Why does the array keep reverting to 0

Question

I have a 1 dimensional array in which ive initialized as 0 but for some reason when i go inside a loop and try to increase its contents by one the value at position 0 keeps reverting to 0 even after i increase it by 1.

#include <stdio.h>
#include <stdlib.h>
#define TOTAL_V 3
#define NUM_CANDIDATES 7

int hex_age(unsigned short hex){
    unsigned short age = hex >> 9;
    if (age >18 && age <101)
        return age;
    else return 0;
}
int hex_gender(unsigned short hex){
    unsigned short gender = hex >> 7 & 3;
    return gender;
}
int hex_vote(unsigned short hex){
    unsigned short vote, tmp = hex & 0x7f , count = 0;
    if (tmp == 0)
        return 7;
    for (int i = 0 ; i<7; i++){
        if (tmp & 1 == 1){
            count++;
            vote = i;
        }
        tmp = tmp >> 1;
    }
    if (count > 1)
        return 7;
    return vote;
}

    
int main() {
    int s_votes = 0, f_votes = 0, v_count[NUM_CANDIDATES] = {0};
    unsigned short **v_info, hex_v_info , age , gender , vote;

    FILE *fp;

    fp = fopen("data1.dat" , "r");
    if (fp == NULL){
        fprintf(stderr ,"apotuxe o anoigmos tou arxeiou");
        exit(-1);
    }

    if (feof(fp)){
        fprintf(stderr, "to arxeio einai adeio");
        exit(-1);
    }


    
    while (fscanf(fp ,"%x", &hex_v_info) != EOF){
        age = hex_age(hex_v_info);
        if(age == 0)
            f_votes++;
        else {
            gender = hex_gender(hex_v_info);
            if (gender == 0)
                f_votes++;
            else{
                vote = hex_vote(hex_v_info);
                if (vote == 7)
                    f_votes++;
                else{
                    if (s_votes == 0){
                            v_info = malloc(sizeof(int *));
                            v_info[s_votes] =malloc(sizeof(int)* TOTAL_V);
                    }
                    else{
                        v_info = realloc(v_info , sizeof(int *)*(s_votes+1));
                        v_info[s_votes] = malloc(sizeof(int)*TOTAL_V);
                    }

                    v_info[s_votes][0] = age;
                    v_info[s_votes][1] = gender;
                    v_info[s_votes][2] = vote;
                    v_count[vote]++;
                    s_votes++;
                    }
                }
            }
    }
    fclose(fp);
    for (int i = 0; i<s_votes; i++)
        free(v_info);
    return 0;
}

and for some reason when i use calloc to create the array it doesnt have that problem. Does anyone know why that happens

Answer 1

1. You declare unsigned short hex_v_info (2 bytes on my system), then read data from with fscanf(fp ,"%x", &hex_v_info) where the format string %x expect the address of an int (4 bytes on my system). This will certainly overwrite data unexpectedly.

2. unsigned short **v_info but you store an array of int [s_votes]. If your pointers are not uniform this will be a problem.

3. realloc(NULL, 1) is well defined so just use instead of malloc() of the first element. You need to assign the result of realloc()` to e temporary variable, however, to be able handle NULL. Otherwise you lose data & leak memory.

4. free(v_info); results in a double free if s_votes > 1, and you still leak the memory you allocate at v_info[i].

#include <stdio.h>
#include <stdlib.h>
#define TOTAL_V 3
#define NUM_CANDIDATES 7

// 0x1111 1110 0000 0000
int hex_age(unsigned short hex){
    unsigned short age = hex >> 9;
    if (age >18 && age < 101)
        return age;
    return 0;
}

// 0x0000 0001 1000 0000
int hex_gender(unsigned short hex){
    unsigned short gender = hex >> 7 & 3; // bit 7 and 6
    return gender;
}

// 0x0000 0000 0111 1111
int hex_vote(unsigned short hex){
    unsigned short vote, tmp = hex & 0x7f , count = 0; // bit 11 through 0
    if (tmp == 0)
        return 7;
    for (int i = 0 ; i<7; i++){
        if ((tmp & 1) == 1){
            count++;
            vote = i;
        }
        tmp = tmp >> 1;
    }
    if (count > 1)
        return 7;
    return vote;
}

int main() {
    int s_votes = 0, f_votes = 0, v_count[NUM_CANDIDATES] = {0};
    unsigned short hex_v_info;
    int **v_info = NULL;

    FILE *fp = fopen("data1.dat" , "r");
    if (!fp){
        fprintf(stderr ,"apotuxe o anoigmos tou arxeiou");
        exit(-1);
    }
    for(;;) {
        int rv = fscanf(fp ,"%hx", &hex_v_info);
        if(rv == EOF) break;
        if(rv != 1) {
            printf("err\n");
            return 1;
        }
        unsigned short age = hex_age(hex_v_info);
        if(!age) {
            f_votes++;
            continue;
        }
        unsigned short gender = hex_gender(hex_v_info);
        if (!gender) {
            f_votes++;
            continue;
        }
        unsigned short vote = hex_vote(hex_v_info);
        if (vote == 7) {
            f_votes++;
            continue;
        }
        int **tmp = realloc(v_info, sizeof *tmp * (s_votes + 1));
        if(!tmp) {
            // handle error: free v_info[i] and v_info?
            return 1;
        }
        v_info = tmp;
        v_info[s_votes] = malloc(sizeof **v_info * TOTAL_V);
        v_info[s_votes][0] = age;
        v_info[s_votes][1] = gender;
        v_info[s_votes][2] = vote;
        v_count[vote]++;
        s_votes++;
    }
    fclose(fp);

    printf("f_votes: %d\n", f_votes);
    for(size_t i = 0; i < s_votes; i++) {
        printf("%zu: %d %d %d\n",
            i,
            v_info[i][0],
            v_info[i][1],
            v_info[i][2]
        );
    }
    for (int i = 0; i< s_votes; i++)
        free(v_info[i]);
    free(v_info);
    return 0;
}

and with input file file:

a081
a082

it appears to process the info data correctly:

f_votes: 0
0: 80 1 0
1: 80 1 1