IPC-sem 信号量

       int semget(key_t key, int nsems, int semflg);       int semop(int semid, struct sembuf *sops, unsigned nsops);The elements of this structure are of type struct sembuf, containing the following members:  unsigned short sem_num;  /* semaphore number */           short          sem_op;   /* semaphore operation */           short          sem_flg;  /* operation flags */       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) */           }; 

[root@localhost ch14]# cat semun.h #if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)    /* union semun is defined by including <sys/sem.h> */#else    /* according to X/OPEN we have to define it ourselves */    union semun {        int val;                    /* value for SETVAL */        struct semid_ds *buf;       /* buffer for IPC_STAT, IPC_SET */        unsigned short int *array;  /* array for GETALL, SETALL */        struct seminfo *__buf;      /* buffer for IPC_INFO */    };#endif[root@localhost ch14]# cat sem1.c /* 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 = 'a';    srand((unsigned int)getpid());        sem_id = semget((key_t)1234, 1, 0666 | IPC_CREAT);    printf("%d\n",sem_id);    if (argc > 1) {        if (!set_semvalue()) {            fprintf(stderr, "Failed to initialize semaphore\n");            exit(EXIT_FAILURE);        }       op_char = 'b';      // op_char = argv[1];        sleep(5);    }/* 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;pause_time = 1;        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;pause_time =1;        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);}[root@localhost ch14]# 

[root@localhost ch14]# ./sem1 9&[2] 7345[root@localhost ch14]# 557056./sem1 557056aaaaaaaabbaabbaabbaabbaabbaabbaab7348 - finishedb[root@localhost ch14]# bbbbbb7345 - finished

应该先运行一个带参数的sem1以设置semun.val的初始值，不过它有一个初始值是信号量为不可用状态，
所以先运行一个没有参数的sem1也行，但它在获取信号量的时候，会被阻塞
直到运行了一个带参数的sem1，来设置了semun.val的初始值，信号量变为可用
然后二者竞争信号量