Linux进程间通信(IPC)编程实践（九）System V信号量---封装一个信号量操作的工具集


System信号量集主要API

#include
#include
#include
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);
/** 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;
}
int semctl(int semid, int semnum, int cmd, ...);

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;
}[cpp] view plaincopy在CODE上查看代码片派生到我的代码片
int semop(int semid, struct sembuf *sops, unsigned nsops);[cpp] view plaincopy在CODE上查看代码片派生到我的代码片
//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) */
};

 

/** 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;
}

 

下面我们封装一个信号量操作函数工具，将主要的操作封装起来，可以像命令一样使用。
 
/** 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
#include 

#include 
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#include 
#include


#include
#include 
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#include 
#include
#include 
#include
#include 
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#include 
#include
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#include
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 #set-value" << endl;
cerr << "./semtool -g #get-value" << endl;
cerr << "./semtool -f #print-mode" << endl;
cerr << "./semtool -m #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：ftok函数

系统建立IPC通讯（如消息队列、共享内存时）必须指定一个ID值。通常情况下，该id值通过ftok函数得到。

ftok原型如下：

key_t ftok( char * fname, int id )
fname就时你指定的文件名(该文件必须是存在而且可以访问的)，id是子序号，虽然为int，但是只有8个比特被使用(0-255)。
返回值：

当成功执行的时候，一个key_t值将会被返回，否则 -1 被返回。

在我们获取到key之后，就可以使用该key作为某种方法的进程间通信的key值。

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