Compile time error while running multiple c files - c

Question

#Edited
I am tested on simple two files now that are add.h and add.c.
I made a Makefile in order to compile my program. Here is my makefile.
# Make file for running the project
CC=gcc
CFLAGS= -Wall -g
LDFLAGS = -include
OBJFILES = add.o
LIB = add.h
TARGET = add
all: ${TARGET}
%.o: %.c
${CC} ${CFLAGS} -c -o $# $<
${TARGET}: ${OBJFILES}
${CC} ${CFLAGS} -o ${TARGET} ${OBJFILES}
clean:
rm -f $(OBJFILES) $(TARGET) *~
when I run
make add
I get the following error:
gcc -Wall -g -c -o add.o add.c
gcc -Wall -g -o add add.o
Undefined symbols for architecture x86_64:
"_b", referenced from:
_main in add.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [add] Error 1
here are snippets of my code
add.c
#include <stdio.h>
#include <stdlib.h>
#include "add.h"
int main(void) {
int a = 10, b=20;
add(a, b);
return 0;
}
add.h
#ifndef __ADD_H_
#define __ADD_H_
extern int a,b;
int add(a,b)
{
return a+b;
}
#endif // __ADD_H_

Answer 1

You are lacking a rule to actually build your object files. Right now you only have one to link them all together once they already exist. Try adding:
%.o: %.c
${CC} ${CFLAGS} -c -o $# $<
This tells make how to build object files out of source files.

Answer 2

You are misusing the extern keyword, losing track of your variables and making your code too complicated.
Let's try something very simple:
int main(void) {
int a = 10; b=20;
return 0;
}
This fails. The compiler complains about the statement b=20;, since it has never heard of this b. The semicolon that made this a separate statement was either a typo or a conceptual error caused by declaring extern int b elsewhere. There is no need for extern here, at least not yet.
This:
int main(void) {
int a=10, b=20;
return 0;
}
works.
Now for an add function.
int add(int a, int b)
{
return a+b;
}
int main(void) {
int a = 10, b=20;
add(a,b);
return 0;
}
Note that the a and b in add are not the same variables as the a and b in main. This is crucial; do not proceed until you understand it.
Now add a declaration of the add function:
int add(int a, int b); // <- declaration
int add(int a, int b) // <- definition
{
return a+b;
}
The declaration can be moved into a header file (add.h); the definition belongs in a source file (add.c).
Finally, I would advise you to add a line to the makefile:
add.o: add.h

Answer 3

Aren't you missing an #endif at the end of the last header file queue.h ?
This can be for sure an issue when compiling. Moreover, aren't you missing the main_application.h header file you are then including within the stack.h one for example?
Have a look at this: https://medium.com/#m.muizzsuddin_25037/error-ld-symbol-not-found-for-architecture-x86-64-a5e5b648ffc seems helpful here and point our attention to the header files again!..