C++ :: ReadFile Function Cannot Read Entire Executable File

I am writing a program which compresses files into .zip files.

Here's my problem: Whenever I want to compress an executable file, my readFile function does not read the entire file. When I extract the .exe I get a very tiny and incomplete file.

Here's the function I use to read files:

std::string miniz_wrapper::readFile(FILE* f, int MAX_FILEBUFFER)
//MAX_FILEBUFFER has a default value of 65536 {
char* tmp;
std::string tmp_s;
int count = 0;

[Code] .....

Prior to reading, every file is opened using fopen with the mode "rb".