Linux IPC实践 --System V信号量(1)

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_UNDO
int 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 -g
BIN = semtool
SOURCES = $(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