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