When debugging, I found out that part of the pointer I need is erased and this error is caused. But I don't understand why the "i" or "j" declaration does this...

My code

#include <stdio.h>
#include <stdbool.h>
#include <malloc.h>

typedef unsigned long bitset_index_t;

typedef struct
{
    bitset_index_t index;
    unsigned short *vyraz;

} bitset_t;

void bitset_alloc(bitset_t **pole, unsigned int velikost)
{
    if (velikost < 1)
    {
        // todo error
    }
    bitset_t new_bitset = {.index = 0, .vyraz = NULL};
    new_bitset.vyraz = malloc(sizeof(unsigned short) * velikost);
    if (new_bitset.vyraz == NULL)
    {
        // todo error
    }
    memset(new_bitset.vyraz, 0, sizeof(unsigned short) * (velikost));

    for (unsigned int i = 0; i < velikost;   i)
    {

        new_bitset.vyraz[i] = 0;
        new_bitset.index  ;
    }
    *pole = &new_bitset;
}

void Eratosthenes(bitset_t *pole)
{
    pole->vyraz[0] = 1;
    pole->vyraz[0] = 1;

    unsigned long velikost = pole->index;
    for (unsigned long i = 2; i < velikost; i  )
    {
        for (unsigned long j = i; j * i < velikost   1; j  )
        {
            if (pole->vyraz[i])
                break;
            pole->vyraz[i * j] = 1;
        }
    }
    unsigned short count_prvocisel = 0;
    for (unsigned long i = pole->index; i > 0; i--)
    {
        if (count_prvocisel < 10)
        {
            if (pole->vyraz[i])
                continue;
            printf("%lu ", i);
            count_prvocisel  ;
        }
        else
            break;
    }
}

int main()
{
    bitset_t *test = NULL;

    bitset_alloc(&test, 10);

    Eratosthenes(test);
}

this is the data before the first cycle enter image description here

this is at the time of the error enter image description here

CodePudding user response：

You have 'returned' the address of a local variable

void bitset_alloc(bitset_t** pole, unsigned int velikost)
{
    if (velikost < 1)
    {
        // todo error
    }
    bitset_t new_bitset = { .index = 0, .vyraz = NULL }; <<<<========
    new_bitset.vyraz = malloc(sizeof(unsigned short) * velikost);
    if (new_bitset.vyraz == NULL)
    {
        // todo error
    }
    memset(new_bitset.vyraz, 0, sizeof(unsigned short) * (velikost));

    for (unsigned int i = 0; i < velikost;   i)
    {

        new_bitset.vyraz[i] = 0;
        new_bitset.index  ;
    }
    *pole = &new_bitset; <<<<<=========

}

'new_bitset' is a local variable that will be released once this function exits, saving its value - effectively returning it - is invalid and results in UB.

Either malloc it too or create it in 'main'