A toy that stores some global variables by writing the binary itself

README.md

Global Variable Storage

Background

VBA could store some data by simply putting those data into Excel cells or Powerpoint TextBoxes, which is kinda counterintuitive for a programming language to have persist value storage even after stopped running, though perfectly reasonable. Could we do similar thing in the magical C++? This heavily platform dependent toy (ELF + POSIX) achieves this.

Working principle

It simply utilizes the .data segment by either specifying some variables to be put there, or just utilize the fact that initialized values are put there. Some magical operations used:

• The variables __data_start and edata provided by linker scripts
• Read output of awk with popen

License

MIT License

toy.cpp

#include <cstdio>
#include <string>
#include <vector>
#include <utility>
#include <iostream>
#include <tuple>
#include <fstream>
// #define DEBUGOUT
#define DATASTART __data_start
#define DATAEND edata
#define NOINITSTORE __attribute__((section(".data")))
extern char DATASTART;
extern char DATAEND;
static std::vector<std::tuple<char* /*ptr*/,size_t /*size*/,char* /*fileOffset*/>> preserveList;
NOINITSTORE int foo = 0, arr[10];
int bar = 456, foobar; char* baseAddr; char* startAddr = &DATASTART;char *endAddr = &DATAEND;
static std::string exeRelPath;
template<class T>
void preserveVal(T& val) {
	char* addr = (char*)&val;
	if(addr >= startAddr && addr < endAddr)
		preserveList.push_back(std::make_tuple(addr, sizeof(val), addr - startAddr + baseAddr));
}
void listPreserve() {
#ifdef DEBUGOUT
	for(auto x : preserveList)
		printf("%x %d %x\n", std::get<0>(x), std::get<1>(x), std::get<2>(x));
#endif
}
void writePreserve(const char* fileName) {
	std::fstream fs;
	fs.open(fileName, std::ios::in | std::ios::out | std::ios::binary);
	for(auto item : preserveList) {
		fs.seekp((long)std::get<2>(item));
		fs.write(std::get<0>(item),std::get<1>(item));
	}
	fs.close();
}
void initPreserve(char** &argv) {
	exeRelPath = (std::string)(argv[0]);
#ifdef DEBUGOUT
	printf("%s\n",argv[0]);
	printf("%s\n", ("objdump -h " + exeRelPath + "| grep \"\\.data\" | awk '{print $6}'").c_str());
#endif
	FILE* f = popen(("objdump -h " + exeRelPath + " | grep \"\\.data\" | awk '{print $6}'").c_str(),"r");
	fscanf(f, "%x", &baseAddr);
	pclose(f);
#ifdef DEBUGOUT
	printf("%x\n", baseAddr);
#endif
}
void finalizePreserve() {
	system(("cp " + exeRelPath + " " + exeRelPath + ".tmp").c_str());
	writePreserve((exeRelPath+".tmp").c_str());
	system(("rm -rf " + exeRelPath).c_str());
	system(("cp " + exeRelPath + ".tmp" + " " + exeRelPath).c_str());
#ifdef DEBUGOUT
	printf("%x %x %x %x\n",&DATASTART,&foo,&bar,&foobar);
#endif
}
int main(int argc, char* argv[]) {
	printf("%d %d %d\n",foo,bar, arr[3]);
	initPreserve(argv);
	preserveVal(foo); preserveVal(bar); preserveVal(arr);
	listPreserve();
	foo++;bar+=2;
	arr[3]+=5;
	finalizePreserve();
	return 0;
}