Raeding and searching byte array from file

[makitos666]

Hi,

I'm writting a simple code to find in wich offset of a given file there is an array of bytes.
For example, searching 0x66 0x7C 0x1B 0xAA in a file. I wrote the next code, that it's working, but I want to know if this is the right way or if there is any other more eficient way to do that.

This is the function to read a file, always assuming you have enought memory to load it:

  char* read_block_2(std::uint32_t offset, std::uint32_t length, std::string const& filename)
{
    char* data = new char[length];
    std::ifstream is(filename, ios::in | ios::binary | ios::ate);
    if (is.is_open())
    {
        is.seekg(offset);
        is.read(data, (ifstream::pos_type)length);
        is.close();
    }
    else {
        cout << "error";
    }
    return data;
}

And this is my "searcher":

for (int i = offset; i < offset + lenght; i+=1) {
        if(a[i] == 0x66 && a[i + 1] == 0x7C && a[i + 2] == 0x1B && a[i + 3] == 0xAA) {
                 cout << "found at: ";
                 cout << i << "\n";
        }
}

And my other variant:

for (int i = offset; i < offset + lenght; i+=1) {
        unsigned long tag= a[i + 3] << 24 | a[i + 2] << 16 | a[i + 1] << 8 | a[i];
        if(tag == 2853928038){
                 cout << "found at: ";
                 cout << i << "\n";
        }
}

Thanks in advance!

[salem c]

1. Your search loop should stop at length-3, otherwise you're stepping off the end of your data.

2. Use memcmp() to simplify the code.

[makitos666]

Thanks for the first observation xD

This is you mean?

for (int i = offset; i < lenght - 3; i += 1) { 
        char find[] = "\x66\x7c\x1b\xaa";
        int flag = memcmp(&a[i], &find, 4);
        if (flag == 0) {
                cout << "found" << "\n";
        }
}

Thanks ;)

[salem c]

Yes, something like that.

But you can move the find array outside the loop, it never changes.