带管道的非终止C程序

Question

I am attempting to write a C program, using pipes, to play the game of “rock, paper, scissors”, between two children. I have most of the code written and working. When I run the code it plays the game, but refuses to terminate. It locking down the terminal, and forces me to close it and try again. I am not sure as to why this is.

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

    static char* const ARGV[] = {
          "prog","4",NULL
    };

   int main(int argc, char *argv[]) {
   int pipe1[2], pipe2[2], pipe3[2], pipe4[2];
   int pid, pid2, cpid;
   int turns, readbyte;
   char message[100];
   int object, i, status;
   char p1[5], p2[5], choice[5];
   srand(time(NULL));


if((argc != 2) || ((turns = atoi(argv[1])) <= 0)) {
    fprintf(stderr, "Usage: %s turns\n",argv[0]);
    exit(1);
}

if(pipe(pipe1) == -1) {
    perror("Could not create pipe");
    exit(1);
}

if(pipe(pipe2) == -1) {
    perror("Could not create pipe");
    exit(1);
}

if(pipe(pipe3) == -1) {
    perror("Could not reach pipe");
    exit(1);
}

if(pipe(pipe4) == -1) {
    perror("Could not reach pipe");
    exit(1);
}


if(pid = fork()) {
    write(pipe1[1], "Ready", strlen("Ready")+1);
    for(i=1; i <= turns; i++) {
        readbyte = read(pipe2[0],message,100);
        srand(time(NULL));
        object = rand() % 3;
        sprintf(choice,"%d",object);
        write(pipe1[1],choice,strlen(choice)+1);
    }
}

if(pid = fork()) {
    write(pipe3[1], "Ready", strlen("Ready")+1);
    for(i=1; i<=turns; i++){
        readbyte = read(pipe4[0],message,100);
        sleep(1);
        srand(time(NULL));
        object = rand()%3;
        sprintf(choice,"%d",object);
        write(pipe3[1],choice,strlen(choice)+1);
    }
}

cpid = wait(&status);
readbyte = read(pipe3[0],message,100);
readbyte = read(pipe1[0],message,100);
printf("Child 1 PID: %d\n", getpid());
printf("Child 2 PID: %d\n", getpid()-1);

printf("A brawl is surely brewing\n%d Rounds\nFight! \n",turns);
for(i=1; i<=turns; i++){
    write(pipe2[1],"Go",strlen("Go")+1);
    write(pipe4[1],"Go",strlen("Go")+1);
    readbyte = read(pipe1[0],p1,5);
    readbyte = read(pipe3[0],p2,5);

    printf("Round %d:\n",i);

    if((atoi(p1)) == 0){
        printf("Child 1 throws Rock\n");
    }
    else if((atoi(p1)) == 1){
        printf("Child 1 throws Paper\n");
    }
    else{
        printf("Child 1 throws Scissors\n");
    }
    if((atoi(p2)) == 0){
        printf("Child 2 throws Rock\n");
    }
    else if((atoi(p2)) == 1){
        printf("Child 2 throws Paper\n");
    }
    else{
        printf("Child 2 throws Scissors\n");
    }
    if(atoi(p1) == atoi(p2)){
        printf("This round is a tie\n");
    }
    else if((atoi(p1) == 0) && (atoi(p2) == 2)){
        printf("Rock beats Scissors: Child 1 wins!\n");
    }
    else if((atoi(p1) == 0) && (atoi(p2) == 1)){
        printf("Paper beats Rock: Child 2 wins!\n");
    }
    else if((atoi(p1) == 1) && (atoi(p2) == 0)){
        printf("Paper beats Rock: Child 1 wins!\n");
    }
    else if((atoi(p1) == 1) && (atoi(p2) == 2)){
        printf("Scissors beats Paper: Child 2 wins!\n");
    }
    else if((atoi(p1) == 2) && (atoi(p2) == 1)){
        printf("scissors beats Paper: Child 1 wins!\n");
    }
    else if(((atoi(p1)) == 2) && (atoi(p2) == 0)){
        printf("Rock beats Scissors: Child 2 wins!\n");
    }
}
}

Answer 1

Fork() causes the child to inherit the open file descriptors of the parent, including pipe()s. A pipe becomes unreadable (EOF) when all “writers” have closed their pipe. You have extraneous “write” ends of a pipe open, thus it will not close properly.

At a minimum, you need the parent to close pipe1[1], pipe3[1]. A little bit of checking return values probably wouldn't hurt your effort either.

Answer 2

Your forking is confusing, and not the only problem.

you do

fork if parent or error then loop both parent and child form if new parent (old child) or error then loop all three processes wait …

I would advise starting with a simple program with one pipe and one fork (2 processes). Get this to work before adding more.

See this blog post for a tutorial of this simpler use case https://ctrlaltdelor.wordpress.com/2009/06/23/creation-of-an-interprocess-pipe/ then expand for what you need.