c++ - What does this cast from long in a union to char* and back do? -

i examining code of simple executable packer writes section executable unpacks during startup, when stumbled upon piece of code:

void setdistance( unsigned long size ) {     char* set = (((char *)i)+punpacker->virtualaddress);     union     {         short sh[2];         long  l;     } conv;     conv.l      = size;     conv.sh[0]  = 0;      unpacker_set(set, (char *)(&conv.l), 4, text_distance); } 

size distance unpacker code in memory beginning of section supposed unpacked. in loader code defined unsigned long. unpacker_set on other hand has code:

void inline unpacker_set( char* at, char* what, size_t size, unsigned long sig ) {     dword oldprotect;     unsigned char *set  = (unsigned char *)at;      while(*((unsigned long*)(set)) != sig)         set++;      if(virtualprotect(set, size, page_readwrite, &oldprotect) == true)         for(unsigned i=0; i<size; i++)             *(set+i) = *(what+i); } 

although understand second routine replaces value unpacker code, know why hassle union done. appreciated.

probably best way understand code write minimal test case , see does:

#include <iostream>  void f() {   union    {     short sh[2];     long l ;   } conv ;    conv.l = 100000000 ;     std::cout << std::hex << conv.l << std::endl ;    conv.sh[0] = 0 ;    std::cout << std::hex << conv.l << std::endl ; }  int main() {   f() ; } 

the output see follows:

5f5e100 5f50000 

so code intention looks trying mask out higher order bits of size, although ugly , unlikely portable.

as david pointed out should aware of strict aliasing. article type-punning , strict-aliasing better since has solid examples of real world issues using unions. in order assure code works expected assuming gcc or clang need pass in following command line argument -fno-strict-aliasing.