C Pipe to STDIN of Another Program

c, pipe

Solution

I suppose this is what you're going to do: 1. main fork, parent pass message to child via pipe. 2. child receive message from pipe, redirect message to STDIN, execute log. 3. log receive message from STDIN, do something.

the key to do this is dup2 to redirect file descriptor, from pipe to STDIN.

This is the modified simple version:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

int main(int argc, char *argv[]) {
    int fd[2];
    char buf[] = "HELLO WORLD!";
    if(pipe(fd)){
      perror("pipe");
      return -1;
    }
    switch(fork()){
        case -1:
            perror("fork");
            return -1;
        case 0:
            // child
            close(fd[1]);
            dup2(fd[0], STDIN_FILENO);
            close(fd[0]);
            execl("./log", NULL);
        default:
            // parent
            close(fd[0]);
            write(fd[1], buf, sizeof(buf));
            close(fd[1]);
            wait(NULL);
    }
    printf("END~\n");
    return 0;
}

Problem

I can barely understand the man page for pipe, so I kinda need help understanding how to take a piped input in an external executable. I have 2 programs: main.o & log.o I written main.o to fork. Here is what it is doing: - Parent fork will pipe data to the child - Child fork will exec log.o I need the child fork for main to pipe to STDIN of log.o log.o simply takes STDIN & logs with time stamp to a file. My code is composed of some code from various StackOverflow pages I dont remember & the man page for pipe: ``` printf("\n> "); while(fgets(input, MAXINPUTLINE, stdin)){ char buf; int fd[2], num, status; if(pipe(fd)){ perror("Pipe broke, dood"); return 111; } switch(fork()){ case -1: perror("Fork is sad fais"); return 111; case 0: // Child close(fd[1]); // Close unused write end while (read(fd[0], &buf, 1) > 0) write(STDOUT_FILENO, &buf, 1); write(STDOUT_FILENO, "\n", 1); close(fd[0]); execlp("./log", "log", "log.txt", 0); // This is where I am confused _exit(EXIT_FAILURE); default: // Parent data=stuff_happens_here(); close(fd[0]); // Close unused read end write(fd[1], data, strlen(data)); close(fd[1]); // Reader will see EOF wait(NULL); // Wait for child } printf("\n> "); } ```

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