Is it possible to write data into own stdin in Linux

Question

I want to debug my cgi script (C++) from IDE, so I would like to create a \"debug mode\": read file from disk, push it to own stdin, set some environment variables, that corresp

Answer 1

Everybody knows that standard input is a file descriptor defined as STDIN_FILENO. Though its value is not guaranteed to be 0, I never saw anything else. Anyway, there is nothing that prevents you from writing to that file descriptor. For the sake of example, here is a small program that write 10 messages to its own standard input:

#include <unistd.h>
#include <string>
#include <sstream>
#include <iostream>
#include <thread>

int main()
{
    std::thread mess_with_stdin([] () {
            for (int i = 0; i < 10; ++i) {
                std::stringstream msg;
                msg << "Self-message #" << i
                    << ": Hello! How do you like that!?\n";
                auto s = msg.str();
                write(STDIN_FILENO, s.c_str(), s.size());
                usleep(1000);
            }
        });

    std::string str;
    while (getline(std::cin, str))
        std::cout << "String: " << str << std::endl;

    mess_with_stdin.join();
}

Save that into test.cpp, compile and run:

$ g++ -std=c++0x -Wall -o test ./test.cpp -lpthread
$ ./test 
Self-message #0: Hello! How do you like that!?
Self-message #1: Hello! How do you like that!?
Self-message #2: Hello! How do you like that!?
Self-message #3: Hello! How do you like that!?
Self-message #4: Hello! How do you like that!?
Self-message #5: Hello! How do you like that!?
Self-message #6: Hello! How do you like that!?
Self-message #7: Hello! How do you like that!?
Self-message #8: Hello! How do you like that!?
Self-message #9: Hello! How do you like that!?
hello?
String: hello?
$ 

The "hello?" part is something that I typed after all 10 messages were sent. Then you press Ctrl+D to indicate end of input and program exits.

Answer 2

You can't "push to own stdin", but you can redirect a file to your own stdin.

freopen("myfile.txt","r",stdin);