Runtime code generation and execution in C++

Makefile

all: program

program: program.cpp
	g++ -std=c++14 -o program program.cpp -ldl -rdynamic

run: program query.cpp
	./program "`< query.cpp`"

.PHONY: clean
clean:
	rm -f program

program.cpp

#include <chrono>
#include <iostream>
#include <memory>

#include <dlfcn.h>
#include <fcntl.h>
#include <unistd.h>

#include <sys/types.h>
#include <sys/wait.h>

// Interface
struct Query {
    virtual void run() = 0;
};

std::unique_ptr<Query> query;

int main(int argc, char* argv[], char* envp[]) {
    if (argc != 2) {
        exit(EXIT_FAILURE);
    }

    auto start = std::chrono::high_resolution_clock::now();

    std::string code {argv[1]};

    char filename[] = "/tmp/queryXXXXXX.so";
    auto targetfile = mkostemps(filename, 3, O_RDWR | O_CREAT);    

    int pipe_fd[2];
    pipe(pipe_fd);
    
    auto pid = fork();
    
    if (pid) {
        close(pipe_fd[0]);

        write(pipe_fd[1], code.c_str(), code.size());
        close(pipe_fd[1]);
    
        auto wstatus = 0;
        
        do {
            waitpid(pid, &wstatus, 0);
        } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus));
        
        if (WIFSIGNALED(wstatus)) {
            std::cerr << "The compiler has died.\n";
            exit(EXIT_FAILURE);
        }
        
        if (WEXITSTATUS(wstatus) != 0) {
            std::cerr << "The code could not be compiled.\n";
            exit(EXIT_FAILURE);
        }
        
        auto handle = dlopen(filename, RTLD_NOW);
        
    	if (handle == nullptr) {
		    std::cerr << dlerror() << std::endl;
		    exit(EXIT_FAILURE);
    	}
        
        auto end = std::chrono::high_resolution_clock::now();
        std::cout << "Duration: " << std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count() << " ms.\n";
        
        query->run();
    }
    else {
        close(pipe_fd[1]);

        close(0);
        dup2(pipe_fd[0], 0);
    
        char* const nargv[] = {
            "/usr/bin/g++",
            "-shared", "-fPIC", "-pipe",
            "-o", filename,
            "-ldl", "-x", "c++", "-",
            nullptr
        };
        
        execve(nargv[0], nargv, envp);
        perror("execve");
        exit(EXIT_FAILURE);
    }
}

query.cpp

#include <iostream>
#include <memory>

// Interface
struct Query {
    virtual void run() = 0;
};

extern std::unique_ptr<Query> query;


struct MyQuery : Query {
    virtual void run() override {
        std::cout << "MyQuery->run()\n";
    }
};

// Automatically instantiate the query when the code is loaded.
[[gnu::constructor]]
void on_load() {
    query = std::make_unique<MyQuery>();
}