How can I enter continuously by another C++ program in C system / popen command?

Question

I want to build a compile system in an online judge system.

Environment: Ubuntu 12.04 LTS, g++ version 4.9

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My workflow is "Compile cpp" -> "Execute it" -> "Record message".

But I got some problems when "the cpp file exist 'scanf' or 'cin' commands".

Because this is a auto-compile & run program, there is an other input need to load. (Is a string from function call not enter in terminal by myself)

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My problem

How can I run the executeCommand (below code in compiler.cpp), using the string input (below too) to enter for this program. If the executed program exist any scanf, cin or other commands.

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compiler.cpp

This is system command version, can replace to popen command too.

#include <string>
#include <cstdlib>
#include <fstream>
#include <iostream>

using namespace std;

int main(int argc, char ** argv) {
    // Compiler one cpp file.
    string compileCommand = "(g++ --std=c++11 ./main.cpp -o ./main.out) 2> main.err";
    system(compileCommand.c_str());

    // Execute this program.
    string executeCommand = "(time timeout -k1s 0.01s ./main.out) > result.txt 2> time.txt";
    system(executeCommand.c_str());

    // I want the above main.out will scanf from this string.
    string input = "Hello world, this is first line.\nThis is second line.";

    return 0;
}

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main.cpp

#include <stdlib.h>
#include <stdio.h>

int main () {
    char str[256];
    scanf("%s", str);
    printf("%s\n", str);
    return 0;
}

Answer 1

You probably need popen(3) (and you flagged your question as such).

FILE*pcmd = popen("time ./main.out", "w");
if (!pcmd) { perror("popen"); exit(EXIT_FAILURE); };
fprintf(pcmd, "Hello world, this is first line.\n");
fprintf(pcmd, "This is the second line.\n");
fflush(pcmd);
int bad = pclose(pcmd);
if (bad) {fprintf(stderr, "pclose failed %d\n", bad); }; 

Be aware of code injection issues, in particular when passing a computed command to popen or system

You might need some event loop around poll(2). Then use fork, execve, pipe and other syscalls(2) explicitly, so read Advanced Linux Programming

Answer 2

All you need is a pipe, system( "echo YOUR_STRING | ./main.out " )