How to link a subfolder with gcc in C?

Question

I have the below tree:

├── cminpack
│   ├── hybrd.c
│   ├── hybrd.o
│   ├── hybrj1.c
│   ├── hybrj1.o
│   ├── hybrj.c
│   ├── hybrj.o
│   ├── libminpack.a
│   ├── Makefile
│   ├── minpack.h
│   ├── minpack.h.gch
│   ├── readmeC.txt
│   ├── readme.txt
├── greeter.c
├── greeter.h
├── greeter.o
├── swaps.c
├── swaps.h
├── swaps.o

The files:

• cminpack is a package I found and copied and I can make it successfully
• swaps.c is where my main is, I also do have the below in it:

#include "./cminpack/minpack.h"
#include "./greeter.h"

• greeter.c is another file with some functions

In swaps.c I call a function from greeter.c and function hybrd_ from cminpack/hybrd.c. I want to link both greeter and cminpack to my swaps.

I manage to link greeter by running the below gcc command:

sudo gcc -c swaps.c greeter.c ;sudo gcc -o swaps swaps.c greeter.c -lm ;./swaps

Now if I try to link cminpack I am running the below gcc command:

sudo gcc -c swaps.c greeter.c -I./cminpack;sudo gcc -o swaps swaps.c greeter.c -lm -L /cminpack

This leads to the infamous linking error:

/usr/bin/ld: /tmp/ccxwxt6A.o: in function `fsolve':
swaps.c:(.text+0xfb3): undefined reference to `hybrd_'
collect2: error: ld returned 1 exit status

How do I link the subfolder?

(gcc version 10.2.1 20210110 (Debian 10.2.1-6))

Answer 1

In addition to -L cminpack, you should add -lminpack to the gcc command line.

There should be no reason to use sudo for the compilation of this program, if you cannot invoke gcc directly, your system is misconfigured and should be fixed. Running the compiler as root is highly discouraged.

Create a Makefile in the main directory with these lines:

%.o: %.c greeter.h swaps.h cminpack/minpack.h
    gcc $(CFLAGS) -I cminpack -o $@ $*.c

swap: swap.o greeter.o cminpack/libminpack.a
    gcc $(CFLAGS) -o $@ swaps.o greeter.o -L cminpack -lminpack -lm

cminpack/libminpack.a:
    make -C cminpack