How can I handle packed data from Perl/PHP in C++? - php

Question

I got a problem implementing a PHP programm in C++. It is about the PHP/Perl function unpack. I don't know how to do the follwing in C++ (no problem in reading a file... but how do i unpack("C*") the read contents).
<?php
$file = fopen("bitmaskt.dat", "rb");
//create the data stream
$matrix_x = unpack("C*", fread($file, 286));
$matrix_y = unpack("C*", fread($file, 286));
$mask_data = unpack("C*", fread($file, 286));
$reed_ecc_codewords = ord(fread($file, 1));
$reed_blockorder = unpack("C*", fread($file, 128));
fclose($file);
?>
Currently, I'm very hopeless solving this problem on my own - I'm searching for days, all I found are questions... Is there any free unpack() c++ implementation out there? :-(

Answer 1

Perl's documentation for pack covers the templates used for pack and unpack.
Say you generated bitmaskt.dat with
#! /usr/bin/perl
use warnings;
use strict;
open my $fh, ">", "bitmaskt.dat" or die "$0: open: $!";
my #data = (42) x 286;
print $fh pack("C*" => #data);
print $fh pack("C*" => #data);
print $fh pack("C*" => #data);
print $fh pack("C" => 7);
print $fh pack("C*" => (1) x 128);
close $fh or warn "$0: close";
You might read it with
#include <fstream>
#include <iostream>
#include <iterator>
#include <vector>
typedef unsigned char datum_t;
typedef std::vector<datum_t> buf_t;
std::istream &read_data(std::istream &in, buf_t &buf, size_t n)
{
std::istreambuf_iterator<char> it(in.rdbuf()), eos;
while (it != eos && n-- != 0)
buf.push_back(static_cast<datum_t>(*it++));
return in;
}
For example:
int main()
{
std::ifstream bm("bitmaskt.dat", std::ifstream::binary | std::ifstream::in);
struct {
buf_t buf;
size_t len;
std::string name;
} sections[] = {
{ buf_t(), 286, "matrix_x" },
{ buf_t(), 286, "matrix_y" },
{ buf_t(), 286, "mask_data" },
{ buf_t(), 1, "reed_ecc_codewords" },
{ buf_t(), 128, "reed_blockorder" },
};
const int n = sizeof(sections) / sizeof(sections[0]);
for (int i = 0; n - i > 0; i++) {
if (!read_data(bm, sections[i].buf, sections[i].len)) {
std::cerr << "Read " << sections[i].name << " failed" << std::endl;
return 1;
}
}
const int codeword = 3;
std::cout << (unsigned int) sections[codeword].buf[0] << '\n';
return 0;
}
Output:
7

Answer 2

I don't know about any general implementation of unpack for C++, but that doesn't seem to be the thing you need anyway.
if matrix_x is defined somewhere as unsigned char matrix_x[286] and you have an opened input stream inFile
then what you need to do is inFile.get(matrix_x, 286). This reads 286 bytes from the input and places them in the array pointed to by matrix_x.