Using C function in another function [closed] - c

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I know that this has been answered, but I still can't get it to work.
first.c
#include <stdio.h>
#include "second.h"
int main(void){
printf("%d\n", addone(5));
return 0;
}
second.c
int addone(int a){
return ++a;
}
second.h
int addone(int a);
When I run gcc -o executable first.c -Wall then it shows me undefined reference to addone

Answer 1

#include <stdio.h>
#include "second.h"
include those headers in second.c file too and then write Makefile and compile it with make.
Makefile:
CFLAGS = -g -O -Wall
OBJ = first.o second.o
# An explicit rule is required to link.
# Compilation is handled automatically.
first: $(OBJ)
$(CC) $(CFLAGS) -o first $(OBJ)
# Declare that both object files depend on the header file.
first.o second.o: second.h
# Conventionally 'make clean' removes what 'make' creates.
# Not strictly required.
.PHONY: clean
clean:
-rm -f first $(OBJ)
You can't just copy and paste the above; you must also change both indented lines (the ones with shell commands on them) so that the indentation is a single hard TAB character. If you don't do this you will get a cryptic error message:
Makefile:5: *** missing separator. Stop.
first.c:
#include <stdio.h>
#include "second.h"
int main(void){
printf("%d\n", addone(5));
return 0;
}
second.c
#include <stdio.h>
#include "second.h"
int addone(int a){
return ++a;
}
second.h
#include <stdio.h>
int addone(int a);