c++读取文件栈溢出，麻烦各位帮我看看什么原因

#include <fstream>
#include <iostream>

using namespace std;

void enCode(char *buf, int size) {
    for (int index = 0; index <= size; ++index)
        ++(*buf++);
}

void dnCode(char *buf, int size) {
    for (int index = 0; index <= size; ++index)
        --(*buf++);
}

int main(int argc, char **argv)
{
    if (argc != 3|| strcmp(argv[1] ,"-e") && strcmp(argv[1] ,"-d") )
    {
        cout << "program name : -e|-d :filename" << endl;
        return 1;
    }

    fstream fin(argv[2], ios::binary | ios::in | ios::out);
   
    if (!fin) {
        cout << "读取文件错误"  << endl;
        return 2;
    }

    char buf[100];

    int pos1 = 0, pos2 = 0 ,bytes;
    void(*p)(char *, int) = argv[1][1] == 'e' ? enCode : dnCode;
    bool ok = true;
    while (ok) {
        fin.seekg(pos1);
        fin.read(buf, sizeof(buf));
        if (!fin) {
            fin.clear();
            ok = false;
        }
        else
            pos2 = fin.tellg();

        bytes = fin.gcount();
        p(buf, bytes);

        fin.seekp(pos1);
        fin.write(buf, fin.gcount());
        
        pos1 = pos2;

        
    }
   
    return 0;
}