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I have two programs. The following code is an example I came up with to understand the basics before implementing the method into my main program. The child process is not editable and is an executable (as I do not have access to the source code for my main program). The code for the child process code for my example:

#include <iostream>
#include <string>

using namespace std;

bool is_number(const std::string& s)
{
    string::const_iterator it = s.begin();
    while (it != s.end() && std::isdigit(*it)) ++it;
    return !s.empty() && it == s.end();
}

int main() {
    cout << "Enter some positive numbers" << endl;
    string testInput = "";

    while(true) {
        cin >> testInput;
        if(is_number(testInput)) {
            testInput += " is a positive number";
            cout << testInput << endl;
        }
        else {
            cout << "invalid" << endl;
        }
    }

    return EXIT_SUCCESS;                              //never exits
}

The code for the parent function:

#include <cstdio>
#include <iostream>
#include <memory>
#include <stdexcept>
#include <string>
#include <cstring>
#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!");
    }
    for (int returnNum = 0; returnNum < 5; returnNum++) {
        if(fgets(buffer.data(), buffer.size(), pipe.get()) == nullptr)
            break;
        result += buffer.data();
    }
    return result;
}

int main() {
    std::cout << "Result: " << exec(".\\child.exe") << "." << std::endl;

    system("PAUSE");
    return EXIT_SUCCESS;
}

The parent function code was adapted from an answer given to How do I execute a command and get the output of the command within C++ using POSIX?. My understanding is the parent function opens the executable and allows me to send commands via the parent functions command line (not really sure how these are passed to the child process but it does work). If the child function was not in the infinite while loop, the result would be printed to the parent terminal.

Note that I will always need to call the child function a known number of times (hence the for loop). I also don't need this code to be perfect as it will just be me using the program.

Why is the result never returned even after 5 commands? How do I get the result to return? How do I send commands in the code of my parent program instead of typing them into the terminal of the parent function?

Reuben Walker
  • 11
  • 2
  • I do not know _popen well, but it seems you have opened a pipe that gets the output of the child process and forwards it to the parent process. If you want to pass arguments to the child process from the parent process, use _popen(cmd, "w") instead. (You may be able to keep your existing pipe if you want the result of the child process, but as I said, I do not know _popen very well.) – Thomas Bécavin Oct 18 '19 at 14:58
  • I strongly suspect that you are confused by terminology. The parent's **command line** would look like `Parent.EXE arg1 arg2 arg3`. Those command-line arguments would be passed via `int main(int argc, char**argv)`. That is **not** how you send commands. Instead, I suspect you send commands via standard in. – MSalters Oct 18 '19 at 15:00

0 Answers0