How to compile library on C using GCC?

Question

I made a library with the files pila.h and pila.c. I compile the file pila.c with gcc pila.c -c and this library works fine. I have tested it.

Then I made another library. This library has the files pila_funciones_extra.h and pila_funciones_extra.c. In this library I need to include the first library. In the file pila_funciones_extra.h I put the next line to include it:

#include "pila.h"

and in the file pila_funciones_extra.c I put the next line:

#include "pila_funciones_extra.h"

as it has to be.

But when I try to compile the file pila_funciones_extra.c the compiler doensn't recognize the inclusion of the library pila. It says that the functions, structures, constants and macros that are defined in the library pila haven't been defined.

I tried to compile it with gcc pila_funciones_extra.c -c and gcc pila_funciones_extra.c -c pila.o but it doesn't work.

I made sure that all the files are in the same folder.

I'm working on Ubuntu.

Can anyone tell me the right way to compile it?

Answer 1

First, always take the habit to compile with -Wall (and perhaps even also -Wextra to get even more warnings) option to gcc; it gives you almost all warnings, and you should improve your code till no warnings are given.

Then you often want to be able to debug your code, so also pass -g to gcc. Once you are confident with your code you could ask gcc to produce optimized machine code with -O2. Learn to use the gdb debugger.

So compile your first library, assuming its source files first1.c and first2.c are in FirstLib/ directory, with e.g.

cd FirstLib/
gcc -Wall -g -c first1.c -o first1.o
gcc -Wall -g -c first2.c -o first2.o

At this point, you should use a Makefile and learn how to use make, in particular because you want to get your libfirst.a with

ar ruv libfirst.a first1.o first2.o
ranlib libfirst.a

Then you can pass -L../FirstLib -lfirst as the last options to the gcc command compiling and linking your program using your libfirst.a

Then compile your second library by having your Makefile in directory SecondLib/ which very probably should contain

# in SecondLib/Makefile
CC=gcc
CFLAGS= -I../FirstLib/ -Wall -g
## add your other stuff for make

etc etc. You really want to learn how to use make and write your own Makefile-s so take time to read the GNU make documentation.

You may want to pass -H to gcc to have it tell you all the included files, and you may want to also use remake (in addition & replacement of make) to debug your more complex Makefile-s (notably by running remake -x). Here is an example of Makefile; you'll find many others!

Read also the Program Library Howto.