How to navigate through a binary file? - c

Question

I have a text file written 10some information#11some more information#12and some more information#
10,11,12 are sentence id and should always be 2 bytes
I need to build a binary file from this text file, with the help of which I will then need to read sentences by id from a text file
In a binary file, each line should contain the following: sentence id, position of the beginning of the sentence, length of the string (sentence)
I managed to build a binary file, but I didn’t manage to read the information using it, the program shows that there are no such id, but I don’t understand why
#include <iostream>
#include <locale>
#include <string.h>
using namespace std;
int main()
{
FILE *poem, *index;
int N = 4096, len;
char *str= new char[N];
if (!(poem = fopen("poem.txt", "r")))
{
cout << "File not found";
return 0;
}
fgets(str, N, poem);
index = fopen("index.txt", "w+b");
int pos1 = 2, pos_b;
char* pos = str+pos1, *pstr=str;
fwrite(pstr, sizeof(char), 2, index);
fwrite(&pos1, sizeof(int), 1, index);
for (int i = 2; *pstr != '\0'; ++i, pstr++) {
if (*pstr == '#') {
len = pstr - pos;
fwrite(&len, sizeof(int), 1, index);
pos = pstr + pos1;
if (*(pstr+1)!='\0') {
fputs("\n", index);
fwrite(pstr+1, sizeof(char), 2, index);
pos_b = i + 3;
fwrite(&pos_b, sizeof(int), 1, index);
}
}
}
pos_b = 0;
len = 0;
fseek(index, 0, SEEK_SET);
fseek(poem, 0, SEEK_SET);
char* id = new char[3], * id_f = new char[3], * sent = new char[N];
int have_id = 0;
cout << "Enter the id of the offer you want to read" << endl;
cin >> id;
while (!feof(index)) {
fread(id_f, sizeof(char), 2, index);
if (strcmp(id_f,id)==0) {
fread(&pos_b, sizeof(int), 1, index);
fread(&len, sizeof(int), 1, index);
fseek(poem, pos_b, SEEK_SET);
fgets(sent, len + 1, poem);
cout << sent << endl;
have_id = 1;
}
else {
fread(id_f, sizeof(char), 3, index);
}
// fseek(index, 5, SEEK_CUR);
}
if (have_id == 0) {
cout << "There is no such id" << endl;
}
fclose(index);
fclose(poem);
delete[] str;
return 0;
}