CODEWITHSUNDEEP
carraysstructmalloc
Yiannis
Yiannis

Reputation: 960

How to correctly allocate memory for an array of structs in C?

This is the first time I'm using structs as I am still relatively new to C. When I try to input values for 3 structs the program crashes, but it works fine for 1 and 2. I think I may not actually be allocating enough memory. Is this the correct way to allocate memory for an array of structs?

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

#define MIN_SIZE 0
#define MAX_SIZE 100
#define MAX_MONTH_STR 9

//Define a struct data type
typedef struct
{
    char* month;
    int day;
    int year;
}date;

//Method to allocate memory for a list of date structures
date* allocateStruct(int size)
{
    //Declaration of variables
    date *array;
    int i;

    //Allocate memory for rows (to store 'size' many pointers to 'date' struct data type)
    array = malloc(size*sizeof(date*));

    //For-loop to allocate memory for columns (to store 'size' many integers, and initialize them to zero)
    for (i=0; i<size; i++)
    {
        array[i].month = calloc(MAX_MONTH_STR,sizeof(char));
        array[i].day = calloc(1,sizeof(int));
        array[i].year = calloc(1,sizeof(int));
    }

    return array;
}

//Method to free memory allocated
//TO DO. . .

int main()
{
    //Declaration of variables
    int n;
    date* date_list;
    int i, j, k; //used in loops

    //Read input
    do
    {
        //printf("Enter number of dates you want to enter (between 1 and 10000):\n");
        scanf("%d", &n);
    }while(n<MIN_SIZE || n>MAX_SIZE);

    //ALLOCATE MEMORY
    date_list = allocateStruct(n);


    //For-loop to store values in 'date_list'
    for (i=0; i<n; i++)
    {
        //printf("Enter the date (month day year) in the following format: text number number");
        scanf("%s", date_list[i].month);
        scanf("%d", &date_list[i].day);
        scanf("%d", &date_list[i].year); //need & ?
    }

//--------> crashes here if 3 dates are input, ok for 1 and 2

    //Test print
    for (i=0; i<n; i++)
    {
        //printf("Enter the date (month day year) in the following format: text number number");
        printf("%s ", date_list[i].month);
        printf("%d ", date_list[i].day);
        printf("%d\n", date_list[i].year); //need & ?
    }

    return 0;
}

Upvotes: 6

Views: 18669

Answers (1)

user1350184
user1350184

Reputation:

You are trying to allocate a array with the size of the pointer to the date struct instead of the actual size of the date struct.

Change date* to date:

array = malloc(size*sizeof(date));

Furthermore you don't need to allocate the day and year variables, because the malloc allocates them for you.

for (i=0; i<size; i++)
{
     array[i].month = calloc(MAX_MONTH_STR,sizeof(char));
     array[i].day = 0;
     array[i].year = 0;
}

Upvotes: 7

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