How to allocate memory to a string array in C - malloc error

Question

Problem: I want to allocate memory to the elements of a fixed sized string array, however, I experience a crash 75% of the time.

That is 25% of the time my program runs flawlessly, but the error is one I have not experienced before

#define PACKAGE_COUNT 60
#define MAX_COUNT     5

const char *colors[] = { "blue", "red", "yellow", "purple", "orange" };

char **generatePackage() {


  char **generatedPackage = malloc(PACKAGE_COUNT);
  int randomIndex         = 0;

  for (int i = 0; i <= PACKAGE_COUNT; ++i) {

    randomIndex         = rand() / (RAND_MAX / MAX_COUNT + 1);
    generatedPackage[i] = malloc(sizeof(char) * sizeof(colors[randomIndex])); 
    // ERROR

    strcpy((generatedPackage[i]), colors[randomIndex]);

    // printf("generatePackage - %d: %s \n", i + 1, generatedPackage[i]);
  }

  return generatedPackage;
}

Your for loop counts the array entries from 1 to PACKAGE_COUNT but they are indexed between 0 and (PACKAGE_COUNT-1) as is the case with all arrays in C. Changing ++i (which adds one to i and then uses the result) to (i++) which uses your result then increments i will resolve this. — Jon Guiton
– Jon Guiton, Commented May 12, 2019 at 19:22
malloc(sizeof(char) * sizeof(colors[randomIndex])) should be malloc(strlen(colors[randomIndex]) + 1). Additionally, you need to allocate space for the pointers, as @xing points out — torstenvl
– torstenvl, Commented May 12, 2019 at 19:22
Please don't post images of text — post the text. That's unreadable on a mobile device (actually, it's also unreadable on a full-size browser on a Mac, too). — Jonathan Leffler
– Jonathan Leffler, Commented May 12, 2019 at 20:00
@JonathanLeffler noted, the images were there to assist to show the location of the error on the IDE. — lifewithelliott
– lifewithelliott, Commented May 12, 2019 at 20:39

AdminXVII · Accepted Answer · 2019-05-12 19:48:42Z

You did not take into account the fact that the size of a pointer is not one. Hence, only a fraction of the size was allocated. Thus, some pointers had not enough memory to be allocated, causing the program to crash only when accessing one of those.

#define PACKAGE_COUNT 60
#define MAX_COUNT     5

const char *colors[] = { "blue", "red", "yellow", "purple", "orange" };

char **generatePackage() {
  char **generatedPackage = malloc(PACKAGE_COUNT * sizeof(char*)); // The size of a pointer is not one, so this a multiplicative factor must be applied
  int randomIndex         = 0;

  for (int i = 0; i < PACKAGE_COUNT; ++i) { // the upper bound is i < PACKAGE_COUNT, not i <= PACKAGE_COUNT

    randomIndex         = rand() / (RAND_MAX / MAX_COUNT + 1);
    generatedPackage[i] = malloc(sizeof(char) * (strlen(colors[randomIndex]) + 1)); // You probably want to allocate enough space for the string, rather than enough space for the pointer, so you must use strlen

    strcpy((generatedPackage[i]), colors[randomIndex]);

    // printf("generatePackage - %d: %s \n", i + 1, generatedPackage[i]);
  }

  return generatedPackage;
}

Collectives™ on Stack Overflow

How to allocate memory to a string array in C - malloc error

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related