进程间通信（IPC）:信号量（Semaphore）
假定有一个信号量变量sv，
P(sv)：用于等待，如果sv大于0，就给它减去1，如果它的值等于0，就挂起该进程的执行
V(sv)：用于发送信号，如果有其他进程因等待sv而挂起，就让它恢复运行，如果没有进程因等待sv而被挂起，就给它加1
semaphore sv=1;
loop forever{
P(sv);
critical code section;
V(sv);
noncritical code section;
}
信号量函数定义如下所示：
#include<sys/sem.h>
int semctl(int sem_id, int sem_num, int command, ...);//用来直接控制信号量信息
int semget(key_t key, int num_sems, int sem_flags);//创建一个新信号量或取得一个已有信号量的键
int semop(int sem_id, struct sembuf *sem_ops, size_t num_sem_ops);//用于改变信号量的值
/* After the #includes, the function prototypes and the global variable, we come to the main function. There the semaphore is created with a call to semget, which returns the semaphore ID. If the program is the first to be called (i.e. it's called with a parameter and argc > 1), a call is made to set_semvalue to initialize the semaphore and op_char is set to X. */#include <unistd.h>#include <stdlib.h>#include <stdio.h>#include <sys/sem.h>#include "semun.h"static int set_semvalue(void);static void del_semvalue(void);static int semaphore_p(void);static int semaphore_v(void);static int sem_id;int main(int argc, char *argv[]){ int i; int pause_time; char op_char = 'O'; srand((unsigned int)getpid()); sem_id = semget((key_t)1234, 1, 0666 | IPC_CREAT); if (argc > 1) { if (!set_semvalue()) { fprintf(stderr, "Failed to initialize semaphore\n"); exit(EXIT_FAILURE); } op_char = 'X'; sleep(2); }/* Then we have a loop which enters and leaves the critical section ten times. There, we first make a call to semaphore_p which sets the semaphore to wait, as this program is about to enter the critical section. */ for(i = 0; i < 10; i++) { if (!semaphore_p()) exit(EXIT_FAILURE); printf("%c", op_char);fflush(stdout); pause_time = rand() % 3; sleep(pause_time); printf("%c", op_char);fflush(stdout);/* After the critical section, we call semaphore_v, setting the semaphore available, before going through the for loop again after a random wait. After the loop, the call to del_semvalue is made to clean up the code. */ if (!semaphore_v()) exit(EXIT_FAILURE); pause_time = rand() % 2; sleep(pause_time); } printf("\n%d - finished\n", getpid()); if (argc > 1) { sleep(10); del_semvalue(); } exit(EXIT_SUCCESS);}/* The function set_semvalue initializes the semaphore using the SETVAL command in a semctl call. We need to do this before we can use the semaphore. */static int set_semvalue(void){ union semun sem_union; sem_union.val = 1; if (semctl(sem_id, 0, SETVAL, sem_union) == -1) return(0); return(1);}/* The del_semvalue function has almost the same form, except the call to semctl uses the command IPC_RMID to remove the semaphore's ID. */static void del_semvalue(void){ union semun sem_union;if (semctl(sem_id, 0, IPC_RMID, sem_union) == -1) fprintf(stderr, "Failed to delete semaphore\n");}/* semaphore_p changes the semaphore by -1 (waiting). */static int semaphore_p(void){ struct sembuf sem_b; sem_b.sem_num = 0; sem_b.sem_op = -1; /* P() */ sem_b.sem_flg = SEM_UNDO; if (semop(sem_id, &sem_b, 1) == -1) { fprintf(stderr, "semaphore_p failed\n"); return(0); } return(1);}/* semaphore_v is similar except for setting the sem_op part of the sembuf structure to 1, so that the semaphore becomes available. */static int semaphore_v(void){ struct sembuf sem_b; sem_b.sem_num = 0; sem_b.sem_op = 1; /* V() */ sem_b.sem_flg = SEM_UNDO; if (semop(sem_id, &sem_b, 1) == -1) { fprintf(stderr, "semaphore_v failed\n"); return(0); } return(1);}