试题--创建三个进程/线程，依次输出 A、B、C

这是一道机试题，大概的预期执行结果如下图所示

最近刚好在学习linux编程，便使用多线程及多进程分别实现了一遍，其中多线程较为简单，使用0/1信号量在线程间实现生产者/消费者即可；多进程则稍微复杂一些，信号量必须设置为进程间通信，且存放在共享内存中，才能被多个进程访问。

多线程的实现代码如下：

 /*================================================================
  *   Copyright (C) 2019 Ltd. All rights reserved.
  *
  *   File Name ：fork_test.c
  *   Author    ：Hamilton
  *   Date      ：2019-06-05
  *   Descriptor：
  *
  ================================================================*/

 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
 #include <unistd.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <semaphore.h>
 #include <sys/mman.h>
 #include <sys/stat.h>        /* For mode constants */
 #include <fcntl.h>           /* For O_* constants */
 #include <stdbool.h>
 #include<pthread.h>

 #define SHARED_MEM_NAME "/PRINTABC"

 struct shared_memory {
     sem_t sem[];
 };

 const char ch[] = {'A', 'B', 'C'};
 int fd_shm = -;
 bool finish = false;
 struct shared_memory *shared_mem_ptr = NULL;

 void err_check(int ret)
 {
     if (ret < )
     {
         perror("error: %d. \n");
         exit(ret);
     }
 }

 void err_exit(char *str)
 {
     perror(str);
     exit();
 }

 void* thread_handler(void* arg)
 {
     int index = *(char*)arg - 'A';
     int pre = index ? (index - ) : ;

     usleep();

     while (!finish)
     {
         sem_wait(&shared_mem_ptr->sem[pre]);
         printf("%c \n", ch[index]);
         sem_post(&shared_mem_ptr->sem[index]);
         usleep();
     }
     pthread_exit(NULL);
 }

 void sig_handler(int signo)
 {
     if (signo == SIGINT)
     {
         printf("received SIGINT\n");
         finish = true;
     }
 }

 int main()
 {
     int err, i = ;

     pthread_t tid[];

     if (signal(SIGINT, sig_handler) == SIG_ERR)
         printf("\ncan't catch SIGINT\n");

     // Get shared memory
     if ((fd_shm = shm_open (SHARED_MEM_NAME, O_RDWR | O_CREAT, )) < )
         err_exit ("shm_open");

     if (ftruncate (fd_shm, sizeof (struct shared_memory)) == -)
         err_exit ("ftruncate");

     if ((shared_mem_ptr = mmap (NULL, sizeof (struct shared_memory), PROT_READ | PROT_WRITE, MAP_SHARED,
                     fd_shm, )) == MAP_FAILED)
         err_exit ("mmap");

     err_check(sem_init(&shared_mem_ptr->sem[], , ));
     err_check(sem_init(&shared_mem_ptr->sem[], , ));
     err_check(sem_init(&shared_mem_ptr->sem[], , ));

     while(i < )
     {
         err = pthread_create(&(tid[i]), NULL, &thread_handler, (void*)&ch[i]);
         if (err != )
             printf("\ncan't create thread :[%s]", err);
         else
             printf("\n Thread created successfully\n");
         i++;
     }

     for (i = ; i < ; i++)
     {
         pthread_join(tid[i], NULL);
         sem_destroy(&shared_mem_ptr->sem[i]);
     }
     shm_unlink(SHARED_MEM_NAME);
     printf("all threads have finished.\n");
     return ;
 }

多进程的实现代码如下：

 /*================================================================
  *   Copyright (C) 2019 Ltd. All rights reserved.
  *
  *   File Name ：fork_test.c
  *   Author    ：Hamilton
  *   Date      ：2019-06-05
  *   Descriptor：
  *
  ================================================================*/

 #include <sys/types.h>
 #include <sys/wait.h>
 #include <unistd.h>
 #include <unistd.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <semaphore.h>
 #include <sys/mman.h>
 #include <sys/stat.h>        /* For mode constants */
 #include <fcntl.h>           /* For O_* constants */
 #include <stdbool.h>

 #define SHARED_MEM_NAME "/PRINTABC"

 struct shared_memory {
     sem_t sem[];
 };

 const char ch[] = {'A', 'B', 'C'};
 int fd_shm = -;
 bool finish = false;
 struct shared_memory *shared_mem_ptr = NULL;

 void err_check(int ret)
 {
     if (ret < )
     {
         perror("error: %d. \n");
         exit(ret);
     }
 }

 void err_exit(char *str)
 {
     perror(str);
     exit();
 }

 void process_handler(int index)
 {
     int pre = index ? (index - ) : ;

     while (!finish)
     {
         sem_wait(&shared_mem_ptr->sem[pre]);
         printf("%c \n", ch[index]);
         sem_post(&shared_mem_ptr->sem[index]);
         usleep();
     }
 }

 void sig_handler(int signo)
 {
     if (signo == SIGINT)
     {
         printf("received SIGINT\n");
         finish = true;
     }
 }

 int main()
 {
     pid_t pid;

     if (signal(SIGINT, sig_handler) == SIG_ERR)
         printf("\ncan't catch SIGINT\n");

     // Get shared memory
     if ((fd_shm = shm_open (SHARED_MEM_NAME, O_RDWR | O_CREAT, )) < )
         err_exit ("shm_open");

     if (ftruncate (fd_shm, sizeof (struct shared_memory)) == -)
         err_exit ("ftruncate");

     if ((shared_mem_ptr = mmap (NULL, sizeof (struct shared_memory), PROT_READ | PROT_WRITE, MAP_SHARED,
                     fd_shm, )) == MAP_FAILED)
         err_exit ("mmap");

     err_check(sem_init(&shared_mem_ptr->sem[], , ));
     err_check(sem_init(&shared_mem_ptr->sem[], , ));
     err_check(sem_init(&shared_mem_ptr->sem[], , ));

     pid = fork();

     if (pid < )
     {
         printf("Fork error.\n\n");
         exit();
     }
     else if (pid == )
     {
         printf("Process A, pid[%d]. \n", (int)getpid());
         usleep();
         process_handler();
     }
     else
     {
         pid = fork();

         if (pid < )
         {
             printf("Fork error.\n\n");
             exit();
         }
         else if (pid == )
         {
             printf("Process B, pid[%d]. \n", (int)getpid());
             process_handler();
         }
         else
         {
             printf("Process C, pid[%d]. \n", (int)getpid());
             process_handler();
             wait(NULL);
             sem_destroy(&shared_mem_ptr->sem[]);
             sem_destroy(&shared_mem_ptr->sem[]);
             sem_destroy(&shared_mem_ptr->sem[]);
             shm_unlink(SHARED_MEM_NAME);
             printf("all processes have finished.\n");
         }
     }
     return ;
 }

编译命令为：

gcc threadabc.c -o threadabc -lpthread -lrt

或者

gcc forkabc.c -o forkabc -lpthread -lrt

因使用到了多线程/进程，需连接 -lpthread；使用到了POSIX的信号量/共享内存相关，则需要连接 -lrt。关于linux多线程/多进程的开发及API接口的使用，可翻阅我近期摘抄的一些文章。

运行结果：

pi@raspberrypi:~/code/ipc/print_abc $ ./threadabc

 Thread created successfully

 Thread created successfully

 Thread created successfully
A
B
C
A
B
C
A
B
C
^Creceived SIGINT
all threads have finished.
pi@raspberrypi:~/code/ipc/print_abc $ ./forkabc
Process C, pid[].
Process A, pid[].
Process B, pid[].
A
B
C
A
B
C
A
B
C
A
B
C
^Creceived SIGINT
received SIGINT
received SIGINT
all processes have finished.
pi@raspberrypi:~/code/ipc/print_abc $