Pipes, dup2 and exec()

Question

I have to write a shell that can run pipes. For example commands like ls -l | wc -l\". I have successfully parsed the command given by the user as below:

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Answer 1

Hmm, close enough. You miss to handle close on some file descriptor after fork.

Here is some reference:

1. About pipe, http://unixwiz.net/techtips/remap-pipe-fds.html
2. About file descriptor, http://www.usna.edu/Users/cs/aviv/classes/ic221/s14/lec/09/lec.html

Here's my code:

#include                                //
#include                                //
#include                               //
#include                               //
#include                            //
#include                             //
#include                             //
#include                              //
#include                               //
                                                 //
#define INPUT_END 1                              // INPUT_END means where the pipe takes input
#define OUTPUT_END 0                             // OUTPUT_END means where the pipe produces output
                                                 //
int main(int argc, char* argv[])                 //
{                                                //
    pid_t pid1;                                  // [STDIN -> terminal_input, STDOUT -> terminal_output]                       (of the parent process)
    pid_t pid2;                                  //
    int fd[2];                                   //
                                                 //
    pipe(fd);                                    // [STDIN -> terminal_input, STDOUT -> terminal_output, fd[0] -> pipe_input, fd[1] -> pipe_output]
    pid1 = fork();                               //
                                                 //
    if(pid1==0)                                  //
    {                                            // I am going to be the wc process (i.e. taking input from the pipe)
        close(fd[INPUT_END]);                    // [STDIN -> terminal_input, STDOUT -> terminal_output, fd[1] -> pipe_output] (of the WC process)
        dup2(fd[OUTPUT_END], STDIN_FILENO);      // [STDIN -> pipe_output, STDOUT -> terminal_output, fd[1] -> pipe_output]    (of the WC process)
        close(fd[OUTPUT_END]);                   // [STDIN -> pipe_output, STDOUT -> terminal_output]                          (of the WC process)
        execlp("wc", "wc", "-l",(char*) NULL);   //
    }                                            //
    else                                         //
    {                                            //
        pid2=fork();                             //
                                                 //
        if(pid2==0)                              //
        {                                        // I am going to be the ls process (i.e. producing output to the pipe)
            close(fd[OUTPUT_END]);               // [STDIN -> terminal_input, STDOUT -> terminal_output, fd[0] -> pipe_input] (of the ls process)
            dup2(fd[INPUT_END], STDOUT_FILENO);  // [STDIN -> terminal_input, STDOUT -> pipe_input, fd[0] -> pipe_input]      (of the ls process)
            close(fd[INPUT_END]);                // [STDIN -> terminal_input, STDOUT -> pipe_input]                           (of the ls process)
            execlp("ls","ls","-l",(char*) NULL); //
        }                                        //
                                                 //
        close(fd[OUTPUT_END]);                   // [STDIN -> terminal_input, STDOUT -> terminal_output, fd[0] -> pipe_input] (of the parent process)
        close(fd[INPUT_END]);                    // [STDIN -> terminal_input, STDOUT -> terminal_output]                      (of the parent process)
        waitpid(-1, NULL, 0);                    // As the parent process - we wait for a process to die (-1) means I don't care which one - it could be either ls or wc
        waitpid(-1, NULL, 0);                    // As the parent process - we wait for the another process to die.
                                                 // At this point we can safely assume both process are completed
    }                                            //
}                                                //