进程间通信的例子-信号量

消息队列，信号量，共享内存
vim-------------------
im中翻页的命令

    整页翻页 ctrl-f ctrl-b
    f就是forword b就是backward

    翻半页
    ctrl-d ctlr-u
    d=down u=up

    滚一行
    ctrl-e ctrl-y

    zz 让光标所杂的行居屏幕中央
    zt 让光标所杂的行居屏幕最上一行 t=top
    zb 让光标所杂的行居屏幕最下一行 b=bottom
-------------------------------

/* After the #includes, the function prototypes and the global variable, we come to the
 main function. There the semaphore is created with a call to semget, which returns the
 semaphore ID. If the program is the first to be called (i.e. it's called with a parameter
 and argc > 1), a call is made to set_semvalue to initialize the semaphore and op_char is
 set to X. */

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

#include <sys/sem.h>

#include "semun.h"

static int set_semvalue(void);  //给信号赋值，设置属性等
static void del_semvalue(void);
static int semaphore_p(void);
static int semaphore_v(void);

static int sem_id;


int main(int argc, char *argv[])
{
    int i;
    int pause_time;
    char op_char = 'O';

    srand((unsigned int)getpid());
    
    sem_id = semget((key_t)1234, 1, 0666 | IPC_CREAT);  //初始化信号量（相当于open）

    if (argc > 1)    //是起始程序
     {
        if (!set_semvalue()) {
            fprintf(stderr, "Failed to initialize semaphore\n");
            exit(EXIT_FAILURE);
        }
        op_char = 'X';
        sleep(2);
    }

/* Then we have a loop which enters and leaves the critical section ten times.
 There, we first make a call to semaphore_p which sets the semaphore to wait, as
 this program is about to enter the critical section. */

    for(i = 0; i < 10; i++) {        

        if (!semaphore_p())
        	 exit(EXIT_FAILURE);
        printf("%c", op_char);fflush(stdout);
        pause_time = rand() % 3;
        sleep(pause_time);
        printf("%c", op_char);fflush(stdout);

/* After the critical section, we call semaphore_v, setting the semaphore available,
 before going through the for loop again after a random wait. After the loop, the call
 to del_semvalue is made to clean up the code. */

        if (!semaphore_v()) 
        	exit(EXIT_FAILURE);
        
        pause_time = rand() % 2;
        sleep(pause_time);
    }    

    printf("\n%d - finished\n", getpid());

    if (argc > 1) {    
        sleep(10);
        del_semvalue();
    }
        
    exit(EXIT_SUCCESS);
}

/* The function set_semvalue initializes the semaphore using the SETVAL command in a
 semctl call. We need to do this before we can use the semaphore. */

//给信号量赋初值
static int set_semvalue(void)
{
    union semun sem_union;

    sem_union.val = 1;
    if (semctl(sem_id, 0, SETVAL, sem_union) == -1) //semctl()的二参是下标
    	return(0);
    return(1);
}

/* The del_semvalue function has almost the same form, except the call to semctl uses
 the command IPC_RMID to remove the semaphore's ID. */

//销毁信号量
static void del_semvalue(void)
{
    union semun sem_union;
    
    if (semctl(sem_id, 0, IPC_RMID, sem_union) == -1)
        fprintf(stderr, "Failed to delete semaphore\n");
}

/* semaphore_p changes the semaphore by -1 (waiting). */

static int semaphore_p(void)
{
    struct sembuf sem_b;
    
    sem_b.sem_num = 0;
    sem_b.sem_op = -1; /* P() */
    sem_b.sem_flg = SEM_UNDO;
    if (semop(sem_id, &sem_b, 1) == -1) {
        fprintf(stderr, "semaphore_p failed\n");
        return(0);
    }
    return(1);
}

/* semaphore_v is similar except for setting the sem_op part of the sembuf structure to 1,
 so that the semaphore becomes available. */

static int semaphore_v(void)
{
    struct sembuf sem_b;
    
    sem_b.sem_num = 0;
    sem_b.sem_op = 1; /* V() */
    sem_b.sem_flg = SEM_UNDO;
    if (semop(sem_id, &sem_b, 1) == -1) {
        fprintf(stderr, "semaphore_v failed\n");
        return(0);
    }
    return(1);
}

root@ubuntu:~/test/SourceCode/ch14# cat semun.h 
#if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)
    /* union semun is defined by including <sys/sem.h> */
#else
    /* according to X/OPEN we have to define it ourselves */
    union semun {
        int val;                    /* value for SETVAL */
        struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */
        unsigned short int *array;  /* array for GETALL, SETALL */
        struct seminfo *__buf;      /* buffer for IPC_INFO */
    };
#endif

先执行
stty -tostop
这个是确保产生的tty的输出的后台程序不会引发系统生成的一个信号
再
./sem1 1 &
./sem1


打印结果就是

root@ubuntu:~/test/SourceCode/ch14# ./sem1 1&  
[1] 14437
root@ubuntu:~/test/SourceCode/ch14# ./sem1
OOXXOOXXOOXXOOXXOOXXXXOOXXOOXXOOXXOOXX
14437 - finished
OO
14438 - finished
root@ubuntu:~/test/SourceCode/ch14# 

发现了没？XX和OO都是成对出现的

信号量就是一个全局的锁，在多进程并行的时候，保持数据一致性的一个东西哦