Compiling and running · Programming in C

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Compiling and running

Recognise which stage fails, read what the preprocessor does, split a program into modules properly, and write a Makefile. Chapter 1.

Unless stated otherwise, we compile with gcc -std=c11 -Wall -Wextra -Werror.

Exercise · Who is complaining?

Each of these programs refuses to become an executable. For each one, say at which stage the error is detected (preprocessor, compilation, linking), and why.

First program:

#include <stdio.h>
#include "absent.h"

int main(void)
{
    return 0;
}

Second program:

#include <stdio.h>

int main(void)
{
    int x = 3
    printf("%d\n", x);
    return 0;
}

Third program:

#include <stdio.h>

int carre(int x);

int main(void)
{
    printf("%d\n", carre(3));
    return 0;
}

Fourth program:

#include <stdio.h>

int main(void)
{
    int x;
    printf("%d\n", x);
    return 0;
}

Exercise · The preprocessor only does text

#define CARRE(x) x * x
#define N 1 + 2

int main(void)
{
    int a = CARRE(N);
    int b = CARRE(3) / CARRE(3);
    return a + b;
}

Write what the preprocessor produces for the two declarations (you can check with gcc -E -P).
What are the values of a and b? And the program's exit code (echo $? after running it)?
Fix both macros to get what was expected: a is 9 and b is 1.

Exercise · A definition in a header

A student wrote this module, and includes it in main.c and in un.c:

/* outils.h */
#ifndef OUTILS_H
#define OUTILS_H

int compteur = 0;

int double_de(int x)
{
    return 2 * x;
}

#endif

Each file compiles (gcc -c) without error, but linking fails:

multiple definition of `compteur'; un.o: first defined here
multiple definition of `double_de'; un.o: first defined here

The include guard was supposed to prevent double definitions. Why is it not enough?
Reorganise the module into outils.h and outils.c. compteur must remain a variable shared by the whole program, which double_de increments on every call.

Exercise · No guard

/* point.h */
struct point {
    int x, y;
};

/* segment.h */
#include "point.h"

struct segment {
    struct point a, b;
};

/* dessin.c */
#include "point.h"
#include "segment.h"

int main(void)
{
    struct segment s = {{0, 0}, {1, 1}};
    return s.b.x;
}

gcc -c dessin.c fails. What message should you expect, and why?
Fix it without touching dessin.c.