I’m trying to build a program which needs 2 source codes.
Here is the simplified source codes and a makefile.
And I copied some ideas here to write this simple makefile.

But still don’t fully understand why I see this error.

[Source code 1 : main.c]

#include <stdio.h>
#include <stdlib.h>

#include "data.h"

int main(void)
{
   test_cfg.a1 = 1;
   printf("test_cfg.a1 = %dn", test_cfg.a1);

   func();
   return 0;
}

[Source code 2 : func.c]

#include <stdio.h>
#include <stdlib.h>

#include "data.h"

int func(void)
{
   test_cfg.b1 = 12;
   printf("test_cfg.b1 = %dn", test_cfg.b1);

   return 0;
}

[Header file : data.h]

#ifndef _DATA_H_
#define _DATA_H_

struct {
  int a1;
  int b1;
  int c1;
} test_cfg;

int func(void);
#endif /* end of _DATA_H_ */

[Makefile]

CC = gcc
AR = ar
ARFLAGS = rcs

CFLAGS += -Wall -std=c99

SOURCE=./main.c ./func.c
OBJECTS=$(SOURCE:.c=.o)

all: app_test

app_test:$(OBJECTS)
    $(CC) -o $@ $(OBJECTS)

.c.o:
    $(CC) $(CFLAGS) $<

.PHONY: all clean

clean:
    rm -f *.o app_test

[Output]

$ make
gcc -Wall -std=c99 main.c
/usr/bin/ld: /tmp/ccjahoYC.o: in function `main':
main.c:(.text+0x2f): undefined reference to `func'
collect2: error: ld returned 1 exit status
make: *** [Makefile:16: main.o] Error 1