C 中的动态 Memory 分配（返回值 3221225477 & 分段错误：11）

Question

Can anyone explain what is the wrong of the code below? İf I give the "line" variable as 1,2,3 or 4, it runs correctly. But after 4 it just shows a count of row correctly, the count of columns is not correct.

output of dev-c/c++ on windows >> "return value 3221225477"

output of gcc on mac >> "Segmentation fault: 11"

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

int main(){

int **matris;
int size;
int i, j;

printf( "Size >> " );
scanf( "%d", &size );

matris = (int **)malloc( size * sizeof(int) );

if( matris == NULL )
    printf( "It's required more memory'!" );

for( i = 0; i < size; i++ ) {
    matris[i] = malloc( size * sizeof(int) );
    if( matris[i] == NULL )
        printf( "It's required more memory'!!" );
}

for( i = 0; i < size; i++ ) {
    for( j = 0; j < size; j++ ){    
        matris[i][j]=i+1;
        printf( "%d ", matris[i][j] );
    }
    printf( "\n" );
}


for( i = 0; i < size; i++ ) {
    free( matris[i] );
}

free( matris );

return 0;
}

Answer 1

matris = (int **)malloc( size * sizeof(int) );

with especially ( size * sizeof(int) ) is wrong. You need to allocate memory for objects of type int* (pointer to int ), not int .

This is a problem if sizeof(int) < sizeof(int*) on your implementation -> Translated: The size of an object of type pointer to int is bigger than the size of an object of type int , which is quite common.

Then you get a segmentation fault, because you access memory beyond the bounds of the pointer array matris in the following code.

Use:

matris = malloc ( sizeof(int*) * size );

or even better

matris = malloc ( sizeof(*matris) * size );

---

Side Notes:

• You don't need to cast the return of malloc() . The returned pointer is automatically aligned. Do I cast the result of malloc?

• sizeof(*matris) requires less maintenance and is more safe if the type of matris and the pointed objects changes.

• Set the sizeof operation first to ensure size_t arithmetic, since size is of type int .

Answer 2

The line matris = (int **)malloc( size * sizeof(int) ); is incorrect as you want to create an array of int pointers and not of int. Changing the int to int* works.