Allocating dynamic memory using malloc in C

Question

I'm new to C and have been trying to tackle this question. It's a continuation of the last thread I made. I made some progress but still have so much to learn and fix.

In short:

In this question a "vector" is a one dimensional array of integers. Therefore an array of vectors would be a two dimensional array that holds one dimensional arrays inside him.

I need to use these variables:

int** vectors- the 2D array
int size -an integer that represents how many vectors exist inside **vectors
int* sizes-a 1D array of integers that represents the length of the vectors

I need to write the following functions:

int init(int ***vectors, int **sizes, int size)
the function allocated memory to **vectors and *sizes with size and initializes vectors to be full of NULLs,and sizes to be full of zeros.
int set(int **vectors, int *sizes, int index, int *tmp, int tmp_size)
the function receives an array of nulls (**vectors)), frees the vector inside **vectors whose index is index and allocates memory for a new vector, whose length is tmp_size and places inside it *tmp's elements.

This is my code:

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
int init(int*** vectors, int** sizes, int size)
{
    int i, k,j;
    *sizes = (int*)malloc(size * sizeof(int));
    if (*sizes == NULL)
        return 0;
    for (j = 0; j < size; j++)
    {
        (*sizes)[j] = 0;
    }

    *vectors = (int**)malloc(size * sizeof(int*));
    if (*vectors == NULL)
        return 0;
    for (i = 0; i < size; i++)
    {
        (vectors)[i] = NULL;
    }
    return 1;
}
int set(int **vectors, int *sizes, int index, int *tmp, int tmp_size)
{
    if ((vectors)[index] != NULL)
    {
        free((vectors)[index]);
    }
    (vectors)[index] = (int*)malloc(tmp_size * sizeof(int));
    if ((vectors)[index] == NULL)
        return 0;
    for (int b = 0; b < tmp_size; b++)
    {
        (vectors)[index][b] = tmp[b];
    }
    sizes[index] = tmp_size;
    return 1;
}
int main()
{
    int size, i, length, indexhere;
    int** vectors = NULL;
    int* sizes = NULL;
    int* tmp = NULL;
    int* p = &vectors;
    int tempindex;
    printf("\nPlease enter an amount of vectors:\n");
    scanf("%d", &size);
    init(p, &sizes, size);

        printf("Enter index\n");
        scanf("%d", &indexhere);
        printf("Enter Length\n");
        scanf("%d", &length);
        tmp = (int*)malloc(length * sizeof(int));
            printf("Enter elements:\n");
            for (int g = 0; g < length; g++)
                scanf("%d", &tmp[g]);
            set(&vectors, sizes, indexhere, tmp, length);

    system("pause");
    return 0;
}

Could someone explain please why the program always crashes?

This is the perfect time to learn how to debug your programs. More specifically, if you run your program in a debugger it will catch the crash, and allow you to walk up and down the function call stack, so you can locate exactly where it happens in your code. You can also examine variables and their values to make sure they are okay. — Some programmer dude
– Some programmer dude, Commented May 6, 2018 at 20:03
If, after examining in a debugger, you still can't figure it out, then you need to create a Minimal, Complete, and Verifiable Example and show us, and tell us where in your code the crash happens. — Some programmer dude
– Some programmer dude, Commented May 6, 2018 at 20:04
So you think the problem is in main() but do not show the code of that? Please make a minimal reproducible example. — Yunnosch
– Yunnosch, Commented May 6, 2018 at 20:12
Don't the compiler shout warnings at you? The variable p is not of the correct type. And you pass the wrong type as first argument to set as well (which is the probable cause of your crash). Listen to your compiler. — Some programmer dude
– Some programmer dude, Commented May 6, 2018 at 20:31
Then you should enable more warnings. The compiler is definitely able to detect such things. — Some programmer dude
– Some programmer dude, Commented May 6, 2018 at 20:43

r3mus n0x · Accepted Answer · 2018-05-06 20:59:50Z

2

In init function (vectors)[i] = NULL; should actually be (*vectors)[i] = NULL;
When calling set function from main you should pass vectors instead of &vectors.

There also seems to be several pointer type mismatches in your code, so you should really pay attention to compiler's warnings. This is because C unfortunately allows implicit conversions between incompatible pointers, unlike C++ for example.

edited May 6, 2018 at 20:59

answered May 6, 2018 at 20:54

r3mus n0x

6,1841 gold badge17 silver badges37 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Ultraviolence Over a year ago

thank you,it solved some of my problems, thank you so much for taking the time in reading my wall text and responding.

Some programmer dude · Accepted Answer · 2018-05-06 20:44:26Z

1

You call set like this

set(&vectors, sizes, indexhere, tmp, length);

but the first argument is declared as an int **. By passing &vector you're passing a pointer to vector, i.e. something of type int ***. This mismatch will lead to undefined behavior and probable crashes.

answered May 6, 2018 at 20:44

Some programmer dude

411k36 gold badges420 silver badges655 bronze badges

1 Comment

Ultraviolence Over a year ago

thanks for taking the time in explaining what was wrong,it helped me fix some of the problems.

малин чекуров · Accepted Answer · 2018-05-06 21:48:56Z

1

Here is a complete working example.

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

void destroyVectors(int **vectors, int size)
{
for (int i = 0; i < size; i++)
{
    free(vectors[i]);
}
}

int init(int*** vectors, int** sizes, int size)
{
int i, j;
*sizes = (int*)malloc(size * sizeof(int));
if (*sizes == NULL)
    return 0;
for (j = 0; j < size; j++)
{
    (*sizes)[j] = 0;
}

*vectors = (int**)malloc(size * sizeof(int*));
if (*vectors == NULL)
    return 0;
for (i = 0; i < size; i++)
{
    (*vectors)[i] = NULL;
}
return 1;
}
int set(int **vectors, int *sizes, int index, int *tmp, int tmp_size)
{
if ((vectors)[index] != NULL)
{
    free((vectors)[index]);
}
(vectors)[index] = (int*)malloc(tmp_size * sizeof(int));
if ((vectors)[index] == NULL)
    return 0;
for (int b = 0; b < tmp_size; b++)
{
    (vectors)[index][b] = tmp[b];
}
sizes[index] = tmp_size;
return 1;
}

int main()
{
int size = 0, length = 0, indexhere = 0;
int** vectors = NULL;
int* sizes = NULL;
int* tmp = NULL;

printf("\nPlease enter an amount of vectors:\n");
scanf("%d", &size);
init(&vectors, &sizes, size);

printf("Enter index\n");
scanf("%d", &indexhere);

printf("Enter Length\n");
scanf("%d", &length);

tmp = (int*)malloc(length * sizeof(int));
printf("Enter elements:\n");
for (int g = 0; g < length; g++)
    scanf("%d", &tmp[g]);

set(vectors, sizes, indexhere, tmp, length);

for(int i = 0; i < length; ++i)
    printf("byte: %d\n", vectors[indexhere][i]);

printf("sizes index: %d\n", sizes[indexhere]);

free(tmp);
free(sizes);
destroyVectors(vectors, size);

return 0;
}

answered May 6, 2018 at 21:48

малин чекуров

5715 silver badges6 bronze badges

1 Comment

Ultraviolence Over a year ago

thank you for the time taken into fixing and replying,much appreciated.i managed to fix my program with everyone's suggestions and even the other functions work,but more ways of writing the same thing are always appreciated and welcomed.

Collectives™ on Stack Overflow

Allocating dynamic memory using malloc in C

3 Answers 3

1 Comment

1 Comment

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related