函數(shù)原型

/* Semaphore control operation.  */
extern int semctl (int __semid, int __semnum, int __cmd, ...) __THROW;

/* Get semaphore.  */
extern int semget (key_t __key, int __nsems, int __semflg) __THROW;

/* Operate on semaphore.  */
extern int semop (int __semid, struct sembuf *__sops, size_t __nsops) __THROW;

semget 函數(shù)介紹

int semget (key_t __key, int __nsems, int __semflg)绍撞；
如果key不存在藕筋，且?guī)в衏teate標(biāo)志則創(chuàng)建，如果已經(jīng)存在則直接返回id

• key：ipc的鍵骚露，用于與semid匹配
• nsems：信號(hào)集中信號(hào)的數(shù)量
• flag：創(chuàng)建的控制選項(xiàng)

semctl 函數(shù)介紹

int semctl (int __semid, int __semnum, int __cmd, ...);

• semid 通過(guò)semget 返回的id
• semnum 要操作第幾個(gè)信號(hào)量
• cmd 命令，支持的命名如下

semop 函數(shù)介紹

int semop (int __semid, struct sembuf *__sops, size_t __nsops)
對(duì)信號(hào)量集中的信號(hào)量進(jìn)行加減操作

• semid: semphore 句柄
• ops 操作選項(xiàng)

struct sembuf {
    unsigned short sem_num; /* 需要操作的信號(hào)量編號(hào) */
    short sem_op; /* 對(duì)信號(hào)量值進(jìn)行加減的值 */
    short sem_flg; /* 操作選項(xiàng) */
}

如果sem_op 為正值聚谁，表示對(duì)信號(hào)量進(jìn)行v操作母剥，會(huì)將信號(hào)量的值加上sem_op的值
如果sem_op 為負(fù)值，表示對(duì)信號(hào)量進(jìn)行p操作垦巴，會(huì)將信號(hào)量的值減上sem_op絕對(duì)值,
如果減上之后小于0媳搪，如果指定了IPC_NOWAIT，則semop返回出錯(cuò)返回EAGAIN骤宣，如果沒(méi)有指定則會(huì)阻塞

/* Mode bits for `msgget', `semget', and `shmget'.  */
#define IPC_CREAT   01000       /* Create key if key does not exist. */
#define IPC_EXCL    02000       /* Fail if key exists.  */
#define IPC_NOWAIT  04000       /* Return error on wait.  */
后面3位是用戶(hù)秦爆、用戶(hù)組、其他用戶(hù)的讀寫(xiě)執(zhí)行權(quán)限憔披，如 0777
創(chuàng)建的時(shí)候要帶上IPC_CREAT 和IPC_EXCL標(biāo)志

/**
 * @file sem_lsw.c
 * @author lisiwen (lisiwen945@163.com)
 * @brief 實(shí)現(xiàn)兩個(gè)進(jìn)程的同步讀寫(xiě)
 * @version 0.1
 * @date 2021-01-28
 *
 * @copyright Copyright (c) 2021
 *
 */
#include <apue.h>
#include <sys/sem.h>

union semun {
    int val;
    struct semid_ds *buf;
    unsigned short *array;
};

#define SEM_SEND 0
#define SEM_RECV 1

key_t key = 128;

// 創(chuàng)建信號(hào)量集等限，初始化semums個(gè)信號(hào)燈 和信號(hào)燈的值
int SemCreate(int value, int semnums) {
    // 創(chuàng)建信號(hào)箱集
    int semid = semget(key, semnums, IPC_CREAT | IPC_EXCL | 0777);
    if (semid < 0) {
        return -1;
    }
    union semun un;
    unsigned short *array = (unsigned short *)calloc(semnums, sizeof(unsigned short));
    for (int i = 0; i < semnums; i++) {
        array[i] = value;
    }
    un.array = array;
    // 初始化信號(hào)量集中所有信號(hào)量燈的值
    if (semctl(semid, 0, SETALL, un) < 0) {
        perror("semctl error");
        return -1;
    }
    free(array);
    printf("create semaphore %d success\n", semid);
    return semid;
}

// 對(duì)信號(hào)量集中的具體的信號(hào)燈作PV操作, semnum 是要操作的第幾個(gè)信號(hào)
// p 操作對(duì)信號(hào)的value -1
// v 操作對(duì)信號(hào)的value +1
void SemP(int semnum) {
    int semid = semget(key, 2, 0777);
    if (semid < 0) {
        perror("semget error\n");
        return;
    }
    // 定義sembuf類(lèi)型的結(jié)構(gòu)體數(shù)組，放置若干個(gè)結(jié)構(gòu)體變量
    // SEM_UNDO表示進(jìn)程退出如果沒(méi)有釋放芬膝，則代為釋放
    struct sembuf ops[] = {{semnum, -1, SEM_UNDO}};
    if (semop(semid, ops, sizeof(ops) / sizeof(struct sembuf)) < 0) {
        perror("semop error");
    }

}
void SemV(int semnum) {
    int semid = semget(key, 2, 0777);
    if (semid < 0) {
        perror("semget error\n");
        return;
    }
    struct sembuf ops[] = {{semnum, 1, SEM_UNDO}};
    if (semop(semid, ops, sizeof(ops) / sizeof(struct sembuf)) < 0) {
        perror("semop error");
    }
}

// 銷(xiāo)毀信號(hào)量集
void SemDel() {
    int semid = semget(key, 2, 0777);
    if (semid < 0) {
        perror("semget error\n");
        return;
    }
    // 0 表示對(duì)所有信號(hào)量進(jìn)行操作
    if (semctl(semid, 0, IPC_RMID, NULL) < 0) {
        perror("semop error");
    }
    printf("del semaphore %d success\n", semid);
}

int main(int argc, char* argv[])
{
    if (argc < 2) {
        printf("%s arg", argv[0]);
    }

    if (strcmp(argv[1], "send") == 0) {
        for (int i = 0; i < 10; i++) {
            sleep(1);
            printf("send num %d\n", i);
            SemV(SEM_SEND);
            SemP(SEM_RECV);
        }
    } else if (strcmp(argv[1], "recv") == 0) {
        for (int i = 0; i < 10; i++) {
            SemP(SEM_SEND);
            printf("recv num %d\n", i);
            SemV(SEM_RECV);
        }
    } else if (strcmp(argv[1], "create") == 0) {
        int value = 0; // 初始value為0
        int semnums = 2; // 構(gòu)建兩個(gè)信號(hào)量
        SemCreate(value, semnums);
    } else if (strcmp(argv[1], "del") == 0) {
        SemDel();
    }
}