# Exercise 1

After trying`pipe_creation.c`,`pipe_withfork.c`and`pipe_withfork2.c`, you should be able to answer questions**Q1-6**. According to**Q4**, we know that when pipe is empty, the reader process blocks until writer process write something into pipe. Similarly, when pipe is full. the writer process blocks until reader process reads something from pipe.[pipe-ex1.c](https://eric-lo.github.io/3150/6-IPC1/code/pipe-ex1.c)use this property to synchronize processes. In`pipe-ex1.c`, parent and child processes need communicate with each other according to the following rules:

* Parent sends a message (a character "p") to child.
* Child sends acknowledgment message (a character "c") to parent on receiving "p", otherwise, child waits and does nothing.
* When parent receives "c", it sends the next "p" to child, otherwise, parent waits and does nothing.

Before`fork()`, we create two pipes as shown in the following figure. Parent writes the character "p" across the top pipe when TELL\_CHILD is called, child writes "c" across the bottom pipe when TELL\_PARENT is called.

![](/files/-Lp-5vt7s-rY6s3hru-F)

**Exercise-1** Complete `TELL\_PARENT`,`TELL\_CHILD`,`WAIT\_PARENT`,`WAIT\_CHILD` to make parent and child communicate correctly. That is to say,pipe-ex1.cshould output as below.

![](/files/-Lp-5vtI6oHP1mreQw74)

```c
/*pipe-ex1.c*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
static int pipefd1[2],pipefd2[2];
/*Create two pipes*/
void TELL_WAIT(void){
    if(pipe(pipefd1)<0 || pipe(pipefd2)<0){
        perror("pipe");
        exit(EXIT_FAILURE);
    }
}

void TELL_PARENT(void){
  /* send parent a message through pipe, need you to complete*/
    printf("Child send message to parent!\n");
}

void WAIT_PARENT(void){
  /* read message sent by parent from pipe, need you to complete*/
    printf("Child receive message from parent!\n");
}

void TELL_CHILD(void){
  /* send child a message through pipe, need you to complete*/
    printf("Parent send message to child!\n");
}

void WAIT_CHILD(void){
  /* read the message sent by child from pipe, need you to complete*/
    printf("Parent receive message from child!\n");
}

int main(int argc, char* argv[]){
    TELL_WAIT();
    pid_t pid;
    pid = fork();
    //set a timer, process will end after 1 second.
    alarm(10);
    if(pid!=0){
        while(1){
            sleep(rand()%2+1);        
            TELL_CHILD();
            WAIT_CHILD();
        }
    }else{
        while(1){
            sleep(rand()%2+1);
            WAIT_PARENT();
            TELL_PARENT();
        }
      }
    return 0;
}
```

## Answer

```c
/* pipe_ex1_sol.c */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
static int pipefd1[2],pipefd2[2];
void TELL_WAIT(void){
    if(pipe(pipefd1)<0 || pipe(pipefd2)<0){
        perror("pipe");
        exit(EXIT_FAILURE);
    }
}

void TELL_PARENT(void){
    if(write(pipefd2[1],"c",1)!=1){
        perror("write");
        exit(EXIT_FAILURE);
    }

    printf("Child send message to parent!\n");
}

void WAIT_PARENT(void){
    char c;
    if(read(pipefd1[0],&c,1)!=1){
        perror("read");
        exit(EXIT_FAILURE);
    }
    printf("Child receive message from parent!\n");
}

void TELL_CHILD(void){
    if(write(pipefd1[1],"p",1)!=1){
        perror("write");
        exit(EXIT_FAILURE);
    }

    printf("Parent send message to child!\n");
}

void WAIT_CHILD(void){
    char c;
    if(read(pipefd2[0],&c,1)!=1){
        perror("read");
        exit(EXIT_FAILURE);
    }
    printf("Parent receive message from child!\n");
}

int main(int argc, char *argv[]){
    TELL_WAIT();
    pid_t pid;
    pid = fork();
    alarm(10);
    if(pid!=0){
        while(1){
            sleep(rand()%2+1);        
            TELL_CHILD();
            WAIT_CHILD();
        }
    }else{
        while(1){
            sleep(rand()%2+1);
            WAIT_PARENT();
            TELL_PARENT();
        }
      }
    return 0;
}
```


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