Question

I have this program that forks and creates three POSIX pipes for the child’s stdin, stdout, & stderr. After forking, the child and parent closes the appropriate ends of their respective pipes such that the child can only read from the stdin pipe and only write to the stdout and stderr pipes (and the opposite is true of the parent). Next, the child process closes and dups the child’s stdin, stdout, and stderr to its open pipe ends and then uses execvp to execute the program whose name was passed in as the parent’s program arguments. The program when behaves as it should for commands such as cat,ls,rm,banner,& ps without any noticeable issue.

However, when I run the program in the terminal like this:

program grep blah

it takes input but doesn’t output anything to the terminal but when I try:

grep blah

in the terminal grep waits for input and prints it back out if the input contained the word “blah”. So the question is, is this because of something wrong with my program as shown below? Or is it normal for grep to behave this way when it must communicate via POSIX pipes? Thank you for reading.

Here is the program in question (please excuse the formatting first time posting):

#include <stdio.h>
#include <stdlib.h> 
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <limits.h>        
int status, cpid; int child = 1;
char mode = 'c';

sigset_t sigMask;
void childHandler(){
    sigprocmask(SIG_BLOCK,&sigMask,NULL);
    int i = waitpid(-1,&status,WNOHANG);
    if(i){
        fprintf(stderr, "The child <%d> has terminated with code <%d>\n",cpid,         WEXITSTATUS(status));
        child = 0;
        mode = 'c';
    }
    sigprocmask(SIG_UNBLOCK,&sigMask,NULL);
 }


 int DEBUG = 0;
 int SLOW = 1;

 fd_set readfds,writefds,errorfds;
 struct timeval timeout;
 int numready;
 int cin,cout,cerr,in,out,err;
 int dcout = 0;
 int dcerr = 0;
 int p2c_in[2], c2p_out[2], c2p_err[2];
 int maxfd = 2;
 int tout = 0;

void doSelect(){
    FD_ZERO(&writefds);
    FD_ZERO(&readfds);

    FD_SET(fileno(stdin), &readfds);
    FD_SET(fileno(stdout), &writefds);
    FD_SET(fileno(stderr), &writefds);

    FD_SET(p2c_in[1], &writefds);
    FD_SET(c2p_out[0], &readfds);
    FD_SET(c2p_err[0], &readfds);

    if(!tout) {
        numready = select((maxfd + 1),&readfds,&writefds,NULL,NULL);
    }
    else numready = select((maxfd + 1),&readfds,&writefds,NULL,&timeout);

    in = FD_ISSET(fileno(stdin), &readfds);
    out = FD_ISSET(fileno(stdout), &writefds);
    err = FD_ISSET(fileno(stderr), &writefds);

    cin = FD_ISSET(p2c_in[1], &writefds);
    if(dcout) cout = 0;
    else cout = FD_ISSET(c2p_out[0], &readfds);
    if(dcerr) cerr = 0;
    else cerr = FD_ISSET(c2p_err[0], &readfds);

    if(DEBUG){
        fprintf(stderr,"\nstdin: %d\n",in);
        fprintf(stderr,"stdout: %d\n",out);
        fprintf(stderr,"stderr: %d\n",err);
        fprintf(stderr,"p2c_in[1]: %d\n",cin);
        fprintf(stderr,"c2p_out[0]: %d\n",cout);
        fprintf(stderr,"c2p_err[0]: %d\n\n",cerr);
        if(SLOW)sleep(2);
    }
 }



  int main(int agrc, char* argv[]){
    //Naming convention of pipes: int [intial of write-end process]2[intial of read-end process] & 
    //_err represents the file descriptor  which the pipe is suppose to 'filter'

    int val = 0, dblval = 0;
    int pipe_result,signal_result,close_result;


      //This pipe has the parent writing and the child reading from the parent's input this pipe is to filter the child's stdin
     if((pipe_result = pipe(p2c_in)) < 0){
        perror("Creation of the pipe for the child's stdin has failed");
        return pipe_result;
     }
    if(p2c_in[1] > maxfd) maxfd = p2c_in[1];

     //This pipe has the child writing and the parent reading from the child's input this pipe is to filter the child's stdout
    if((pipe_result = pipe(c2p_out)) < 0){
            perror("Creation of the pipe for the child's stdout has failed");
        return pipe_result;
    }
    if(c2p_out[0] > maxfd) maxfd = c2p_out[0];
     //This pipe has the child writing and the parent reading from the child's input this pipe is to filter the child's stderr
    if((pipe_result = pipe(c2p_err)) < 0){
        perror("Creation of the pipe for the child's stderr has failed");
        return pipe_result;
    }
    if(c2p_err[0] > maxfd) maxfd = c2p_err[0];
    if(DEBUG){
        fprintf(stderr,"stdin: %d\n",fileno(stdin));
        fprintf(stderr,"stdout: %d\n",fileno(stdout));
        fprintf(stderr,"stderr: %d\n",fileno(stderr));
        fprintf(stderr,"p2c_in[0]: %d\n",p2c_in[0]);
        fprintf(stderr,"p2c_in[1]: %d\n",p2c_in[1]);
        fprintf(stderr,"c2p_out[0]: %d\n",c2p_out[0]);
        fprintf(stderr,"c2p_out[1]: %d\n",c2p_out[1]);
        fprintf(stderr,"c2p_err[0]: %d\n",c2p_err[0]);
        fprintf(stderr,"c2p_err[1]: %d\n",c2p_err[1]);
    }


    sigemptyset(&sigMask);
    sigaddset(&sigMask,SIGCHLD);
    if((signal_result = signal(SIGCHLD, childHandler)) == SIG_ERR){
        perror("Signal handler binding has failed");
        return signal_result;
    }

    //This creates the child process
    if((cpid = fork()) < 0){
        perror("Creation of the child process has failed");
        return cpid;
    }

    if(!cpid){ //Child-exclusive code
        //This makes it so the child can only read input from the stdin pipe
        if((close_result = close(p2c_in[1])) < 0){
            perror("Closing the write end of the child's stdin pipe has failed");
            exit(close_result);
        }

        //This makes it so the child can only write input to the stdout pipe
        if((close_result = close(c2p_out[0])) < 0){
            perror("Closing the read end of the child's stdout pipe has failed");
            exit(close_result);
        }

        //This makes it so the child can only write input to the stderr pipe
        if((close_result = close(c2p_err[0])) < 0){
            perror("Closing the read end of the child's stderr pipe has failed");
            exit(close_result);
        }

        //This closes the child's standard input
        if((close_result = close(0)) < 0){
            perror("Closing the child's standard input has failed");
            exit(close_result);
        }

        int dup_result;
        if((dup_result = dup(p2c_in[0])) < 0){
            perror("Duplication of the read end of the child's stdin pipe has failed");
            exit(dup_result);
        }

        //This closes the child's standard output
        if((close_result = close(1)) < 0){
            perror("Closing the child's standard output has failed");
            exit(close_result);
        }

        if((dup_result = dup(c2p_out[1])) < 0){
            perror("Duplication of the write end of the child's stdout pipe has failed"); 
            exit(dup_result);
        }

        //This closes the child's standard error
        if((close_result = close(2)) < 0){
            perror("Closing the child's standard error has failed");
            exit(close_result);
        }

        if((dup_result = dup(c2p_err[1])) < 0){
            perror("Duplication of the write end of the child's stderr pipe has failed");
            exit(dup_result);
        }

        execvp(argv[1], &argv[1]);
        perror("execvp has failed:");

        if((close_result = close(p2c_in[0])) < 0){
            perror("Closing the write end of the child's stdin pipe has failed");
            exit(close_result);
        }

        if((close_result = close(c2p_out[1])) < 0){
            perror("Closing the read end of the child's stdout pipe has failed");
            exit(close_result);
        }

        if((close_result = close(c2p_err[1])) < 0){
            perror("Closing the read end of the child's stderr pipe has failed");
            exit(close_result);
        }

        exit(66); //This occurs after p2c_in's write end is closed
    } else { //Parent-exclusive code

        //This makes it so the parent can only write input to the stdin pipe
        if((close_result = close(p2c_in[0])) < 0){
            perror("Closing the read end of the child's stdin pipe has failed");
            return close_result;
        }

        //This makes it so the parent can read only input from the stdout pipe
        if((close_result = close(c2p_out[1])) < 0){
            perror("Closing the write end of the child's stdout pipe has failed");
            return close_result;
        }

        //This makes it so the parent can read only input from the stderr pipe
        if((close_result = close(c2p_err[1])) < 0){
            perror("Closing the write end of the child's stderr pipe has failed");
            return close_result;
        }



        timeout.tv_sec = 1; timeout.tv_usec = 0;
        char mode = 'c';
        char t_mode;
        int maxlines = 20;
        int sig; int lcount = 0; int fill = 0;
        char buf_err;
        char buf_in[MAX_CANON];
        char buf_out;
        int read_resultin,read_resultout,read_resulterr;
                int write_resultin,write_resultout,write_resulterr;

        while(1){
            if(DEBUG)fprintf(stderr,"<%d> While Begin \n",getpid());
            doSelect();

            if(in && cin){
                if(((read_resultin = read(fileno(stdin), &buf_in, MAX_CANON)) > 0) && child){
                    //write(2, &buf_in, read_resultin);
                    if((write_resultin = write(p2c_in[1], &buf_in, read_resultin)) > -1){
                        fill = write_resultin;
                        if(DEBUG)fprintf(stderr,"<%d> Input \n",getpid());
                        doSelect();
                        while(cin && child && (fill < read_resultin)){
                            if(DEBUG)fprintf(stderr,"<%d> Input Loop \n",getpid());
                            write_resultin = write(p2c_in[1], &buf_in, read_resultin - fill);
                            fill += write_resultin;
                            doSelect();
                        }
                        //if(read_resultin < 0) perror("read from stdin without a / failed");
                    }
                    else perror("Reading to child's stdin has failed");

                }

            }
            if(DEBUG)fprintf(stderr,"<%d> Input Loop End \n",getpid());
            doSelect();

            if (out && cout){
                if(DEBUG)fprintf(stderr,"<%d> Output \n",getpid());
                while(out && cout){
                    if(DEBUG)fprintf(stderr,"<%d> Output Loop \n",getpid());
                    if((read_resultout = read(c2p_out[0], &buf_out, 1)) == 1)
                        write(fileno(stdout), &buf_out, 1);
                    else if(!read_resultout && !child/**/){
                        if(DEBUG)fprintf(stderr,"<%d> No More Output \n",getpid());
                        dcout = 1;
                    }
                    else if(read_resultout < 0)
                        perror("Output Read");

                    doSelect();
                }


            }

            if(DEBUG)fprintf(stderr,"<%d> Output End \n",getpid());
            doSelect();
            if (err && cerr){
                /**/if(DEBUG)fprintf(stdout,"<%d> Error \n",getpid());
                while(err && cerr){
                    /**/if(DEBUG)fprintf(stdout,"<%d> Error Loop \n",getpid());
                    if((read_resulterr = read(c2p_err[0], &buf_err, 1)) == 1){
                        write_resulterr = write(fileno(stderr), &buf_err, 1);
                        if(write_resulterr < 0) perror("Error Wirte");
                    }else if(!read_resulterr&& !child/**/){
                        dcerr = 1;
                        if(DEBUG)fprintf(stderr,"<%d> No More Error \n",getpid());
                    }
                    else if(read_resulterr < 1) perror("Error Read");
                    doSelect();
                }
                /**/if(DEBUG)fprintf(stdout,"<%d> Error Loop End \n",getpid());
                if(read_resulterr < 0) perror("Reading from child's stdout has failed");
                /**/if(DEBUG)fprintf(stdout,"<%d> Error End \n",getpid());

            }

            if(!child && dcout && dcerr) break;
        }




        //This closes up the write end of p2c_in that way the child will be able to terminate for it will recieve an EOF
        if((close_result = close(p2c_in[1])) < 0){
            perror("The last closing of the write end of the child's stdin pipe has failed");
            exit(close_result);
        }

        //This closes up the read end of c2p_out
        if((close_result = close(c2p_out[0])) < 0){
            perror("The last closing of the read end of the child's stdout pipe has failed");
            exit(close_result);
        }

        //This closes up the read end of c2p_err
        if((close_result = close(c2p_err[0])) < 0){
            perror("The last closing of the read end of the child's stderr pipe has failed");
            exit(close_result);
        }



    }
   }

Answer 1

Many programs are line-buffered when dealing with terminal input/ouput but use much larger chunks in other cases (such as pipes). grep is one of them.

If you are using grep from GNU coreutils, you can use grep --line-buffered to force grep to produce output line-by-line instead of waiting for more data to queue up. However, you really should fix your program to close its end of the input pipe when you receive EOF on stdin: then grep will notice an EOF on its end of the input pipe, and flush its remaining output.