CODEWITHSUNDEEP
cstructmemory-managementmallocrealloc
av1028
av1028

Reputation: 1

How do I reallocate a array of structures in a function

I am trying to allocate a dynamic array of Country objects for my school project. I have malloc'd the array in main() function and I am reallocating it in a add_country() function. but it seems to give me realloc invalid ponter error. Could someone help? This is the minimal reproducable code.

    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    int count = 0;
    typedef struct test_Country
    {
    char name[20];
    int gold;
    int silver;
    int bronze;
    } test_Country;

    test_Country *addtest_Country(test_Country test_Country_obj, test_Country*array)
    {
    int flag = 0;
    printf("%s\n", "before realloc");
    test_Country *new_array;
    new_array = realloc(array, sizeof(test_Country *) * (count + 1));
    printf("%s\n", "after realloc");
    //array[count].name = (char *)malloc(strlen(test_Country_obj.name) + 1);
    if (count == 0)
    {
        strcpy(new_array[0].name, test_Country_obj.name);
    }
    else
    {

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

            if (strcasecmp(new_array[i].name, test_Country_obj.name) == 0)
            {
                printf("%s", "test_Country already added\n");
                flag = 1;
                break;
            }
        }
    }
    if (flag == 0)
    {
        strcpy(new_array[count].name, test_Country_obj.name);
        test_Country_obj.gold = 0;
        test_Country_obj.silver = 0;
        test_Country_obj.bronze = 0;
        new_array[count] = test_Country_obj;
        count = count + 1;
    }
    flag = 0;
    return new_array;
}

    int main()
    {
    char choice;
    test_Country *array = malloc(sizeof(test_Country));
    test_Country test_Country_obj;
    printf("%s", "Enter your choice : ");
    scanf("%s", &choice);
    //fgets(ptr, 80, stdin);
    //sscanf(ptr, "%c %s %d %d %d", &choice, test_Country_obj.name, &test_Country_obj.gold, &test_Country_obj.silver, &test_Country_obj.bronze);
    //printf("%s", &choice);
    while (choice != 'E')
    {

        printf("%s", "Enter test_Country name : ");
        scanf("%s", test_Country_obj.name);
        array = addtest_Country(test_Country_obj, array);
        //printf("%d%s", count, "is count");
        printf("%s", "Enter your choice : ");
        scanf("%s", &choice);
    }
    }

I cant seem to understand what is wrong.

Upvotes: 0

Views: 76

Answers (1)

MikeCAT
MikeCAT

Reputation: 75062

char choice;
scanf("%s", &choice);

is bad. choice has only room for one character, so it can hold only strings upto zero characters. (the one-character room is for terminating null-character). Trying to store strings longer than zero character leads to dangerous out-of-range write and it may destroy data around that.

To avoid out-of-range write, you should allocate enough elements and specify the maximum length to read. The maximum length should be the buffer size minus one for terminating null-character.

char choice[16]; /* allocate enough elements */
scanf("%15s", choice); /* specify the maximum length */

After that, choice in the while and switch should be replaced with choice[0] to judge by the first character. Another way is using strcmp() to check the whole string.

Upvotes: 2

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