In Makefile, how should I put header files in prerequisite list? (header file includes execution cod-CodePudding

Sometimes we have to deal with a header file which contains inline code snippets that use macros or variables defined in previous include files. I've met such a problem while trying to make the C object file dependent on the included header files. So I looked it up on stackoverflow and found this but the poster's question is unnecessarily complex so it seems he didn't get the intended answer. So I made a simple example here.

-- main.c

#include <stdio.h>
#define ADDVAL 32
int val;
void set_val(int);
int main()
{
set_val(10);
#include "print_val.h"
return 0;
}

-- set_val.c

extern int val;
void set_val(int a)
{
val  = a;
}

-- print_val.h

printf("added value = %d\n",val   ADDVAL);

Of course I can compile and run the program like this.

$cc -c main.c -o main.o
$cc -c set_val.c -o set_val.o
$cc main.o set_val.o -o main
$./main
added value = 42

Now using Makefile.. I made this Makefile.

csrcs := $(wildcard *.c)
cobjs := $(patsubst %.c, %.o, $(csrcs))
main: $(cobjs)
    cc $^ -o $@
clean:
    \rm -f *.o main

When I do make, it works just fine.

$make
cc    -c -o set_val.o set_val.c
cc    -c -o main.o main.c
cc set_val.o main.o -o main

But of course the problem is, when I modify print_val.h, it doesn't re-make main.
So I added the header file in the prerequisite list of target 'main' like below.

csrcs := $(wildcard *.c)
cobjs := $(patsubst %.c, %.o, $(csrcs))
main: $(cobjs) print_val.h
    cc $^ -o $@
clean:
    \rm -f *.o main

Now, after cleaning, if I do make, I see errors.

$ make
cc    -c -o set_val.o set_val.c
cc    -c -o main.o main.c
cc set_val.o main.o print_val.h -o main
print_val.h:1:8: error: expected declaration specifiers or ??...?? before string constant
    1 | printf("added value = %d\n",val   ADDVAL);
      |        ^~~~~~~~~~~~~~~~~~~~
print_val.h:1:29: error: unknown type name ??val??
    1 | printf("added value = %d\n",val   ADDVAL);
      |                             ^~~
make: *** [Makefile:4: main] Error 1

This header file contains execution line and this file cannot be compile by itself.
How should I handle this??

CodePudding user response：

It is the main.o file which depends on the header file, not the main executable program file.

So you need to add the dependency for the object file only:

main.o: print_val.h

Just add that line at the end of the of the makefile.