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 $

巴特西

试题--创建三个进程/线程，依次输出 A、B、C

多线程的实现代码如下：

多进程的实现代码如下：

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