How to Execute a Child Process with Piped Stdin/Stdout in Linux?

Executing a Child Process with Piped Stdin/Stdout in Linux

In Linux, executing a child process with pipedstdin/stdout requires a combination of Linux syscalls or POSIX functions. To achieve this, we'll utilize the following techniques:

1. Pipe Creation (pipe syscall): Creates a unidirectional interprocess communication channel, allowing data exchange between parent and child processes.
2. File Descriptor Duplication (dup2 syscall): Duplicates an existing file descriptor, used to redirect input or output streams.
3. fork and exec (fork execve syscalls): Creates a new child process that executes a specified command (in our case, "foo").
4. File Descriptor Management: Closing unused file descriptors to prevent errors and ensure correct resource handling.

Below is a C example that implements these techniques:

#include <iostream>
#include <unistd.h>
#include <stdlib.h>

using namespace std;

int main() {
  // Input string
  string s = "Hello, world!";

  // Create pipes for stdin and stdout
  int stdinPipe[2], stdoutPipe[2];
  pipe(stdinPipe);
  pipe(stdoutPipe);

  // Fork a child process
  int pid = fork();

  if (pid == 0) {  // Child process
    // Redirect stdin and stdout to pipes
    dup2(stdinPipe[0], STDIN_FILENO);  // Read from pipe
    dup2(stdoutPipe[1], STDOUT_FILENO);  // Write to pipe

    // Close unused file descriptors
    close(stdinPipe[1]);
    close(stdoutPipe[0]);

    // Execute "foo" with piped stdin
    execlp("foo", "foo", NULL);

    // Exit child process on failure
    exit(1);
  } else if (pid > 0) {  // Parent process
    // Close unused file descriptors
    close(stdinPipe[0]);
    close(stdoutPipe[1]);

    // Write to stdin pipe
    write(stdinPipe[1], s.c_str(), s.length());
    close(stdinPipe[1]);

    // Read from stdout pipe
    char buffer[256];
    int bytesRead = 0;
    string output;
    while ((bytesRead = read(stdoutPipe[0], buffer, sizeof(buffer))) > 0) {
      output.append(buffer, bytesRead);
    }
    close(stdoutPipe[0]);

    // Print output string
    cout << output << endl;
  }

  return 0;
}

This code snippet:

• Creates pipes for stdin and stdout using pipe.
• Forks a child process using fork.
• In the child process, it redirects stdin and stdout to pipes using dup2 and then executes "foo" with piped stdin using execlp.
• In the parent process, it closes unused file descriptors, writes to stdin pipe, and reads from stdout pipe to capture the output.

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