Question

I’ve written a sample program about structs. I wrote 3 source file and a 1 header file. This is the complete source of the program:

main.c:

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

/*
struct fish
{
    const char *name;
    const char *species;
    int teeth;
    int age;
};
*/

/*
void catalog(struct fish f)
{
    printf("%s is a %s with %i teeth. He is %i.",
           f.name, f.species, f.teeth, f.age);
}


void label(struct fish f)
{
    printf("Name: %s\n", f.name);
    printf("Species: %s\n", f.species);
    printf("Teeth: %i\n", f.teeth);
    printf("Age: %i\n", f.age);
}
*/

int main()
{
    struct fish snappy = {"Snappy", "Piranha", 69, 4};
    catalog(snappy);
    label(snappy);

    return 0;
}

label.c:

#include "fish.h"

void label(struct fish f)
{
    printf("Name: %s\n", f.name);
    printf("Species: %s\n", f.species);
    printf("Teeth: %i\n", f.teeth);
    printf("Age: %i\n", f.age);
}

catalog.c:

#include "fish.h"

void catalog(struct fish f)
{
    printf("%s is a %s with %i teeth. He is %i.",
           f.name, f.species, f.teeth, f.age);
}

I also wrote a makefile:

fish.o: main.c label.c catalog.c fish.h
    gcc -c main.c label.c catalog.c
fish: fish.o
    gcc fish.o -o fish

I compile the program in cmd:

make fish

It says:

gcc -c main.c label.c catalog.c
label.c: In function 'label':
label.c:5:5: warning: incompatible implicit declaration of built-in function 'printf'    [enabled by default]
catalog.c: In function 'catalog':
catalog.c:5:5: warning: incompatible implicit declaration of built-in function 'printf' [enabled by default]
cc   fish.o   -o fish
cc.exe: error: fish.o: No such file or directory
cc.exe: fatal error: no input files
compilation terminated.
make: *** [fish] Error 1

If I remove the comments in the main.c and compile only it, It works, But when I broke into small pieces as mentioned above it doesn’t work.

What’s the wrong?

Answer 1

This linking rule:

fish: fish.o
    gcc fish.o -o fish

is appropriate for building a program fish from a single object file fish.o (which is presumably compiled from fish.c).

You need a different rule:

OBJECTS = main.o label.o catalog.o

fish: ${OBJECTS}
    ${CC} -o $@ ${OBJECTS}

You can add ${CFLAGS} and ${LDFLAGS} and ${LDLIBS} (or ${LDLIBES}) to the link line as appropriate if you want to, but for simple programs, what I showed suffices.

You don’t have to tell make how to convert main.c into main.o; it knows that already. It will use a rule such as:

%.o: %.c
    ${CC} -c ${CFLAGS} $<

to create the .o file from the matching .c file. (This is the GNU Make notation; POSIX make and other classic versions of make use an alternative but loosely equivalent notation .c.o: in place of %.o: %.c.)

If you don’t mind compiling all the sources each time, you can use a similar rule:

SOURCES = main.c label.c catalog.c

fish: ${OBJECTS}
    ${CC} ${CFLAGS} -o $@ ${SOURCES}

This time, ${CFLAGS} is practically mandatory (though you’d just about get away without it in your bare-bones sample program).

---

Note that make is fussy; the command lines such as the ${CC} lines must start with a TAB; blanks will not do.