c++processposixsystemreturn-value

How do I execute a command and get the output of the command within C++ using POSIX?


I am looking for a way to get the output of a command when it is run from within a C++ program. I have looked at using the system() function, but that will just execute a command. Here's an example of what I'm looking for:

std::string result = system("./some_command");

I need to run an arbitrary command and get its output. I've looked at boost.org, but I have not found anything that will give me what I need.


Solution

  • #include <cstdio>
    #include <iostream>
    #include <memory>
    #include <stdexcept>
    #include <string>
    #include <array>
    
    std::string exec(const char* cmd) {
        std::array<char, 128> buffer;
        std::string result;
        std::unique_ptr<FILE, decltype(&pclose)> pipe(popen(cmd, "r"), pclose);
        if (!pipe) {
            throw std::runtime_error("popen() failed!");
        }
        while (fgets(buffer.data(), static_cast<int>(buffer.size()), pipe.get()) != nullptr) {
            result += buffer.data();
        }
        return result;
    }
    

    Pre-C++11 version:

    #include <iostream>
    #include <stdexcept>
    #include <stdio.h>
    #include <string>
    
    std::string exec(const char* cmd) {
        char buffer[128];
        std::string result = "";
        FILE* pipe = popen(cmd, "r");
        if (!pipe) throw std::runtime_error("popen() failed!");
        try {
            while (fgets(buffer, sizeof buffer, pipe) != NULL) {
                result += buffer;
            }
        } catch (...) {
            pclose(pipe);
            throw;
        }
        pclose(pipe);
        return result;
    }
    

    Replace popen and pclose with _popen and _pclose for Windows.


    2024 Edit:

    With newer versions of gnu g++ such as the one in Ubuntu 24.04, the code above results in an error because the std::unique_ptr deleter is ignoring the return value from pclose().

    error: ignoring attributes on template argument ‘int (*)(FILE*)’ [-Werror=ignored-attributes]
    

    Had to modify the code to wrap pclose(). Used a lambda for the wrapper. Now looks like this:

    std::unique_ptr<FILE, void(*)(FILE*)> pipe(popen(cmd.c_str(), "r"),
        [](FILE * f) -> void
        {
            // wrapper to ignore the return value from pclose() is needed with newer versions of gnu g++
            std::ignore = pclose(f);
        });