在Linux中,如何使用带有C的信号量同步两个进程的输出?
我有以下代码:在Linux中,如何使用带有C的信号量同步两个进程的输出?,c,linux,operating-system,semaphore,C,Linux,Operating System,Semaphore,我有以下代码: #include <stdio.h> #include <stdlib.h> #include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> #include <unistd.h> #define SEMAPHORE 0 int main(int argc, char *argv[]) { int semid, pid, j = 1
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <unistd.h>
#define SEMAPHORE 0
int main(int argc, char *argv[])
{
int semid, pid, j = 10;
struct sembuf operation;
key_t key;
key = ftok(argv[0], 'U');
if ((semid = semget(key, 1, IPC_CREAT | 0600)) == -1)
{
perror("Error at semget");
exit(-1);
}
semctl(semid, SEMAPHORE, SETVAL, 1);
/* Child is created */
if ((pid = fork()) == -1)
{
perror("Error at fork");
exit(-1);
}
else if (pid == 0)
{
/* Code corresponding to the child process */
while (j)
{
/* DOWN operation is performed at the semaphore */
operation.sem_flg = 0;
operation.sem_op = -1;
operation.sem_num = SEMAPHORE;
semop(semid, &operation, 1);
printf("I AM THE CHILD PROCESS, IMPRESSION: %d\n", j--);
/* UP operation is performed at the semaphore */
operation.sem_op = 1;
operation.sem_num = SEMAPHORE;
semop(semid, &operation, 1);
}
}
else
{
/* Code corresponding to the parent process */
while (j)
{
/* DOWN operation is performed at the semaphore */
operation.sem_flg = 0;
operation.sem_op = -1;
operation.sem_num = SEMAPHORE;
semop(semid, &operation, 1);
printf("I AM THE FATHER PROCESS, IMPRESSION: %d\n", j--);
/* UP operation is performed at the semaphore */
operation.sem_op = 1;
operation.sem_num = SEMAPHORE;
semop(semid, &operation, 1);
}
}
}
或者先是孩子,然后是父亲。但是,我运行它,它产生的输出如下所示:
I AM THE FATHER PROCESS, IMPRESSION: 10
I AM THE FATHER PROCESS, IMPRESSION: 9
I AM THE FATHER PROCESS, IMPRESSION: 8
I AM THE FATHER PROCESS, IMPRESSION: 7
I AM THE FATHER PROCESS, IMPRESSION: 6
I AM THE FATHER PROCESS, IMPRESSION: 5
I AM THE FATHER PROCESS, IMPRESSION: 4
I AM THE FATHER PROCESS, IMPRESSION: 3
I AM THE FATHER PROCESS, IMPRESSION: 2
I AM THE FATHER PROCESS, IMPRESSION: 1
I AM THE CHILD PROCESS, IMPRESSION: 10
I AM THE CHILD PROCESS, IMPRESSION: 9
I AM THE CHILD PROCESS, IMPRESSION: 8
I AM THE CHILD PROCESS, IMPRESSION: 7
I AM THE CHILD PROCESS, IMPRESSION: 6
I AM THE CHILD PROCESS, IMPRESSION: 5
I AM THE CHILD PROCESS, IMPRESSION: 4
I AM THE CHILD PROCESS, IMPRESSION: 3
I AM THE CHILD PROCESS, IMPRESSION: 2
I AM THE CHILD PROCESS, IMPRESSION: 1
或者有时会有这样的印象,但我没有得到预期的印象:
I AM THE FATHER PROCESS, IMPRESSION: 10
I AM THE FATHER PROCESS, IMPRESSION: 9
I AM THE CHILD PROCESS, IMPRESSION: 10
I AM THE CHILD PROCESS, IMPRESSION: 9
I AM THE CHILD PROCESS, IMPRESSION: 8
I AM THE CHILD PROCESS, IMPRESSION: 7
I AM THE CHILD PROCESS, IMPRESSION: 6
I AM THE CHILD PROCESS, IMPRESSION: 5
I AM THE FATHER PROCESS, IMPRESSION: 8
I AM THE FATHER PROCESS, IMPRESSION: 7
I AM THE FATHER PROCESS, IMPRESSION: 6
I AM THE CHILD PROCESS, IMPRESSION: 4
I AM THE CHILD PROCESS, IMPRESSION: 3
I AM THE CHILD PROCESS, IMPRESSION: 2
I AM THE CHILD PROCESS, IMPRESSION: 1
I AM THE FATHER PROCESS, IMPRESSION: 5
I AM THE FATHER PROCESS, IMPRESSION: 4
I AM THE FATHER PROCESS, IMPRESSION: 3
I AM THE FATHER PROCESS, IMPRESSION: 2
I AM THE FATHER PROCESS, IMPRESSION: 1
我知道进程的执行顺序直接取决于操作系统,但这不是假设当两个进程中的一个进程启动时,另一个进程应该进入运行吗
所以我的问题是,这里到底发生了什么?让p(sem)表示信号量sem的down操作,V(sem)表示信号量sem的up操作
目前,您的代码如下:
// semaphore s1
s1 = 1;
// parent process
P (s1);
// work
V (s1)
// child process
P (s1)
// work
V (s1)
所以这里要说的是,一次只有一个进程可以做“工作”,内核也在做这件事
要同步两个进程的输出,需要执行以下操作
// semaphores, sem-parent, sem-child
sem-parent = 1; sem-child = 0;
// parent process
P (sem-parent);
// work
V (sem-child);
// child process
P (sem-child);
// work
V (sem-parent);
// semaphores, sem-parent, sem-child
sem-parent = 1; sem-child = 0;
// parent process
P (sem-parent);
// work
V (sem-child);
// child process
P (sem-child);
// work
V (sem-parent);