Wrong way to update binary file (don't do this at home)

wrongWay.c

#include <stdio.h>
#include <stddef.h>

typedef struct {
    char name[128];
    int max_hp;
    int current_hp;
} Charactor;

#define SAVE_FILENAME "gamedata.bin"

int main()
{
    FILE *fp = fopen(SAVE_FILENAME, "rwb");
    Charactor c;

    if (!fp) {
            perror(NULL);
            return 1;
    }

    while (fread(&c, sizeof(Charactor), 1, fp)) {
        c.max_hp = 9999;
        fseek(fp, -sizeof(Charactor), SEEK_CUR);
        fwrite(&c, sizeof(Charactor), 1, fp);
        fflush(fp);
    }

    fclose(fp);
    return 0;
}