How to properly access this realloc-ed array?-CodePudding

In this code below I am trying to create an array of ints that can be accessed from the main() function, however, Address-sanitizer gives me stack-buffer-overflow-error and I cannot figure out what I am doing wrong. What am I missing?

#include <stdlib.h>

void reallocFail(int **arrayOfInts) {
    *arrayOfInts = (int *)malloc(sizeof(int));
    for (int i = 1; i <= 10; i  ) {
        *arrayOfInts = (int *)realloc(*arrayOfInts, (i) * sizeof(int));
        *arrayOfInts[i - 1] = i;
    }
}

int main(void) {
    int *arrayOfInts;
    reallocFail(&arrayOfInts);
    return 0;
}

CodePudding user response：

For starters this memory allocation before the for loop

*arrayOfInts = (int*)malloc(sizeof(int));

is redundant. You could just write

*arrayOfInts = NULL;

Also you need to check whether memory allocation was successful.

Also this record

    *arrayOfInts[i-1] = i;

is equivalent to

    *( arrayOfInts[i-1] ) = i;

but you need

    ( *arrayOfInts )[i-1] = i;

The function can look the following way

size_t reallocFail( int **arrayOfInts, size_t n )
{
    *arrayOfInts = NULL;
    size_t i = 0;

    if ( n != 0 )
    {
        int *tmp = NULL;

        do
        {
            tmp = realloc( *arrayOfInts, ( i   1 ) * sizeof( int ) );

            if ( tmp != NULL )
            {
                tmp[i]  = i   1;
                *arrayOfInts = tmp;
            }
        } while ( tmp != NULL &&   i != n );
    }

    return i;
}

And the function can be called for example like

int *arrayOfInts = NULL;

size_t n = reallocFail( &arrayOfInts, 10 );

for ( size_t i = 0; i != n; i   )
{
    printf( "%d ", arrayOfInts[i] );
}

putchar( '\n' );

free( arrayOfInts );

And ignore the down-voting of the answer. There is nothing wrong in the answer.:)

Here is a demonstration program.

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

size_t reallocFail( int **arrayOfInts, size_t n )
{
    *arrayOfInts = NULL;
    size_t i = 0;

    if ( n != 0 )
    {
        int *tmp = NULL;

        do
        {
            tmp = realloc( *arrayOfInts, ( i   1 ) * sizeof( int ) );

            if ( tmp != NULL )
            {
                tmp[i]  = i   1;
                *arrayOfInts = tmp;
            }
        } while ( tmp != NULL &&   i != n );
    }

    return i;
}

int main( void ) 
{
    int *arrayOfInts = NULL;

    size_t n = reallocFail( &arrayOfInts, 10 );

    for ( size_t i = 0; i != n; i   )
    {
        printf( "%d ", arrayOfInts[i] );
    }

    putchar( '\n' );

    free( arrayOfInts );

    return 0;
}

The program output is

1 2 3 4 5 6 7 8 9 10

Of course there is no great sense to reallocate the memory in the loop within the function. The function just demonstrates how to manage the function realloc.

CodePudding user response：

void reallocFail(int **arrayOfInts) 
{
    for (int i = 1; i <= 10; i  ) 
    {
        *arrayOfInts = realloc(*arrayOfInts, i * sizeof(**arrayOfInts));
        arrayOfInts[0][i - 1] = i;
    }
}

int main(void) {
    int *arrayOfInts = NULL;
    reallocFail(&arrayOfInts);
    return 0;
}

But this code can be reduced to one realloc (I understand that you test if realloc works). IT does not check if the realloc has succeeded or failed:

void reallocFail(int **arrayOfInts) 
{
    int *tmp;
    for (int i = 1; i <= 10; i  ) 
    {
        tmp = realloc(*arrayOfInts, i * sizeof(**arrayOfInts));
        if(tmp)
        {
            *arrayOfInts = tmp;
            arrayOfInts[0][i - 1] = i;
        }
        else
            printf("Alloction error\n");
    }
}

CodePudding user response：

There is just a simple typo in *arrayOfInts[i - 1] = i;. suffix operators such as [] bind stronger than prefix operators such as *. Hence you should write:

   (*arrayOfInts)[i - 1] = i;

Note also that you should check for memory reallocation failure and you can initialize *arrayOfInts to NULL as realloc(NULL, size) is equivalent to malloc(size).

Here is a modified version:

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

int reallocFail(int **pp, int n) {
    int i;
    *pp = NULL;
    for (i = 0; i < n; i  ) {
        int *p = realloc(*pp, (i   1) * sizeof(*p));
        if (p == NULL)
            break;
        p[i] = i   1;
        *pp = p;
    }
    return i;
}

int main(void) {
    int *arrayOfInts = NULL;
    int n = reallocFail(&arrayOfInts, 10);
    for (int i = 0; i < n; i  ) {
        printf("%d%c", arrayOfInts[i], " \n"[i == n-1]);
    }
    free(arrayOfInt);
    return 0;
}