Linux互斥与同步应用（五）：system V信号量的互斥与同步
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system V信号量操作类似于posix信号量，但system V信号量的操作要复杂得多，posix信号量使用步骤为sem_init（sem_open）-->sem_wait(sem_post) --> sem_close详见上一节，system V使用不同的函数。
1. 创建和打开信号量函数：semget()。
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semget(key_t key, int nsems, int semflg);
key为ftok返回值或IPC_PRIVATE；
nsems为指定信号量集合中信号量的数量，一旦创建就不能更改，需要大于0，如果等于0则方位一个已存在的集合。
semflg为读写权限值组合。IPC_CREAT（创建新的信号量集合）或IPC_CREAT|IPC_EXCL（当将要创建的信号量集合已经存在时，再试图创建将返回EEXIST）。其实IPC_CREAT和IPC_EXCL的组合和open函数的O_CREAT和O_EXCL组合类似。
2. 对象信号集合进行操作函数：semctl()。
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semctl(int semid, int semnum, int cmd, ...);
semid为信号量标识，函数semget的返回值。
semnum为信号在信号集合中的序号。
cmd为操作的命令：
GETVAL - 返回指定信号量当前值；
SETVAL - 设置指定信号量的值；
IPC_RMIN - 删除信号量集合。
其余命令详见man手册。
初始化一个system V信号量如下：
int sln_seminit(const char *filename, int initval){ int semid; key_t keyid; keyid = ftok(filename, 0); if (keyid < 0) { fprintf(stderr, "ftok: %s\n", strerror(errno)); return -1; } semid = semget(keyid, 1, IPC_CREAT | IPC_EXCL); if (semid >= 0) { // 创建成功，然后修改信号量的初始值为initval printf("=== new sem create! ===\n"); if (semctl(semid, 0, SETVAL, initval) < 0) { printf("semctl: %s\n", strerror(errno)); return -1; } } else if (EEXIST == errno) { //信号量存在，则打开，再次修改信号量的初始值。 printf("=== open a exist sem! ===\n"); semid = semget(keyid, 0, 0); if (semid < 0) { printf("[%d] - semget: %s\n", __LINE__, strerror(errno)); return -1; } if (semctl(semid, 0, SETVAL, initval) < 0) { printf("semctl: %s\n", strerror(errno)); return -1; } } else { //create failed, other reason printf("[%d] - semget: %s, semid: %d\n", __LINE__, strerror(errno), semid); } return semid;}3.信号量PV操作函数：semop()，semtimedop()函数。
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> int semop(int semid, struct sembuf *sops, unsigned nsops); int semtimedop(int semid, struct sembuf *sops, unsigned nsops, struct timespec *timeout);semid为信号量标识，函数semget的返回值；
sops指向信号量操作结构数组指针；
nsops为指向数组中sembuf结构体个数。
struct sembuf { short sem_num; // 要操作的信号量在信号量集里的编号， short sem_op; // 信号量操作 short sem_flg; // 操作表示符};若sem_op 是正数，其值就加到semval上，即释放信号量控制的资源若sem_op 是0，那么调用者希望等到semval变为0，如果semval是0就返回;
若sem_op 是负数，那么调用者希望等待semval变为大于或等于sem_op的绝对值
P、V操作的实现为：
void sln_sem_wait(int semid){ struct sembuf sb; memset(&sb, 0, sizeof(struct sembuf)); sb.sem_num = SLN_SEM_NUM; sb.sem_op = -1; sb.sem_flg = 0; if (semop(semid, &sb, 1) < 0) { printf("shm lock error! semop: %s\n", strerror(errno)); }}void sln_sem_post(int semid){ struct sembuf sb; sb.sem_num = SLN_SEM_NUM; sb.sem_op = 1; sb.sem_flg = 0; //sb.sem_flg = SEM_UNDO; if (semop(semid, &sb, 1) < 0) { printf("shm unlock error! semop: %s\n", strerror(errno)); }}示例代码：
服务进程：
#include <stdio.h>#include <errno.h>#include <sys/types.h>#include <sys/ipc.h> //ftok()#include <string.h>#include <sys/sem.h>#include <sys/shm.h> //shmat()#include <unistd.h> //access()#include <stdlib.h> // exit()#include <signal.h> //signal()#include "shmipc.h"int sem_clt2ser, sem_ser2clt, sem_mutex_id;int sln_shm_get(const char *filename, void **mem, int mem_len){ int shmid; key_t shm_key; shm_key = ftok(filename, 0); if (shm_key < 0) { fprintf(stderr, "ftok: %s\n", strerror(errno)); return -1; } shmid = shmget(shm_key, mem_len, IPC_CREAT); if (shmid < 0) { printf("shmget: %s\n", strerror(errno)); return -1; } *mem = (void *)shmat(shmid, NULL, 0); if ((void *)-1 == *mem) { printf("shmat: %s\n", strerror(errno)); return -1; }// semget(shm_key, 1, IPC_CREAT); return shmid;}void sln_sem_wait_timeout(int semid){ struct sembuf sb; struct timespec tp; memset(&sb, 0, sizeof(struct sembuf)); sb.sem_num = SLN_SEM_NUM; sb.sem_op = -1; sb.sem_flg = 0; tp.tv_sec = time(NULL) + SEM_TIMEOUT_SEC; tp.tv_nsec = 0; if (semtimedop(semid, &sb, 1, &tp) < 0) { printf("shm lock error! semop: %s\n", strerror(errno)); }}void sln_sem_wait(int semid){ struct sembuf sb; memset(&sb, 0, sizeof(struct sembuf)); sb.sem_num = SLN_SEM_NUM; sb.sem_op = -1; sb.sem_flg = 0; if (semop(semid, &sb, 1) < 0) { printf("shm lock error! semop: %s\n", strerror(errno)); }}void sln_sem_post(int semid){ struct sembuf sb; sb.sem_num = SLN_SEM_NUM; sb.sem_op = 1; sb.sem_flg = 0; //sb.sem_flg = SEM_UNDO; if (semop(semid, &sb, 1) < 0) { printf("shm unlock error! semop: %s\n", strerror(errno)); }}void sln_shm_lock(int semid){ struct sembuf sb; memset(&sb, 0, sizeof(struct sembuf)); sb.sem_num = SLN_SEM_NUM; sb.sem_op = -1; //sb.sem_flg = 0; sb.sem_flg = SEM_UNDO; if (semop(semid, &sb, 1) < 0) { printf("shm lock error! semop: %s\n", strerror(errno)); }}void sln_shm_unlock(int semid){ struct sembuf sb; sb.sem_num = SLN_SEM_NUM; sb.sem_op = 1; //sb.sem_flg = 0; sb.sem_flg = SEM_UNDO; if (semop(semid, &sb, 1) < 0) { printf("shm unlock error! semop: %s\n", strerror(errno)); }}int sln_seminit(const char *filename, int initval){ int semid; key_t keyid; keyid = ftok(filename, 0); if (keyid < 0) { fprintf(stderr, "ftok: %s\n", strerror(errno)); return -1; } semid = semget(keyid, 1, IPC_CREAT | IPC_EXCL); if (semid >= 0) { // create success printf("=== new sem create! ===\n"); if (semctl(semid, 0, SETVAL, initval) < 0) { printf("semctl: %s\n", strerror(errno)); return -1; } } else if (EEXIST == errno) { //exist, create failed! printf("=== open a exist sem! ===\n"); semid = semget(keyid, 0, 0); if (semid < 0) { printf("[%d] - semget: %s\n", __LINE__, strerror(errno)); return -1; } if (semctl(semid, 0, SETVAL, initval) < 0) { printf("semctl: %s\n", strerror(errno)); return -1; } } else { //create failed, other reason printf("[%d] - semget: %s, semid: %d\n", __LINE__, strerror(errno), semid); } return semid;}static void sln_create_not_exist_file(const char *filename){ FILE *fp = NULL; if (access(filename, F_OK) < 0) { fp = fopen(filename, "w+"); if (NULL == fp) { fprintf(stderr, "fopen <%s> failed!\n", filename); return; } fclose(fp); }}static void sigint_func(int sig){ if (semctl(sem_clt2ser, 0, IPC_RMID) < 0) { printf("semctl: %s\n", strerror(errno)); } if (semctl(sem_ser2clt, 0, IPC_RMID) < 0) { printf("semctl: %s\n", strerror(errno)); } if (semctl(sem_mutex_id, 0, IPC_RMID) < 0) { printf("semctl: %s\n", strerror(errno)); } exit(0);}int main(int argc, const char *argv[]){ char *str = NULL; sln_create_not_exist_file(SLN_SEM_SERWAIT_FILEPATH); sln_create_not_exist_file(SLN_SEM_CLTWAIT_FILEPATH); sln_create_not_exist_file(SLN_SEM_MUTEX_FILEPATH); sem_clt2ser = sln_seminit(SLN_SEM_SERWAIT_FILEPATH, 0); if (sem_clt2ser < 0) { printf("sync sem init failed!\n"); return -1; } sem_ser2clt = sln_seminit(SLN_SEM_CLTWAIT_FILEPATH, 0); if (sem_clt2ser < 0) { printf("sync sem init failed!\n"); return -1; } sem_mutex_id = sln_seminit(SLN_SEM_MUTEX_FILEPATH, 1); if (sem_mutex_id < 0) { printf("mutex sem init failed!\n"); return -1; } //signal(SIGINT, sigint_func); for (;;) { printf("--------start wait!---semval: %d------\n", semctl(sem_clt2ser, 0, GETVAL, 0)); sln_sem_wait(sem_clt2ser); printf("=====stat handle!========\n"); sln_shm_lock(sem_mutex_id); sln_shm_get(SLN_SEM_MUTEX_FILEPATH, (void **)&str, 32); sleep(6); printf("str: %s\n", str); if (shmdt(str) < 0) { printf("shmdt: %s\n", strerror(errno)); } sln_shm_unlock(sem_mutex_id); printf("=====handle ok!========\n"); sln_sem_post(sem_ser2clt); } return 0;}客户进程代码：
#include <stdio.h>#include <errno.h>#include <string.h>#include <sys/types.h>#include <sys/ipc.h> //ftok()#include <sys/sem.h>#include "shmipc.h"int sln_shm_get(const char *filename, void **mem, int mem_len){ int shmid; key_t shm_key; shm_key = ftok(filename, 0); if (shm_key < 0) { fprintf(stderr, "ftok: %s\n", strerror(errno)); return -1; } shmid = shmget(shm_key, mem_len, IPC_CREAT); if (shmid < 0) { printf("shmget: %s\n", strerror(errno)); return -1; } *mem = (void *)shmat(shmid, NULL, 0); if ((void *)-1 == *mem) { printf("shmat: %s\n", strerror(errno)); return -1; } return shmid;}void sln_sem_wait_timeout(int semid){ struct sembuf sb; struct timespec tp; memset(&sb, 0, sizeof(struct sembuf)); sb.sem_num = SLN_SEM_NUM; sb.sem_op = -1; sb.sem_flg = 0; tp.tv_sec = time(NULL) + SEM_TIMEOUT_SEC; tp.tv_nsec = 0; /* if (semtimedop(semid, &sb, 1, &tp) < 0) { printf("shm lock error! semop: %s\n", strerror(errno)); } */ if (semop(semid, &sb, 1) < 0) { printf("shm lock error! semop: %s\n", strerror(errno)); }}void sln_sem_wait(int semid){ struct sembuf sb; memset(&sb, 0, sizeof(struct sembuf)); sb.sem_num = SLN_SEM_NUM; sb.sem_op = -1; sb.sem_flg = 0; if (semop(semid, &sb, 1) < 0) { printf("shm lock error! semop: %s\n", strerror(errno)); }}void sln_sem_post(int semid){ struct sembuf sb; sb.sem_num = SLN_SEM_NUM; sb.sem_op = 1; sb.sem_flg = 0; //sb.sem_flg = SEM_UNDO; if (semop(semid, &sb, 1) < 0) { printf("shm unlock error! semop: %s\n", strerror(errno)); }}void sln_shm_lock(int semid){ struct sembuf sb; memset(&sb, 0, sizeof(struct sembuf)); sb.sem_num = SLN_SEM_NUM; sb.sem_op = -1; sb.sem_flg = 0; if (semop(semid, &sb, 1) < 0) { printf("shm lock error! semop: %s\n", strerror(errno)); }}void sln_shm_unlock(int semid){ struct sembuf sb; sb.sem_num = SLN_SEM_NUM; sb.sem_op = 1; sb.sem_flg = 0; if (semop(semid, &sb, 1) < 0) { printf("shm unlock error! semop: %s\n", strerror(errno)); }}int sln_semget(const char *filename){ int semid; key_t keyid; keyid = ftok(filename, 0); if (keyid < 0) { fprintf(stderr, "ftok: %s\n", strerror(errno)); return -1; } return semget(keyid, 0, 0);}int main(int argc, const char *argv[]){ char *str = NULL; int sem_clt2ser, sem_ser2clt, sem_mutex_id; sem_clt2ser = sln_semget(SLN_SEM_SERWAIT_FILEPATH); if (sem_clt2ser < 0) { printf("semget: %s\n", strerror(errno)); return -1; } sem_ser2clt = sln_semget(SLN_SEM_CLTWAIT_FILEPATH); if (sem_ser2clt < 0) { printf("semget: %s\n", strerror(errno)); return -1; } sem_mutex_id = sln_semget(SLN_SEM_MUTEX_FILEPATH); if (sem_mutex_id < 0) { printf("mutex sem failed!\n"); return -1; } printf("--------cli wait!-sem_mutex_id: %d--------\n", sem_mutex_id); sln_shm_lock(sem_mutex_id); printf("--------cli wait ok !---------\n"); sln_shm_get(SLN_SEM_MUTEX_FILEPATH, (void **)&str, 32); strcpy(str, argv[1]); sln_shm_unlock(sem_mutex_id); sln_sem_post(sem_clt2ser); sln_sem_wait(sem_ser2clt); printf("=========set: %s=========\n", str); return 0;}源码下载：
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