CODEWITHSUNDEEP
cfor-loopstructprintfmalloc
federico rovere
federico rovere

Reputation: 153

Unexpected result in C program, using Malloc and for loop

i have a trouble with my code in C:

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


int main()
{
    int i1,j1,k1;
    int n_block;
    struct  block
    {
        int i,j,k;
    } *blocks;
    n_block=3;
    i1=4;
    k1=3;
    j1=2;
    blocks=malloc(n_block*sizeof(blocks));
    for(int count=0;count<=n_block-1;count++){
        printf("count %d\n",count);
        blocks[count].i=i1;
        blocks[count].j=j1;
        blocks[count].k=k1;
        printf("results:%d\n",blocks[count].i);
        printf("results:%d\n",blocks[count].j);
        printf("results:%d\n",blocks[count].k);
    }
}

the expected output is:

count 0
results:4
results:2
results:3
count 1
results:4
results:2
results:3
count 2
results:4
results:2
results:3

but I obtain:

count 0
results:4
results:2
results:3
count 1
results:4
results:2
results:3
count 2
results:4
results:2
results:1970496882

I suppose that the last result is the value of the pointer, but why do it happen?

I tried to modify the for loop (without modify the n_block):

for(int count=0;count<=n_block+1;count++)

and I obtain:

count 0
results:4
results:2
results:3
count 1
results:4
results:2
results:3
count 2
results:4
results:2
results:1970496882
count 3
results:4
results:2612
results:10
count 4
results:4
results:2
results:3

so, the question is, why does it happen?

Thanks

Upvotes: 0

Views: 59

Answers (1)

asio_guy
asio_guy

Reputation: 3767

In case of malloc the statement sizeof(blocks) will evaluates to size of a pointer and in your case sizeof(blocks) != sizeof(struct block)

change the malloc to 

 blocks = malloc(n_block * (sizeof( struct block) ) ); // more readable

for loop is over complicated one here, stick to basics

for(int count=0; count < n_block; count++ )

Upvotes: 1

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