they are the same then.

Yes, in your special case they are, but normally a *.a lib does contain multiple objects.

The linker will still treat them differently in a way that often matters (but apparently didn't in your current situation).

A .o file will be linked in even if it doesn't resolve any symbol that was undefined at the point the linker encounters that .o.

A .o wrapped inside a .a will only be linked in if it resolves some undefined symbol.