Linux Make & Makefile

about me

EE degree back in 1982

Z80 was the most popular CPU

Pascal/Fortran/COBOL were popular languages

Apple ][ + BASIC and CP/M

intel 80386SX PC mother board designer

......

Interested in Linux since 2016

Z80 CPU

Apple ][

marconi.jiang@gmail.com

References

- 撰寫Makefile教學

- C example code from Computer Systems - A Programmer's Perspective

WHAT ARE MAKE & MAKEFILE

Make & Makefile

make (Reminder: Linux is case sensitive. "Make" will be treated as Command not found.)

- A compiler tool embedded in Linux Shell (GNU project)

- Usually stored in /usr/bin/

- To prepare to use make, user must create a file called "makefile" that describes the relationships among files in your program and provides commands for updating each file.

makefile

- A user created file for make to follow compiler instructions

- Makefile is saved in the same directory of the source files you intend to compiler. Users might create one or many Makefile's for different projects

- By default, when make looks for the makefile, it tries the following names, in order: GNUmakefile, makefile and Makefile.

- You can also specify your own makefile by -f parameter

$ make
$ make -f MyMakefile

Example of C Code

- Let me use the following C code (show_byes.c) as an example to demonstrate the use of makefile.

    Example : show_byes.c accept your input in 'int' (decimal) format, and display in hex format in int, (type cast to) float, and physical address

    Usage: ./show_byes int_number

#include <stdio.h>
#include <unistd.h>

typedef unsigned char *byte_pointer;

void show_bytes(byte_pointer start, int len) {
    int i;
    for (i = 0; i < len; i++)
        printf(" %.2x", start[i]);
    printf("\n");
}

void show_int(int x) {
    show_bytes((byte_pointer) &x, sizeof(int));
}

void show_float(float x) {
    show_bytes((byte_pointer) &x, sizeof(float));
}

void show_pointer(void *x) {
    show_bytes((byte_pointer) &x, sizeof(void *));
}

int main(int argc, char *argv[]) {

    int ival = atoi(argv[1]);

    float fval = (float) ival;
    int *pval = &ival;

    show_int(ival);
    show_float(fval);
    show_pointer(pval);

    return 0;
}

Using gcc w/o makefile

$ gcc -Wall -o main show_bytes.c
$ ./main  65537
 01 00 01 00
 80 00 80 47
 2c ca 54 51 ff 7f 00 00
$

* the 3rd lines will be different each time you run the program, as the program address changes

It is simple and straight forward....

... for a single program,

but maybe not for complex programs when you don't want to waste time to re-compile all source codes if you only modify few of the programs

make + makefile is smart enough to decide what (modified) needs re-compile and what not (unchanged)

C Example for makefile

#include <stdio.h>
#include <unistd.h>

extern void show_int(int x);

extern void show_float(float x);

extern void show_pointer(void *x);

int main(int argc, char *argv[]) {

    int ival = atoi(argv[1]);

    float fval = (float) ival;
    int *pval = &ival;

    show_int(ival);
    show_float(fval);
    show_pointer(pval);

    return 0;
}

#include <stdio.h>

typedef unsigned char *byte_pointer;

void show_bytes(byte_pointer start, int len) {
    int i;
    for (i = 0; i < len; i++)
        printf(" %.2x", start[i]);
    printf("\n");
}

void show_int(int x) {
    show_bytes((byte_pointer) &x, sizeof(int));
}

void show_float(float x) {
    show_bytes((byte_pointer) &x, sizeof(float));
}

void show_pointer(void *x) {
    show_bytes((byte_pointer) &x, sizeof(void *));
}

Makefile Template

#compiler 
CC = gcc 
#cflags 
CFLAGS = -Wall -ansi -g 
#source 
SRCS = main.c show_bytes.c 
#object 
OBJS = $(SRCS:.c=.o) 

all: main

main: $(OBJS)
	$(CC) $(OBJS) -o $@ 

main.o: main.c

show_bytes.o: show_bytes.c

clean: 
	-rm -f *.o

Using gcc w/ makefile

$ make
gcc -Wall -ansi -g -c -o show_bytes.o show_bytes.c
gcc main.o show_bytes.o -o main 
$ ./main  65537
 01 00 01 00
 80 00 80 47
 2c 1a 86 5e ff 7f 00 00
$

* the 3rd lines will be different each time you run the program, as the program address changes

Using 'make' is more efficient when you get familiar with it