An example of using malloc

This is an example C program demonstrating the use of malloc.

#include <stdio.h>

/* "malloc" is defined in <stdlib.h>. */

#include <stdlib.h>

/* This gives us "isprint". */

#include <ctype.h>

/* Safely show what is in memory. If the character is unprintable,
   print it like {AB}, where AB is the value in hexadecimal. */

static void hex_dump (char * r, int rl)
{
    int i;
    for (i = 0; i < rl; i++) {
        if (isprint (r[i])) {
            printf ("%c", r[i]);
        }
        else {
            printf ("{%X}", (unsigned char) r[i]);
        }
    }
    printf ("\n");
}

/* Fill "r" with random letters. */

static void randomly_fill (char * r, int rl)
{
    int i;

    for (i = 0; i < rl - 1; i++) {
        r[i] = (random () % 26) + 'A';
    }
    
    /* Put a zero byte at the end. */
    
    r[rl - 1] = '\0';
}

int main ()
{
    int j;
    for (j = 0; j < 4; j++) {

        /* Allocate here. */
        char * r;
        /* r's length. */
        int rl;

        /* Set "r" to 0 initially. */

        r = 0;

        /* We want to test with various lengths of memory. */

        rl = (j + 1) * 10;

        printf ("** Get %d bytes with malloc:\n", rl);

        /* Call malloc and get "rl" bytes of memory. */

        r = malloc (rl);

        /* Test whether the call to malloc was successful. */

        if (! r) {

            /* Print an error message to the standard error stream,
               "stderr". */

            fprintf (stderr, "The call to malloc was not successful.\n");

            /* EXIT_FAILURE and "exit" are defined in <stdlib.h>. The
               following tells the calling process that we hit a
               snag. */

            exit (EXIT_FAILURE);
        }

        /* Inspect the memory we have got. */

        printf ("Initially, the memory contains: ");
        hex_dump (r, rl);

        randomly_fill (r, rl);

        printf ("Filled up, the memory contains: ");
        hex_dump (r, rl);

        /* Thanks for the memory. */

        free (r);
    }
    return 0;
}

(download)

The output of the example looks like this:

** Get 10 bytes with malloc:
Initially, the memory contains: {0}{0}{0}{0}{0}{0}{0}{0}{0}{0}
Filled up, the memory contains: NWLRBBMQB{0}
** Get 20 bytes with malloc:
Initially, the memory contains: {FF}{FF}{FF}{FF}{FF}{FF}{FF}{FF}{FF}{FF}{FF}{1F}{FF}{0}{0}{0}NWLR
Filled up, the memory contains: HCDARZOWKKYHIDDQSCD{0}
** Get 30 bytes with malloc:
Initially, the memory contains: HCDARZOWKKYHIDDQSCD{0}BBMQB{0}{0}{0}{0}{0}
Filled up, the memory contains: XRJMOWFRXSJYBLDBEFSARCBYNECDY{0}
** Get 40 bytes with malloc:
Initially, the memory contains: {0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}{0}
Filled up, the memory contains: GGXXPKLORELLNMPAPQFWKHOPKMCOQHNWNKUEWHS{0}

The memory may contain anything initially. For example, the memory may be recycled from a previous call, or it may contain all zeros. To guarantee that the memory is set to zeros initially, use calloc rather than malloc.