Demonstrate C string creation & mutability

stringplay.c

/**
 * Demonstrate types of strings & mutability & storage
 *
 * Joel Burton <joel@joelburton.com>
 **/

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

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

    // Strings as arrays of literals

    char hi1[] = "Hello";
    char hi2[] = "Hello";

    // Strings as arrays of char -- this is just an alternate way to specify
    //   the above strings (they work and act exactly the same)

    char hi3[] = {'H', 'e', 'l', 'l', 'o', '\0'};
    char hi4[] = {'H', 'e', 'l', 'l', 'o', '\0'};

    // Strings as pointers to char literals

    char *hi5 = "Hello";     // could write as:  const char *hi3 = "Hello";
    char *hi6 = "Hello";     //   (since it is constant anyway)

    // Strings as pointers to char stored in heap & dynamically assigned

    char *hi7 = malloc(6);   // same as:  char *hi5;  hi5 = malloc(6)
    char *hi8 = malloc(6);   //   (that may be easier to understand)
    strcpy(hi7, "Hello");
    strcpy(hi8, "Hello");

    // strdup calls malloc and returns a pointer to allocated memory, so
    // we can do the same thing as hi7 and hi8 like this:

    char *hi9 = strdup("Hello");  // same as:  char *hi0;  hi9 = strdup("Hello")
    char *hia = strdup("Hello");  //   (also may be easier to understand)


    // Mutate our strings by changing first character

    hi1[0] = '1';
    hi2[0] = '2';

    hi3[0] = '3';
    hi4[0] = '4';

//  hi5[0] = '5';     // ERROR: can't mutate  (but ok:  hi5 = "New String")
//  hi6[0] = '6';     // ERROR: can't mutate

    hi7[0] = '7';
    hi8[0] = '8';

    hi9[0] = '9';
    hia[0] = 'a';


    // Print results (string @ variable location = location of first char)

    printf("\nArray (stored in stack)\n");
    printf("hi1:  %s  @ %p = %p\n", hi1, &hi1, (void *) hi1);
    printf("hi2:  %s  @ %p = %p\n", hi2, &hi2, (void *) hi2);
    printf("hi3:  %s  @ %p = %p\n", hi3, &hi3, (void *) hi3);
    printf("hi4:  %s  @ %p = %p\n", hi4, &hi4, (void *) hi4);

    printf("\nPointers to literal (pointer in stack points to literal)\n");
    printf("hi5:  %s  @ %p = %p\n", hi5, &hi5, (void *) hi5);
    printf("hi6:  %s  @ %p = %p\n", hi6, &hi6, (void *) hi6);

    printf("\nPointers to heap, strcpy (pointer in stack to heap)\n");
    printf("hi7:  %s  @ %p = %p\n", hi7, &hi7, (void *) hi7);
    printf("hi8:  %s  @ %p = %p\n", hi8, &hi8, (void *) hi8);

    printf("\nPointers to heap, strdup (pointer in stack to heap)\n");
    printf("hi9:  %s  @ %p = %p\n", hi9, &hi9, (void *) hi9);
    printf("hia:  %s  @ %p = %p\n", hia, &hia, (void *) hia);


    // A good scout is tidy
    
    free(hi7);
    free(hi8);
    free(hi9);
    free(hia);
}