Basic makefile program: in main Error undefined reference to "power" - c

Question

Header lab3p2.h
#ifndef LAB3P2_H
#define LAB3P2_H
long power(int integer1, int integer2);
#endif
Power Function: lab3p2f1.c
#include "lab3p2.h"
#include <stdio.h>
long power(int integer1, int integer2){
int i;
long ret =(long) integer1;
if(integer2 ==0)
{
ret = 1;
}else{
for( i =1 ; i < integer2; i++)
{
ret = ret * integer1;
}
}
return ret;
}
Main: lab3p2.c
#include <stdio.h>
#include "lab3p2.h"
/*Takes in integers from the command line and returns the power function result*/
int main(int argc, char **argv){
int a = atoi( *(argv+1));
int b = atoi( *(argv+2));
int c =atoi( *(argv+3));
long functionResult;
functionResult = power(b,c);
printf("The result of the power function is %ld \n", functionResult);
}
MakeFile: makefile
all: lab3p2
mkprog: lab3p2.o lab3p2f1.o
gcc lab3p2.o lab3p2f1.o -o lab3p2
lab3p2.o: lab3p2.c
gcc -ansi -pedantic -c lab3p2.c
lab3p2f1.o: lab3p2f1.c
gcc -ansi -pedantic -c lab3p2f1.c
clean:
rm -rf *.o lab3p2
Why can the main not access the function?
Is something wrong with how I am compiling?
Any help is greatly appreciated.

Answer 1

You're missing the rule for target lab3p2; i.e. mkprog should be lab3p2:
all: lab3p2
lab3p2: lab3p2.o lab3p2f1.o
gcc lab3p2.o lab3p2f1.o -o lab3p2
lab3p2.o: lab3p2.c
gcc -ansi -pedantic -c lab3p2.c
lab3p2f1.o: lab3p2f1.c
gcc -ansi -pedantic -c lab3p2f1.c
clean:
rm -rf *.o lab3p2
With your current Makefile, when you run make without arguments, you will get the following output:
% make
gcc -ansi -pedantic -c lab3p2.c
cc lab3p2.o -o lab3p2
lab3p2.o: In function `main':
lab3p2.c:(.text+0x6b): undefined reference to `power'
collect2: error: ld returned 1 exit status
<builtin>: recipe for target 'lab3p2' failed
make: *** [lab3p2] Error 1
What happens is that make tries to satisfy the first target (i.e. all); this then requires lab3p2. As make cannot find an explicit rule to build lab3p2 it then tries an implicit one - it knows that one can build foo by linking foo.o into a program; thus it runs the command
cc lab3p2.o -o lab3p2
However, this command doesn't link in the lab3p2f1.o where the definition for power resides.
These implicit rules can be quite handy with simple projects; for example for your project, the Makefile for GNU make could be simply written as
CC = gcc
CFLAGS = -ansi -pedantic
all: lab3p2
lab3p2: lab3p2.o lab3p2f1.o
clean:
rm -rf *.o lab3p2
and make would automatically figure out how to build the .o from corresponding .c and that it should use the compiler from the CC variable, and pass arguments from the CFLAGS.