Interprocess communication

One 、 name pipes FIFO

FIFO IPC Mechanism ： Using system files in the file system to identify . It can be used mkfifo Command to create a FIFO file ：

mkfifo tube

ls -l tube

FIFO File has no data block on disk , It is only used to identify one channel in the kernel , Each process can open this file for read/write, It's actually reading and writing kernel channels （ It turns out that file What the structure points to read,write Functions are different from regular files ）, To achieve inter process communication .

Two 、 Shared memory

1. Shared memory

Allow two or more processes to share a given storage area . Because the data does not need to be copied between the client and the server .

2. The first function to call is shmget, Get a shared storage identifier of the specified size .

①key： Used to identify shared memory size Parameter the minimum value of the shared memory segment , If you are creating a new segment , Must specify size. If you are storing an existing segment , take size Designated as 0.

②shmflg：IPC_CREATE and IPC_EXCL, The most important thing is shmflg Specify access rights in , Follow open Of mode Same parameter . Otherwise appear permission denied error .

③ return ： If successful, it is shared memory ID, Error is -1.

④key from ftok() Generate .pathname Must be accessible to the calling process .pathname and proj_id Make up one key.

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>


int main(void) {
    key_t key = ftok("./9_1readfifo.c", 9);
    if (key < 0) {
        perror("ftok");
        exit(1);
    }
    printf("key=0x%x\n", key);

    int shmid = shmget(key, 20, IPC_CREAT  | 0666);
    if (shmid < 0) {
        perror("shmget");
        exit(1);
    }
    printf("shmid=%d\n", key);
    return 0;
}

3. Call once a shared storage segment is created , The process can call shmat Connect it to its address space .

① return ： Pointer to shared storage segment if successful , Error is -1.

② The address of the calling process to which the shared memory segment is connected addr Parameters and flag Specify in SHM_RND Bit related ：

If addr=NULL： This segment connects to the first available address selected by the kernel .

If addr Not NULL, There is no designation SHM_RND： This segment connects to addr At the specified address .

If addr Not 0, And specify SHM_RND： This segment connects to addr-(addr mod SHMLBA) On the address of .

SHM_RND: integer .

SHM_LBA: Low boundary address multiples , Always 2 Power of .

This expression takes the address down to the nearest 1 individual SHMLBA Multiple .

③ General designation addr=0, So that the kernel can choose the address .

 

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>


int main(void) {
    key_t key = ftok("./9_1readfifo.c", 9);
    if (key < 0) {
        perror("ftok");
        exit(1);
    }
    printf("key=0x%x\n", key);

    int shmid = shmget(key, 20, IPC_CREAT  | 0666);
    if (shmid < 0) {
        perror("shmget");
        exit(1);
    }
    printf("shmid=%d\n", key);

    void *shmp = shmat(shmid, NULL, 0);
    if (shmp < 0) {
        perror("shmat");
        exit(1);
    }
    printf("shmp=%p\n", shmp);
    pid_t pid = fork();
    if (pid < 0) {
        perror("fork");
        exit(1);
    }
    if (pid) {
        //parent process
        while (1) {
            scanf("%s", shmp);
            if (!strcmp(shmp, "quit")) break;
        }
        wait(NULL);
    } else {
        //child process
        while (1) {
            if (!strcmp(shmp, "quit")) break;
            if (shmp) {
                printf("child read %s\n", shmp);
                bzero(shmp, 20);
            }
            sleep(1);
        }
    }
    shmdt(shmp);
    return 0;
}

 

4. Call when the operation on the shared storage segment has ended , Call shmdt Disconnect this section .

（ Its identifier and its data structure are not deleted from the system . The identifier remains until a process calls shmctl With command IPC_RMID Specifically delete it ）

shmaddr Parameters ： Previous call shmat Returns a value of ： Return success as 0, Error for -1.

5. call shmctl Perform multiple operations on shared storage segments

3、 ... and 、 Message queue

1. The system kernel maintains a queue for storing messages , Different users can send messages to the queue or receive messages in the queue .

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#include <sys/msg.h>

int main() {
    key_t key = ftok("./tube", 9);
    printf("mqid=%#x\n", key);
    int mqid = msgget(key, IPC_CREAT | 0666);
    printf("mqid=%d\n", mqid);
    return 0;
}

2. Send a message to the queue . This operation will not be restarted after being interrupted （ In signal processing SA_RESTART）

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#include <sys/msg.h>

#define MSGLEN 20

typedef struct msgbuf
{
    long mtype;
    char mtext[MSGLEN];
}MSG;


int main() {
    key_t key = ftok("./tube", 9);
    printf("mqid=%#x\n", key);
    int mqid = msgget(key, IPC_CREAT | 0666);
    printf("mqid=%d\n", mqid);

    MSG msg;
    msg.mtype = 1;
    strncpy(msg.mtext, "how are you?\n", MSGLEN);
    msgsnd(mqid, &msg, MSGLEN, 0);

    msg.mtype = 2;
    strncpy(msg.mtext, "haha\n", MSGLEN);
    msgsnd(mqid, &msg, MSGLEN, 0);
    return 0;
}

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>
#include <sys/msg.h>

#define MSGLEN 20

typedef struct msgbuf
{
    long mtype;
    char mtext[MSGLEN];
}MSG;


int main() {
    key_t key = ftok("./tube", 9);
    printf("mqid=%#x\n", key);
    int mqid = msgget(key, IPC_CREAT | 0666);
    printf("mqid=%d\n", mqid);

    MSG msg;
    msgrcv(mqid, &msg, MSGLEN, 2, 0);
    printf("msg.type=%ld\nmsg,text=%s\n", msg.mtype, msg.mtext);

    msgrcv(mqid, &msg, MSGLEN, 1, 0);
    printf("msg.type=%ld\nmsg,text=%s\n", msg.mtype, msg.mtext);
    return 0;
}