#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
key_t semid;
struct sembuf sem_down = {0, -1, 0};
struct sembuf sem_up = {0, 1, 0};
union semun {
int val;
struct semid_ds *buf;
unsigned short *array;
struct seminfo *_buf;
};
void down()
{
semop(semid, &sem_down, 1);
}
void up()
{
semop(semid, &sem_up, 1);
}
int main()
{
int ret = -1;
union semun arg;
arg.val = 1;
semid = semget((key_t)22345, 1, IPC_CREAT|0666);
if(semid == -1) {
perror("semget error");
return 1;
}
ret = semctl(semid, 1, SETVAL, arg);
if(ret == -1) {
perror("semctl error");
return 1;
}
if(semid != -1) {
down();
printf("hello world.\n");
sleep(10);
up();
}
return 0;
}
以上代碼編譯可以通過,但是運行的時候,出現semctl error: Invalid argument
錯誤。
問題出在semctl
函數的第二個參數處。
有關semctl
函數的使說明,man手冊的說明如下:
semctl() performs the control operation specified by cmd on the semaphore set identified by semid, or on the semnum-th semaphore of
that set. (The semaphores in a set are numbered starting at 0.)
注意括號裏邊的描述,信號在信號量集中的編號是從0開始的,而我們這裏的第二個參數是1,並且創建的信號量集中只有一個信號,因此會出現參數無效的錯誤。
修改完成之後如下:
ret = semctl(semid, 0, SETVAL, arg);