c - C-->如何使用信号量协调进程行为？

问题描述

想请大家帮忙理解一下C Semaphores。基本上我想创建两个 ProcessesChild和Parent. 当然他们的行为是不同的：

1）子进程做一些事情sleep()并在先前创建的消息队列上发送消息

2）父进程也做了一些sleep()，但随后读取队列上发送的消息。

当然，我需要子进程成为第一个运行的进程。我设法使用wait和waitpid系统调用来做到这一点，现在我想使用Semaphore. 下面是我设法编写的一些代码：

A）我正在使用的功能semaphore.h

//In order to create a Semaphore

int createSemaphore(key_t semaphoreKey){
    int semaphoreId = semget(semaphoreKey, 1, 0666 | IPC_CREAT);
    if(semaphoreId == -1){
        printf(RED "Semaphore Creation failed\n"RESET);
        return -1;
    }
    return semaphoreId;
}

void semaphoreWait(int semaphoreId, int semaphoreNumber){
    struct sembuf buffer;
    buffer.sem_num = semaphoreNumber;
    buffer.sem_op = -1;
    buffer.sem_flg = 0;
    int done = semop(semaphoreId, &buffer, 1);
    if(done == -1){
        printf(RED "Wait on Semaphore %d failed\n" RESET, semaphoreNumber);
        return;
    }
}

void semaphoreSignal(int semaphoreId, int semaphoreNumber){
    struct sembuf buffer;
    buffer.sem_num = semaphoreNumber;
    buffer.sem_op = 1;
    buffer.sem_flg = 0;
    int done = semop(semaphoreId, &buffer, 1);
    if(done == -1){
        printf(RED "semaphoreSignal Failed on Semaphore %d\n" RESET, semaphoreNumber);
        return;
    }
}

B）我正在使用的功能messageQueue.h

typedef struct message{
    long type;
    char text[255];
}message;

//Creates a Message Queue and returns its ID
int createMessageQueue(key_t semaphoreKey){
    int queueId;
    queueId = msgget(semaphoreKey, 0666 | IPC_CREAT);
    return queueId;
}

//Sends a Message on Message Queue queueId
void sendMessage(int queueId, message* messaggio){
    int sent = msgsnd(queueId, messaggio, strlen(messaggio->text) + 1, 0);
    if(sent == -1){
        printf(RED "Sending the message %s failed on Message Queue %d\n"RESET, messaggio->text, queueId);
        return;
    }
}

//Receives a message from Message Queue queueId
void receiveMessage(int queueId, message* messaggio, int type){
    int received = msgrcv(queueId, messaggio, 255, type, 0);
    if(received == -1){
        printf(RED "Receiving message on Message Queue %d failed\n" RESET, queueId);
        return;
    }
}
void setText(message *mex, char text[], int size){
    for(int i = 0; i < size; i++){
        mex->text[i] = text[i];
    }
}

C）最后我的main.c文件：

#include "semaphore.h"
#include "messageQueue.h"

int main(int argc, char *argv[]){
    pid_t piddo;
    message mex, mex2;
    mex2.type = 1;
    key_t semKey = ftok("/temp", 0);
    key_t mexKey = ftok("/temp", 1);
    char text[] = "This is a message sent from Child to Parent";
    int semId = createSemaphore(semKey);
    int mexId = createMessageQueue(mexKey);
    switch(piddo = fork()){
        case -1:
            printf(RED "Fork ERROR\n" RESET);
            exit(EXIT_FAILURE);
        case 0:
            printf(CYAN "Child Process %d has started\n" RESET, getpid());
            printf(CYAN "Doing some useless sleep\n" RESET);
            for(int i = 0; i < 4; i++){
                printf(CYAN "Sleep %d\n" RESET, i);
                sleep(1);
            }
            printf(CYAN "Useless Sleep Finished\n" RESET);
            printf(CYAN "Now I setup Message to send\n" RESET);
            mex.type = 1;
            setText(&mex, text, strlen(text));
            printf(CYAN "Settings saved, Now I send Message\n" RESET);
            sendMessage(mexId, &mex);
            printf(CYAN "Message succesfully sent, Child Process %d has finished\n" RESET, getpid());
            //Now I want Parent Process to know Child has finished
            semaphoreSignal(semId, 0);
            exit(EXIT_SUCCESS);
        default:
            printf(YELLOW "Parent Process %d started\n" RESET, getpid());
            printf(YELLOW "I have to wait for Child Process\n" RESET);
            //With this wait Parent Process should wait for Child to have finished
            semaphoreWait(semId, 0);
            //Now Parent knows Child has finished
            printf(YELLOW "Looks like Child Process has finished\n" RESET);
            printf(YELLOW "Doing some useless sleep\n" RESET);
            for(int i = 0; i < 4; i++){
                printf(YELLOW "Sleep %d\n" RESET, i);
                sleep(1);
            }
            printf(YELLOW "Useless sleep finished\n" RESET);
            printf(YELLOW "Receiving Message sent by Child...\n" RESET);
            receiveMessage(mexId, &mex2, 1);
            printf(YELLOW "Message Received: \n" RESET);
            printf(YELLOW "%s\n" RESET, mex2.text);
            printf(YELLOW "All Done, Parent Process %d has finished\n" RESET, getpid());
            semaphoreSignal(semId, 0);
            exit(EXIT_SUCCESS);
    }
}

正如你们可能注意到的那样，这个文件中Child的Parent进程并没有main.c像我想要的那样同步。我的问题是：如何确保 Child 在 Parent 开始之前完成他的代码要求的所有事情？谢谢:)

标签： cmessage-queuemessagesemaphore

c - C-->如何使用信号量协调进程行为？

问题描述

解决方案

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