Linux Debugging Questions & Answers - System-V IPCs

This set of Linux Debugging questions and answers focuses on the System-V IPCs i.e. Message Queues, Shared Memory and Semaphores.

1. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<fcntl.h>
```
```
   #include<sys/stat.h>
```
```
   #include<semaphore.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       sem_t* sem_id;
```

       sem_id = sem_open("sem_value",O_CREAT,0666,0);

```
       if(sem_id == SEM_FAILED)
```
```
           perror("sem_open");
```
```
       sem_wait(sem_id);
```
```
       printf("Sanfoundry\n");
```
```
       if(sem_close(sem_id) == -1)
```
```
           perror("sem_close");    
```
```
       return 0;
```
```
   }
```

a) this program will print the string “Sanfoundry”
b) this process will block
c) segmentaion fault
d) none of the mentioned
View Answer

Answer: b
Explanation: The initial value of semaphore is 0. As we call the sem_wait() in this program, it blocks the process.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lpthread
[root@localhost sanfoundry]# ./san
^Z
[37]+ Stopped ./san
[root@localhost sanfoundry]#

2. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<fcntl.h>
```
```
   #include<sys/stat.h>
```
```
   #include<semaphore.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       sem_t* sem_id;
```
```
       int value;
```

       sem_id = sem_open("sem_value",O_CREAT,0666,0);

```
       if(sem_id == SEM_FAILED)
```
```
           perror("sem_open");
```

       if(sem_getvalue(sem_id,&value) == -1)

```
           perror("sem_getvalue");
```
```
       printf("%d\n",value);
```
```
       sem_wait(sem_id);
```
```
       printf("Sanfoundry\n");
```
```
       if(sem_close(sem_id) == -1)
```
```
           perror("sem_close");
```
```
       return 0;
```
```
   }
```

a) 0
b) Sanfoundry
c) Both 0 and Sanfoundry
d) None of the mentioned
View Answer

Answer: a
Explanation: The sem_getvalue() is used to get the current value of the semaphore.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lpthread
[root@localhost sanfoundry]# ./san
0
^Z
[58]+ Stopped ./san
[root@localhost sanfoundry]#

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3. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<fcntl.h>
```
```
   #include<sys/stat.h>
```
```
   #include<semaphore.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       sem_t* sem_id;
```

       sem_id = sem_open("sem_value",O_CREAT,0666,0);

```
       if(sem_id == SEM_FAILED)
```
```
           perror("sem_open");
```
```
       sem_post(sem_id);
```
```
       printf("Sanfoundry\n");
```
```
       if(sem_close(sem_id) == -1)
```
```
           perror("sem_close");
```
```
       return 0;
```
```
   }
```

a) this process will block
b) this program will print the string “Sanfoundry”
c) segmentation fault
d) none of the mentioned
View Answer

Answer: b
Explanation: The initial value of semaphore is 0 but the sem_post() increments the value of semaphore by 1.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lpthread
[root@localhost sanfoundry]# ./san
Sanfoundry
[root@localhost sanfoundry]#

4. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<fcntl.h>
```
```
   #include<sys/stat.h>
```
```
   #include<semaphore.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       sem_t* sem_id;
```

       sem_id = sem_open("sem_value",O_CREAT,0666,0);

```
       if(sem_id == SEM_FAILED)
```
```
           perror("sem_open");
```
```
       if(sem_close(sem_id) == -1)
```
```
           perror("sem_close");
```
```
       sem_wait(sem_id);
```
```
       printf("Sanfoundry\n");
```
```
       return 0;
```
```
   }
```

a) this process will block
b) this program will print the string “Sanfoundry”
c) segmentation fault
d) none of the mentioned
View Answer

Answer: c
Explanation: The sem_wait() is trying to use the semaphore when it is already closed.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lpthread
[root@localhost sanfoundry]# ./san
Segmentation fault (core dumped)
[root@localhost sanfoundry]#

5. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<fcntl.h>
```
```
   #include<sys/stat.h>
```
```
   #include<semaphore.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       sem_t* sem_id;
```
```
       int value;
```

       sem_id = sem_open("new_13",O_CREAT,0666,3);

```
       if(sem_id == SEM_FAILED)
```
```
           perror("sem_open");
```
```
       sem_wait(sem_id);
```
```
       sem_wait(sem_id);
```
```
       sem_wait(sem_id);
```
```
       sem_wait(sem_id);
```
```
       sem_post(sem_id);
```
```
       sem_post(sem_id);
```
```
       sem_getvalue(sem_id,&value);
```
```
       printf("%d\n",value);
```
```
       if(sem_close(sem_id) == -1)
```
```
           perror("sem_close");
```
```
       return 0;
```
```
   }
```

a) 2
b) 3
c) 0
d) none of the mentioned
View Answer

Answer: d
Explanation: This process will block when the sem_wait() has been called last time in this program.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lpthread
[root@localhost sanfoundry]# ./san
^Z
[64]+ Stopped ./san
[root@localhost sanfoundry]#

6. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<fcntl.h>
```
```
   #include<sys/stat.h>
```
```
   #include<sys/mman.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       int s_id;
```

       s_id = shm_open("shared_mem",O_CREAT|O_RDWR,0666);

```
       printf("%d\n",s_id);
```

       if(shm_unlink("shared_mem") == -1)

```
           perror("shm_unlink");
```
```
       return 0;
```
```
   }
```

a) -1
b) 1
c) 2
d) 3
View Answer

Answer: d
Explanation: On success the shm_open() returns a nonnegative file descriptor.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lrt
[root@localhost sanfoundry]# ./san
3
[root@localhost sanfoundry]

7. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<fcntl.h>
```
```
   #include<sys/stat.h>
```
```
   #include<sys/mman.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       int s_id;
```
```
       int *ptr;
```

       s_id = shm_open("shared_mem",O_CREAT|O_RDWR,0666);

```
       if(s_id == -1)
```
```
           perror("shm_open");
```

       ptr = mmap(NULL,100,PROT_WRITE,MAP_PRIVATE,s_id,0);

```
       if(ptr == MAP_FAILED);
```
```
           perror("mmap");
```
```
       if(munmap(ptr,100) == -1)
```
```
           perror("munmap");
```

       if(shm_unlink("shared_mem") == -1)

```
           perror("shm_unlink");
```
```
       return 0;
```
```
   }
```

a) mmap: Success
b) mmap: Failure
c) munmap: Success
d) munmap: Failure
View Answer

Answer: a
Explanation: Memory of the 100 bytes is mapped successfully as shared memory.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lrt
[root@localhost sanfoundry]# ./san
mmap: Success
[root@localhost sanfoundry]#

8. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<fcntl.h>
```
```
   #include<sys/stat.h>
```
```
   #include<sys/mman.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       int s_id;
```
```
       int *ptr;
```

       s_id = shm_open("shared_mem",O_CREAT|O_RDWR,0666);

```
       if(s_id == -1)
```
```
           perror("shm_open");
```

       ptr = mmap(NULL,100,PROT_WRITE,MAP_PRIVATE,s_id,0);

```
       if(ptr == MAP_FAILED);
```
```
           perror("mmap");
```

       ptr = mmap(ptr,100,PROT_WRITE,MAP_PRIVATE,s_id,0);

```
       if(ptr == MAP_FAILED);
```
```
           perror("mmap");
```
```
       if(munmap(ptr,100) == -1)
```
```
           perror("munmap");
```

       if(shm_unlink("shared_mem") == -1)

```
           perror("shm_unlink");
```
```
       return 0;
```
```
   }
```

a) mmap: Success
mmap: Success
b) mmap: Success
mmap: Failure
c) segmentation fault
d) none of the mentioned
View Answer

Answer: a
Explanation: None.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lrt
[root@localhost sanfoundry]# ./san
mmap: Success
mmap: Success
[root@localhost sanfoundry]#

9. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<fcntl.h>
```
```
   #include<errno.h>
```
```
   #include<sys/stat.h>
```
```
   #include<sys/mman.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       int s_id;
```

       s_id = shm_open("shared_mem",O_CREAT|O_RDWR,0666);

```
       if(s_id != EACCES)
```

           perror("Permission granted\n");

```
       return 0;
```
```
   }
```

a) Permission granted
: Success
b) Permission granted
c) segmentation fault
d) none of the mentioned
View Answer

Answer: a
Explanation: The shm_open() returns the error EACCES, when the permission is denied.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lrt
[root@localhost sanfoundry]# ./san
Permission granted
: Success
[root@localhost sanfoundry]#

10. What is the output of this program?

```
    #include<stdio.h>
```
```
    #include<fcntl.h>
```
```
    #include<errno.h>
```
```
    #include<sys/stat.h>
```
```
    #include<sys/mman.h>
```
```
 
```
```
    int main()
```
```
    {
```
```
        int s_id;
```

        s_id = shm_open("shared_memory",O_TRUNC,0666);

```
        if(s_id == -1)
```
```
            perror("shm_open\n");
```
```
        return 0;
```
```
    }
```

a) this program will give an error because OTRUNC is not a valid flag
b) this program will give an error
c) this program will give segmentation fault
d) none of the mentioned
View Answer

Answer: b
Explanation: There is no shared memory object “shared_memory” already present in the system.
Output:
[root@localhost sanfoundry]# gcc -o san san.c -lrt
[root@localhost sanfoundry]# ./san
shm_open
: No such file or directory
[root@localhost sanfoundry]#

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