有P1，P2，P3三个进程，P1和P2负责从键盘接收字符串，均发送给P3，P3接收到字符串，根据发送方分别显示”P3 received *** from P1（或P2）" ；分别用管道通信，消息队列和共享存储三种通信方式实现。

管道通信方式实现：

P1和P2创建两个无名管道，分别用于向P3发送数据。P3创建两个无名管道分别用于接收来自P1和P2的数据。

P1和P2创建进程后，循环读取键盘输入字符串，将字符串写入到对应的管道中。P3在循环中不断从两个管道中读取数据，并根据发送方的不同显示不同的信息。

P1代码示例：

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

#define BUF_SIZE 256

int main(void) {
    int fd[2];
    char buf[BUF_SIZE];

    if (pipe(fd) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    while (1) {
        printf("Enter a string:");
        fgets(buf, BUF_SIZE, stdin);
        write(fd[1], buf, strlen(buf) + 1);
    }

    close(fd[1]);

    return 0;
}

P2代码示例：

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

#define BUF_SIZE 256

int main(void) {
    int fd[2];
    char buf[BUF_SIZE];

    if (pipe(fd) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    while (1) {
        printf("Enter a string:");
        fgets(buf, BUF_SIZE, stdin);
        write(fd[1], buf, strlen(buf) + 1);
    }

    close(fd[1]);

    return 0;
}

P3代码示例：

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

#define BUF_SIZE 256

int main(void) {
    int fd1[2], fd2[2];
    char buf[BUF_SIZE];
    int n;

    if (pipe(fd1) == -1 || pipe(fd2) == -1) {
        perror("pipe");
        exit(EXIT_FAILURE);
    }

    while (1) {
        n = read(fd1[0], buf, BUF_SIZE);
        if (n > 0) {
            printf("P3 received %s from P1\n", buf);
        }

        n = read(fd2[0], buf, BUF_SIZE);
        if (n > 0) {
            printf("P3 received %s from P2\n", buf);
        }
    }

    close(fd1[0]);
    close(fd2[0]);

    return 0;
}

消息队列通信方式实现：

P1和P2创建消息队列，并循环读取键盘输入的字符串，将字符串发送到消息队列中。P3在循环中不断从消息队列中接收消息，并根据发送方的不同显示不同的信息。

P1代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/msg.h>

#define BUF_SIZE 256

struct message {
    long type;
    char text[BUF_SIZE];
};

int main(void) {
    int msgid;
    struct message msg;

    msgid = msgget(IPC_PRIVATE, 0644);
    if (msgid == -1) {
        perror("msgget");
        exit(EXIT_FAILURE);
    }

    while (1) {
        printf("Enter a string:");
        fgets(msg.text, BUF_SIZE, stdin);
        msg.type = 1;
        if (msgsnd(msgid, &msg, sizeof(msg.text), 0) == -1) {
            perror("msgsnd");
            exit(EXIT_FAILURE);
        }
    }

    if (msgctl(msgid, IPC_RMID, NULL) == -1) {
        perror("msgctl");
        exit(EXIT_FAILURE);
    }

    return 0;
}

P2代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/msg.h>

#define BUF_SIZE 256

struct message {
    long type;
    char text[BUF_SIZE];
};

int main(void) {
    int msgid;
    struct message msg;

    msgid = msgget(IPC_PRIVATE, 0644);
    if (msgid == -1) {
        perror("msgget");
        exit(EXIT_FAILURE);
    }

    while (1) {
        printf("Enter a string:");
        fgets(msg.text, BUF_SIZE, stdin);
        msg.type = 2;
        if (msgsnd(msgid, &msg, sizeof(msg.text), 0) == -1) {
            perror("msgsnd");
            exit(EXIT_FAILURE);
        }
    }

    if (msgctl(msgid, IPC_RMID, NULL) == -1) {
        perror("msgctl");
        exit(EXIT_FAILURE);
    }

    return 0;
}

P3代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/msg.h>

#define BUF_SIZE 256

struct message {
    long type;
    char text[BUF_SIZE];
};

int main(void) {
    int msgid;
    struct message msg;
    int n;

    msgid = msgget(IPC_PRIVATE, 0644);
    if (msgid == -1) {
        perror("msgget");
        exit(EXIT_FAILURE);
    }

    while (1) {
        n = msgrcv(msgid, &msg, sizeof(msg.text), 0, 0);
        if (n == -1) {
            perror("msgrcv");
            exit(EXIT_FAILURE);
        }

        if (msg.type == 1) {
            printf("P3 received %s from P1\n", msg.text);
        } else if (msg.type == 2) {
            printf("P3 received %s from P2\n", msg.text);
        }
    }

    if (msgctl(msgid, IPC_RMID, NULL) == -1) {
        perror("msgctl");
        exit(EXIT_FAILURE);
    }

    return 0;
}

共享存储通信方式实现：

P1和P2创建共享内存，并循环读取键盘输入的字符串，将字符串写入到共享内存中。P3在循环中不断从共享内存中读取数据，并根据发送方的不同显示不同的信息。

P1代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <sys/stat.h>

#define BUF_SIZE 256

int main(void) {
    int fd;
    char *ptr;
    struct stat buf;

    fd = shm_open("/shm", O_CREAT | O_RDWR, 0664);
    if (fd == -1) {
        perror("shm_open");
        exit(EXIT_FAILURE);
    }

    if (fstat(fd, &buf) == -1) {
        perror("fstat");
        exit(EXIT_FAILURE);
    }

    if (buf.st_size == 0) {
        if (ftruncate(fd, BUF_SIZE) == -1) {
            perror("ftruncate");
            exit(EXIT_FAILURE);
        }
    }

    ptr = mmap(NULL, BUF_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (ptr == MAP_FAILED) {
        perror("mmap");
        exit(EXIT_FAILURE);
    }

    while (1) {
        printf("Enter a string:");
        fgets(ptr, BUF_SIZE, stdin);
    }

    if (munmap(ptr, BUF_SIZE) == -1) {
        perror("munmap");
        exit(EXIT_FAILURE);
    }

    if (shm_unlink("/shm") == -1) {
        perror("shm_unlink");
        exit(EXIT_FAILURE);
    }

    return 0;
}

P2代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <sys/stat.h>

#define BUF_SIZE 256

int main(void) {
    int fd;
    char *ptr;
    struct stat buf;

    fd = shm_open("/shm", O_CREAT | O_RDWR, 0664);
    if (fd == -1) {
        perror("shm_open");
        exit(EXIT_FAILURE);
    }

    if (fstat(fd, &buf) == -1) {
        perror("fstat");
        exit(EXIT_FAILURE);
    }

    if (buf.st_size == 0) {
        if (ftruncate(fd, BUF_SIZE) == -1) {
            perror("ftruncate");
            exit(EXIT_FAILURE);
        }
    }

    ptr = mmap(NULL, BUF_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (ptr == MAP_FAILED) {
        perror("mmap");
        exit(EXIT_FAILURE);
    }

    while (1) {
        printf("Enter a string:");
        fgets(ptr, BUF_SIZE, stdin);
    }

    if (munmap(ptr, BUF_SIZE) == -1) {
        perror("munmap");
        exit(EXIT_FAILURE);
    }

    if (shm_unlink("/shm") == -1) {
        perror("shm_unlink");
        exit(EXIT_FAILURE);
    }

    return 0;
}

P3代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <sys/stat.h>

#define BUF_SIZE 256

int main(void) {
    int fd;
    char *ptr;
    int n;

    fd = shm_open("/shm", O_RDWR, 0664);
    if (fd == -1) {
        perror("shm_open");
        exit(EXIT_FAILURE);
    }

    ptr = mmap(NULL, BUF_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (ptr == MAP_FAILED) {
        perror("mmap");
        exit(EXIT_FAILURE);
    }

    while (1) {
        n = read(STDIN_FILENO, ptr, BUF_SIZE);
        if (n > 0) {
            printf("P3 received %s from %s\n", ptr, (ptr[0] == 'P') ? "P1" : "P2");
        }
    }

    if (munmap(ptr, BUF_SIZE) == -1) {
        perror("munmap");
        exit(EXIT_FAILURE);
    }

    return 0;
}