c++ - Piping for input/output

Question

This question follows from my attempt to implement the instructions in:

Linux Pipes as Input and Output

How to send a simple string between two programs using pipes?

http://tldp.org/LDP/lpg/node11.html

My question is along the lines of the question in: Linux Pipes as Input and Output, but more specific.

Essentially, I am trying to replace:

/directory/program < input.txt > output.txt

using pipes in C++ in order to avoid using the hard drive. Here's my code:

//LET THE PLUMBING BEGIN 
int fd_p2c[2], fd_pFc[2], bytes_read;
    // "p2c" = pipe_to_child, "pFc" = pipe_from_child (see above link)
pid_t childpid;
char readbuffer[80];
string program_name;// <---- includes program name + full path
string gulp_command;// <---- includes my line-by-line stdin for program execution
string receive_output = "";

pipe(fd_p2c);//create pipe-to-child
pipe(fd_pFc);//create pipe-from-child
childpid = fork();//create fork

if (childpid < 0)
{
    cout << "Fork failed" << endl;
    exit(-1);
}
else if (childpid == 0)
{
    dup2(0,fd_p2c[0]);//close stdout & make read end of p2c into stdout
    close(fd_p2c[0]);//close read end of p2c
    close(fd_p2c[1]);//close write end of p2c
    dup2(1,fd_pFc[1]);//close stdin & make read end of pFc into stdin
    close(fd_pFc[1]);//close write end of pFc
    close(fd_pFc[0]);//close read end of pFc

    //Execute the required program
    execl(program_name.c_str(),program_name.c_str(),(char *) 0);
    exit(0);
}
else
{
    close(fd_p2c[0]);//close read end of p2c
    close(fd_pFc[1]);//close write end of pFc

    //"Loop" - send all data to child on write end of p2c
    write(fd_p2c[1], gulp_command.c_str(), (strlen(gulp_command.c_str())));
    close(fd_p2c[1]);//close write end of p2c

    //Loop - receive all data to child on read end of pFc
    while (1)
    {        
        bytes_read = read(fd_pFc[0], readbuffer, sizeof(readbuffer));

        if (bytes_read <= 0)//if nothing read from buffer...
            break;//...break loop

        receive_output += readbuffer;//append data to string
    }
    close(fd_pFc[0]);//close read end of pFc
}

I am absolutely sure that the above strings are initialized properly. However, two things happen that don't make sense to me:

(1) The program I am executing reports that the "input file is empty." Since I am not calling the program with "<" it should not be expecting an input file. Instead, it should be expecting keyboard input. Furthermore, it should be reading the text contained in "gulp_command."

(2) The program's report (provided via standard output) appears in the terminal. This is odd because the purpose of this piping is to transfer stdout to my string "receive_output." But since it is appearing on screen, that indicates to me that the information is not being passed correctly through the pipe to the variable. If I implement the following at the end of the if statement,

cout << receive_output << endl;

I get nothing, as though the string is empty. I appreciate any help you can give me!

EDIT: Clarification

My program currently communicates with another program using text files. My program writes a text file (e.g. input.txt), which is read by the external program. That program then produces output.txt, which is read by my program. So it's something like this:

my code -> input.txt -> program -> output.txt -> my code

Therefore, my code currently uses,

system("program < input.txt > output.txt");

I want to replace this process using pipes. I want to pass my input as standard input to the program, and have my code read the standard output from that program into a string.

Answer 1

Your primary problem is that you have the arguments to dup2() reversed. You need to use:

dup2(fd_p2c[0], 0);   // Duplicate read end of pipe to standard input
dup2(fd_pFc[1], 1);   // Duplicate write end of pipe to standard output

I got suckered into misreading what you wrote as OK until I put error checking on the set-up code and got unexpected values from the dup2() calls, which told me what the trouble was. When something goes wrong, insert the error checks you skimped on before.

You also did not ensure null termination of the data read from the child; this code does.

Working code (with diagnostics), using cat as the simplest possible 'other command':

#include <unistd.h>
#include <string>
#include <iostream>
using namespace std;

int main()
{
    int fd_p2c[2], fd_c2p[2], bytes_read;
    pid_t childpid;
    char readbuffer[80];
    string program_name = "/bin/cat";
    string gulp_command = "this is the command data sent to the child cat (kitten?)";
    string receive_output = "";

    if (pipe(fd_p2c) != 0 || pipe(fd_c2p) != 0)
    {
        cerr << "Failed to pipe
";
        exit(1);
    }
    childpid = fork();

    if (childpid < 0)
    {
        cout << "Fork failed" << endl;
        exit(-1);
    }
    else if (childpid == 0)
    {
        if (dup2(fd_p2c[0], 0) != 0 ||
            close(fd_p2c[0]) != 0 ||
            close(fd_p2c[1]) != 0)
        {
            cerr << "Child: failed to set up standard input
";
            exit(1);
        }
        if (dup2(fd_c2p[1], 1) != 1 ||
            close(fd_c2p[1]) != 0 ||
            close(fd_c2p[0]) != 0)
        {
            cerr << "Child: failed to set up standard output
";
            exit(1);
        }

        execl(program_name.c_str(), program_name.c_str(), (char *) 0);
        cerr << "Failed to execute " << program_name << endl;
        exit(1);
    }
    else
    {
        close(fd_p2c[0]);
        close(fd_c2p[1]);

        cout << "Writing to child: <<" << gulp_command << ">>" << endl;
        int nbytes = gulp_command.length();
        if (write(fd_p2c[1], gulp_command.c_str(), nbytes) != nbytes)
        {
            cerr << "Parent: short write to child
";
            exit(1);
        }
        close(fd_p2c[1]);

        while (1)
        {
            bytes_read = read(fd_c2p[0], readbuffer, sizeof(readbuffer)-1);

            if (bytes_read <= 0)
                break;

            readbuffer[bytes_read] = '';
            receive_output += readbuffer;
        }
        close(fd_c2p[0]);
        cout << "From child: <<" << receive_output << ">>" << endl;
    }
    return 0;
}

Sample output:

Writing to child: <<this is the command data sent to the child cat (kitten?)>>
From child: <<this is the command data sent to the child cat (kitten?)>>

---

Note that you will need to be careful to ensure you don't get deadlocked with your code. If you have a strictly synchronous protocol (so the parent writes a message and reads a response in lock-step), you should be fine, but if the parent is trying to write a message that's too big to fit in the pipe to the child while the child is trying to write a message that's too big to fit in the pipe back to the parent, then each will be blocked writing while waiting for the other to read.