Linux IPC实践(11) --System V信号量(1)

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信号量API

#include <sys/types.h>#include <sys/ipc.h>#include <sys/sem.h>int semget(key_t key, int nsems, int semflg);int semctl(int semid, int semnum, int cmd, ...);int semop(int semid, struct sembuf *sops, unsigned nsops);

semget

int semget(key_t key, int nsems, int semflg);

创建/访问一个信号量集

参数:

key: 信号集键(key)

nsems:信号集中信号量的个数

semflg: 由九个权限标志构成，它们的用法和创建文件时使用的mode模式标志一致

返回值：成功返回一个非负整数，即该信号集的标识码；失败返回-1;

此时创建的信号量集中的每一个信号量都会有一个默认值: 0, 如果需要更改该值, 则需要调用semctl函数->更改初始值;

/** 示例1: 封装一个创建一个信号量集函数 该信号量集包含1个信号量;权限为0666**/int sem_create(key_t key){    int semid = semget(key, 1, IPC_CREAT|IPC_EXCL|0666);    if (semid == -1)        err_exit("sem_create error");    return semid;}

/** 示例2: 打开一个信号量集nsems(信号量数量)可以填0,semflg(信号量权限)也可以填0, 表示使用默认的权限打开**/int sem_open(key_t key){    int semid = semget(key, 0, 0);    if (semid == -1)        err_exit("sem_open error");    return semid;}

shmctl

int semctl(int semid, int semnum, int cmd, ...);

控制信号量集

参数

semid:由semget返回的信号集标识码

semnum:信号集中信号量的序号(注意: 从0开始The semaphores in a set are numbered starting at 0.)

cmd:将要采取的动作(常用取值如下)

如果该函数需要第四个参数(有时是不需要第四个参数的, 取决于cmd的取值), 则程序中必须定义如下的联合体:

union semun{    int              val;    /* Value for SETVAL */    struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */    unsigned short  *array;  /* Array for GETALL, SETALL */    struct seminfo  *__buf;  /* Buffer for IPC_INFO (Linux-specific)*/};

//struct semid_ds : Linux内核为System V信号量维护的数据结构struct semid_ds{    struct ipc_perm sem_perm;  /* Ownership and permissions */    time_t          sem_otime; /* Last semop time */    time_t          sem_ctime; /* Last change time */    unsigned long   sem_nsems; /* No. of semaphores in set */};

/** 示例1: 将信号量集semid中的第一个信号量的值设置成为value(SETVAL)注意: semun联合体需要自己给出(从man-page中拷贝出来即可)**/union semun{    int              val;    /* Value for SETVAL */    struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */    unsigned short  *array;  /* Array for GETALL, SETALL */    struct seminfo  *__buf;  /* Buffer for IPC_INFO                                           (Linux-specific) */};int sem_setval(int semid, int value){    union semun su;    su.val = value;    if (semctl(semid, 0, SETVAL, su) == -1)        err_exit("sem_setval error");    return 0;}

/** 示例2: 获取信号量集中第一个信号所关联的值(GETVAL)注意: 此时第四个参数可以不填, 而信号量所关联的值可以通过semctl的返回值返回(the value of semval.)**/int sem_getval(int semid){    int value = semctl(semid, 0, GETVAL);    if (value == -1)        err_exit("sem_getval error");    return value;    return 0;}

/** 示例3: 删除一个信号量集(注意是删除整个集合)IPC_RMID  Immediately  remove(立刻删除)  the  semaphore  set,  awakening  all  processes blocked in semop(2) calls on the set (with an error return and errno set to  EIDRM)[然后唤醒所有阻塞在该信号量上的进程]. The  argument  semnum  is ignored[忽略第二个参数].**/int sem_delete(int semid){    if (semctl(semid, 0, IPC_RMID) == -1)        err_exit("sem_delete error");    return 0;}//测试代码int main(int argc,char *argv[]){    int semid = sem_create(0x1234);//创建一个信号量集    sem_setval(semid, 500);//设置值    cout << sem_getval(semid) << endl;//获取值    sleep(10);    sem_delete(semid);//删除该集合}

/**示例4: 获取/设置信号量的权限注意:一定要设定struct semid_ds结构体, 以指定使用semun的哪个字段**/int sem_getmode(int semid){    union semun su;    // 注意: 下面这两行语句一定要设定.    // (告诉内核使用的semun的哪个字段)    struct semid_ds sd;    su.buf = &sd;    //    if (semctl(semid, 0, IPC_STAT, su) == -1)        err_exit("sem_getmode error");    printf("current permissions is: %o\n", su.buf->sem_perm.mode);    return 0;}int sem_setmode(int semid, char *mode){    union semun su;    // 注意: 下面这两行语句一定要设定.    // (告诉内核使用的semun的哪个字段)    struct semid_ds sd;    su.buf = &sd;    //    sscanf(mode, "%o", (unsigned int *)&su.buf->sem_perm.mode);    if (semctl(semid, 0, IPC_SET, su) == -1)        err_exit("sem_setmode error");    return 0;}

semop

int semop(int semid, struct sembuf *sops, unsigned nsops);

用来操纵一个信号量集, 以实现P,V操作

参数:

semid:是该信号量的标识码，也就是semget函数的返回值

sops:是个指向一个结构数组(如果信号量集中只有一个信号量的话, 只有一个结构体也可)的指针

nsops:所设置的信号量个数(如果nsops>1话, 需要将sops[第二个参数]设置成为一个结构数组, 具体参考Man-Page给出的示例代码), 第三个参数其实也指出第二个参数所表示对象的个数;

//sembuf结构体struct sembuf{    unsigned short sem_num;  /*semaphore number:信号量的编号(从0开始)*/    short          sem_op;   /* semaphore operation(+1, 0, -1) */    short          sem_flg;  /* operation flags: 常用取值为SEM_UNDO(解释见下) */};

sem_op是信号量一次PV操作时加减的数值，一般只会用到两个值，一个是“-1”，也就是P操作，等待信号量变得可用；另一个是“+1”，也就是V操作，发出信号量已经变得可用, 该参数还可以等于0, 表示进程将阻塞直到信号量的值等于0;

sem_flg有三个取值: SEM_UNDO(在进程结束时, 将该进程对信号量的操作复原[即:取消该进程对信号量所有的操作], 推荐使用), IPC_NOWAIT(非阻塞)或0(默认操作, 并不撤销操作);

/** 示例: P,V操作封装 **可以将sembuf的第三个参数设置为IPC_NOWAIT/0, 以查看程序的状态的变化**/int sem_P(int semid){    struct sembuf sops = {0, -1, SEM_UNDO};    if (semop(semid, &sops, 1) == -1)        err_exit("sem_P error");    return 0;}int sem_V(int semid){    struct sembuf sops = {0, +1, SEM_UNDO};    if (semop(semid, &sops, 1) == -1)        err_exit("sem_V error");    return 0;}

/** 信号量综合运用示例:编译完成之后, 直接运行./semtool, 程序将打印该工具的用法;下面的这些函数调用, 只不过是对上面所封装函数的稍稍改动, 理解起来并不困难;**///semtool.cpp#include "Usage.h"int main(int argc,char *argv[]){    int opt = getopt(argc, argv, "cdpvs:gfm:");    if (opt == '?')        exit(EXIT_FAILURE);    else if (opt == -1)    {        usage();        exit(EXIT_FAILURE);    }    key_t key = ftok(".", 's');    int semid;    switch (opt)    {    case 'c':        sem_create(key);        break;    case 'd':        semid = sem_open(key);        sem_delete(semid);        break;    case 'p':        semid = sem_open(key);        sem_P(semid);        sem_getval(semid);        break;    case 'v':        semid = sem_open(key);        sem_V(semid);        sem_getval(semid);        break;    case 's':        semid = sem_open(key);        sem_setval(semid, atoi(optarg));        sem_getval(semid);        break;    case 'g':        semid = sem_open(key);        sem_getval(semid);        break;    case 'f':        semid = sem_open(key);        sem_getmode(semid);        break;    case 'm':        semid = sem_open(key);        sem_setmode(semid, argv[2]);        sem_getmode(semid);        break;    default:        break;    }    return 0;}

//Usage.h#ifndef USAGE_H_INCLUDED#define USAGE_H_INCLUDED#include <iostream>#include <string>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <string.h>#include <sys/types.h>#include <sys/stat.h>#include <sys/ipc.h>#include <sys/wait.h>#include <sys/time.h>#include <sys/msg.h>#include <sys/shm.h>#include <sys/mman.h>#include <sys/sem.h>#include <fcntl.h>#include <signal.h>#include <unistd.h>#include <grp.h>#include <pwd.h>#include <time.h>#include <errno.h>#include <mqueue.h>using namespace std;inline void err_quit(std::string message);inline void err_exit(std::string message);void usage(){    cerr << "Usage:" << endl;    cerr << "./semtool -c        #create" << endl;    cerr << "./semtool -d        #delte" << endl;    cerr << "./semtool -p        #signal" << endl;    cerr << "./semtool -v        #wait" << endl;    cerr << "./semtool -s <val>  #set-value" << endl;    cerr << "./semtool -g        #get-value" << endl;    cerr << "./semtool -f        #print-mode" << endl;    cerr << "./semtool -m <mode> #set-mode" << endl;}int sem_create(key_t key){    int semid = semget(key, 1, IPC_CREAT|IPC_EXCL|0666);    if (semid == -1)        err_exit("sem_create error");    return semid;}int sem_open(key_t key){    int semid = semget(key, 0, 0);    if (semid == -1)        err_exit("sem_open error");    return semid;}union semun{    int              val;    /* Value for SETVAL */    struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */    unsigned short  *array;  /* Array for GETALL, SETALL */    struct seminfo  *__buf;  /* Buffer for IPC_INFO (Linux-specific) */};int sem_getmode(int semid){    union semun su;    // 注意: 下面这两行语句一定要设定.    // (告诉内核使用的semun的哪个字段)    struct semid_ds sd;    su.buf = &sd;    //    if (semctl(semid, 0, IPC_STAT, su) == -1)        err_exit("sem_getmode error");    printf("current permissions is: %o\n", su.buf->sem_perm.mode);    return 0;}int sem_setmode(int semid, char *mode){    union semun su;    // 注意: 下面这两行语句一定要设定.    // (告诉内核使用的semun的哪个字段)    struct semid_ds sd;    su.buf = &sd;    //    sscanf(mode, "%o", (unsigned int *)&su.buf->sem_perm.mode);    if (semctl(semid, 0, IPC_SET, su) == -1)        err_exit("sem_setmode error");    return 0;}int sem_getval(int semid){    int value = semctl(semid, 0, GETVAL);    if (value == -1)        err_exit("sem_getval error");    cout << "current value: " << value << endl;    return value;}int sem_setval(int semid, int value){    union semun su;    su.val = value;    if (semctl(semid, 0, SETVAL, su) == -1)        err_exit("sem_setval error");    return 0;}int sem_delete(int semid){    if (semctl(semid, 0, IPC_RMID) == -1)        err_exit("sem_delete error");    return 0;}// 为了能够打印信号量的持续变化, 因此sem_flg我们并没用SEM_UNDO// 但是我们推荐使用SEM_UNDOint sem_P(int semid){    struct sembuf sops = {0, -1, 0};    if (semop(semid, &sops, 1) == -1)        err_exit("sem_P error");    return 0;}int sem_V(int semid){    struct sembuf sops = {0, +1, 0};    if (semop(semid, &sops, 1) == -1)        err_exit("sem_V error");    return 0;}inline void err_quit(std::string message){    std::cerr << message << std::endl;    exit(EXIT_FAILURE);}inline void err_exit(std::string message){    perror(message.c_str());    exit(EXIT_FAILURE);}#endif // USAGE_H_INCLUDED

附-Makefile

.PHONY: clean all CC = g++ CPPFLAGS = -Wall -gBIN = semtoolSOURCES = $(BIN.=.cpp)all: $(BIN)%.o: %.c     $(CC) $(CPPFLAGS) -c $^ -o $@main: main.o    $(CC) $(CPPFLAGS) $^ -lrt -o $@clean:    -rm -rf $(BIN) *.o bin/ obj/ core

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