Unexpected result in C program, using Malloc and for loop

Question

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

sizeof(blocks) is the size of a pointer. You sure that's what you wanted? Doesn't look like it. Try blocks=malloc(n_block * sizeof *blocks); ? — WhozCraig
– WhozCraig, Commented Oct 10, 2018 at 10:09
not a problem, but for(int count=0;count<=n_block-1;count++) is usually written as for (int count = 0; count < n_block; count++) — pmg
– pmg, Commented Oct 10, 2018 at 10:13
Your code as is gives some strange output. A version with the correct sizeof gives the expected output. — pmg
– pmg, Commented Oct 10, 2018 at 10:19
@pmg yes, i think that the problem was on the sizeof(block) as suggested in the answer by u_ — federico rovere
– federico rovere, Commented Oct 10, 2018 at 10:30

asio_guy · Accepted Answer · 2018-10-10 10:16:50Z

1

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++ )

answered Oct 10, 2018 at 10:16

asio_guy

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2 Comments

federico rovere Over a year ago

Ok, now it works. Why sizeof(blocks) != sizeof(struct block) ?

Osiris Over a year ago

sizeof *blocks would maybe be better, in case the type of blocks changes.

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