#include <stdio.h>

int readfile_n(FILE *file) {
    char a = 0;
    int n = 0;
    do
    {
        fscanf_s(file, "%c", &a);
        n++;
    } while (a != "0");
    return n;
}

char* fcounter(FILE* file) {
    int* counter = (int*)malloc(readfile_n(file) * sizeof(char));
    for (int i = 0; i < readfile_n(file); i++) {
        fscanf_s(file, "%c", &counter[i]);
    }
    return counter;
}

typedef struct symbols_s {
    int start;
    int finish;
}symbols;

int start(symbols s) {
    printf("From wich symbol: ");
    scanf_s("%d", s.start);
    return s.start;
}

int finish(symbols s) {
    printf("To wich symbol: ");
    scanf_s("%d", s.finish);
    return s.finish;
}

 int lenght(symbols s, FILE* file) {
     int* counter = fcounter(file);
     if (finish(s) - start(s) + 1 > readfile_n(file) )
     {
         printf("Error");
     }
     int len = 0;
     for (int i = start(s); i != finish(s); i++)
     {
         len = i;
     }
     return len;
}

 int main() {
     FILE *file;
     fopen_s(&file, "C:\\Users\\refle\\Desktop\\info.txt", "r");
     symbols s={};
     lenght(s, file);
     free(fcounter(file));
     fclose(file);
     getch();
     return 0;
 }