CODEWITHSUNDEEP
cvalgrind
Daniel
Daniel

Reputation: 2974

Invalid read of size 4

I have the following program:

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

typedef struct a{
    int a;
} Player;

typedef struct b{
    Player players[5];
}* Formation;

int main()
{
    Player a; a.a = 1;
    Player b; b.a = 2;
    Player c; c.a = 3;
    Player d; d.a = 4;
    Player e; e.a = 5;

    Formation team = malloc(sizeof(*team));
    team->players[0] = a;
    team->players[1] = b;
    team->players[2] = c;
    team->players[3] = d;
    team->players[4] = e;

    for (int i = 0; i < 5; i++){
        team->players[i] = team->players[i + 1];
    }

    Player empty;
    team->players[4] = empty;
    for (int i = 0; i < 4; i++){
        printf("\n%d\n", team->players[i].a);
    }
}

In essence, I create five different players, each with a different a value, then put them inside a dynamically allocated Formation's players array. I then remove the first player by shifting all the values in the array left and placing an empty player in the last element of the array. When I run, it prints (as expected) 2345.

But when I run valgrind on the program, I get:

==25919== Invalid read of size 4
==25919==    at 0x4005DF: main (in /u1/023/sdkl1456/mtm/ex1/test/test)
==25919==  Address 0x4c22054 is 0 bytes after a block of size 20 alloc'd
==25919==    at 0x4A069EE: malloc (vg_replace_malloc.c:270)
==25919==    by 0x400589: main (in /u1/023/sdkl1456/mtm/ex1/test/test)
==25919==

So apparently my method of removing the first player is incorrect. How can I remove the first player without memory issues?

Upvotes: 1

Views: 1568

Answers (1)

ouah
ouah

Reputation: 145829

for (int i = 0; i < 5; i++){
    team->players[i] = team->players[i + 1];
}

You are reading one past the last element as your array has only 5 elements.

Upvotes: 6

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