Pointers and arrays · Programming in C

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Pointers and arrays

Follow pointers by hand, predict sizeof values, find classic bugs, then write code with addresses - minmax, strings, matrices. Chapter 2.

Unless stated otherwise, we assume a 64-bit PC running Linux: an int is 4 bytes, a pointer 8. Programs are compiled with gcc -std=c11 -Wall -Wextra -Werror.

Exercise · Follow the arrows

What does this program print? Draw the variables and the pointers after each line.

#include <stdio.h>

int main(void)
{
    int a = 1, b = 2;
    int *p = &a, *q = &b;

    *p = *q + 1;
    p = q;
    *p = 10;
    *q = *q + a;
    printf("a = %d, b = %d, *p = %d\n", a, b, *p);
    return 0;
}

Exercise · How many bytes?

What does this program print?

#include <stdio.h>
#include <string.h>

int main(void)
{
    char s[] = "abc";
    char *p = s;
    int t[5];
    char m[10] = "abc";

    printf("%zu %zu %zu %zu %zu %zu %zu\n", sizeof s, sizeof p, strlen(s),
           sizeof t, sizeof t / sizeof t[0], sizeof m, strlen(m));
    return 0;
}

We then add this function, called with f(t):

void f(int u[])
{
    printf("%zu\n", sizeof u / sizeof u[0]);
}

What would it print? Why does gcc -Wall -Werror refuse to compile it?

Exercise · Three bugs

Each of these programs is wrong. Say why, what gcc -Wall -Wextra says about it, and fix it.

First program:

#include <stdio.h>

int main(void)
{
    int n;
    if (scanf("%d", n) != 1)
        return 1;
    printf("%d\n", n);
    return 0;
}

Second program:

int main(void)
{
    int *p;
    *p = 3;
    return 0;
}

Third program:

#include <stdio.h>

int somme(int t[])
{
    int s = 0;
    for (size_t i = 0; i < sizeof t / sizeof t[0]; i++)
        s += t[i];
    return s;
}

int main(void)
{
    int t[4] = {1, 2, 3, 4};
    printf("%d\n", somme(t));
    return 0;
}

Exercise · Two results

A C function returns only one value. Write

void minmax(const int t[], size_t n, int *min, int *max);

which stores in *min and *max the smallest and the largest element of the array t of n elements (n ≥ 1). Call it from main on {3, -1, 4, 1, -5, 9, 2, 6}.