80 Linux Debugging Interview Questions and Answers

1 :: Which one of the following string will print first by this program?
#include<stdio.h>
#include<pthread.h>

void fun_t(void arg);
void fun_t(void arg)
{
printf("Googlen");
pthread_exit("Bye");
}
int main()
{
pthread_t pt;
void *res_t;
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
printf("Linuxn");
if(pthread_join(pt,&res_t) != 0)
perror("pthread_join");
return 0;
}
a) Linux
b) Google
c) it can not be predicted
d) none of the mentioned

b) Google
Explanation:It depends upon the scheduler.
Output:
[root@localhost Google]# gcc -o san san.c -lpthread
[root@localhost Google]# ./san
Google
Linux
[root@localhost threads]#

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2 :: What is the output of this program?

#include<stdio.h>
#include<pthread.h>

void fun_t(void arg);
void fun_t(void arg)
{
int ret;
ret = pthread_exit("Bye");
printf("%dn",ret);
}
int main()
{
pthread_t pt;
void *res_t;
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
if(pthread_join(pt,&res_t) != 0)
perror("pthread_join");
return 0;
}
a) 0
b) 1
c) -1
d) none of the mentioned

d) none of the mentioned
Explanation:
The function pthread_exit() does not return any value. Hence this program will give an error.
Output:
[root@localhost Google]# gcc -o san san.c -lpthread
san.c: In function 'fun_t':
san.c:8:6: error: void value not ignored as it ought to be
[root@localhost google]#

3 :: What is the output of this program no 1?
#include<stdio.h>
#include<pthread.h>

void fun_t(void arg);
void fun_t(void arg)
{
printf("googlen");
pthread_exit("Bye");
}
int main()
{
pthread_t pt;
void *res_t;
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
return 0;
}
a) this program will print the string "google"
b) this program will print nothing
c) segmentation fault
d) run time error

b) this program will print nothing
Explanation:The pthread_join() function waits for the thread to terminate.
Output:
[root@localhost google]# gcc -o san san.c -lpthread
[root@localhost google]# ./san
[root@localhost google]#

4 :: What is the output of this program no 2?
#include<stdio.h>
#include<pthread.h>

void fun_t(void arg);
void fun_t(void arg)
{
printf("%dn",a);
pthread_exit("Bye");
}
int main()
{
int a;
pthread_t pt;
void *res_t;
a = 10;
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
if(pthread_join(pt,&res_t) != 0)
perror("pthread_join");
return 0;
}
a) 10
b) 0
c) -1
d) none of the mentioned

d) none of the mentioned
Explanation:
Each thread has its own stack so local variables are not shared among thread. Hence this program will give an error.
Output:
[root@localhost google]# gcc -o san san.c -lpthread
san.c: In function 'fun_t':
san.c:7:16: error: 'a' undeclared (first use in this function)
san.c:7:16: note: each undeclared identifier is reported only once for each function it appears in
[root@localhost google]#

5 :: What is the output of this program no 3?
#include<stdio.h>
#include<pthread.h>

int a;
void fun_t(void arg);
void fun_t(void arg)
{
printf("%dn",a);
pthread_exit("Bye");
}
int main()
{
pthread_t pt;
void *res_t;
a = 10;
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
if(pthread_join(pt,&res_t) != 0)
perror("pthread_join");
return 0;
}
a) 10
b) 0
c) -1
d) none of the mentioned

a) 10
Explanation:
Thread of the same process shares the global variables.
Output:
[root@localhost google]# gcc -o san san.c -lpthread
[root@localhost google]# ./san
10
[root@localhost google]#

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6 :: What is the output of this program no 4?
#include<stdio.h>
#include<pthread.h>

int a;
void fun_t(void arg);
void fun_t(void arg)
{
a = 20;
pthread_exit("Bye");
}
int main()
{
pthread_t pt;
void *res_t;
a = 10;
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
if(pthread_join(pt,&res_t) != 0)
perror("pthread_join");
printf("%dn",a);
return 0;
}
a) 10
b) 20
c) segmentation fault
d) none of the mentioned

b) 20
Explanation:
In this program the value of variable "a" is changed by the thread "fun_t".
Output:
[root@localhost google]# gcc -o san san.c -lpthread
[root@localhost google]# ./san
20
[root@localhost google]#

7 :: Which one of the following statement is not true about this program?
#include<stdio.h>
#include<pthread.h>

void fun_t(void arg);
void fun_t(void arg)
{
printf("%dn",getpid());
pthread_exit("Bye");
}
int main()
{
pthread_t pt;
void *res_t;
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
if(pthread_join(pt,&res_t) != 0)
perror("pthread_join");
printf("%dn",getpid());
return 0;
}
a) both printf statements will print the same value
b) both printf statements will print the different values
c) this program will print nothing
d) none of the mentioned

a) both printf statements will print the same value
Explanation:
All the threads of the same process have same PID.
Output:
[root@localhost google]# gcc -o san san.c -lpthread
[root@localhost google]# ./san
12981
12981
[root@localhost google]#

8 :: What is the output of this program no 6?
#include<stdio.h>
#include<pthread.h>
#include<fcntl.h>

int fd;
void fun_t(void arg);
void fun_t(void arg)
{
char buff[10];
int count;
count = read(fd,buff,10);
printf("%dn",count);
pthread_exit("Bye");
}
int main()
{
pthread_t pt;
void *res_t;
fd = open("san.c",O_RDONLY);
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
if(pthread_join(pt,&res_t) != 0)
perror("pthread_join");
return 0;
}
a) 10
b) 0
c) -1
d) segmentation fault

a) 10
Explanation:
Open file descritpors can be shares between threads of the same process
Output:
[root@localhost google]# gcc -o san san.c -lpthread
[root@localhost google]# ./san
10
[root@localhost google]#

9 :: What is the output of this program no 7?
#include<stdio.h>
#include<pthread.h>
#include<fcntl.h>

void fun_t(void arg);
void fun_t(void arg)
{
pthread_exit("Bye");
printf("googlen");
}
int main()
{
pthread_t pt;
void *res_t;
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
if(pthread_join(pt,&res_t) != 0)
perror("pthread_join");
printf("%sn",res_t);
return 0;
}
a) google
b) Bye
c) segementation fault
d) run time error

b) Bye
Output:
[root@localhost google]# gcc -o san san.c -lpthread
[root@localhost google]# ./san
Bye
[root@localhost google]#

10 :: What is the output of this program no 8?
#include<stdio.h>
#include<pthread.h>

void fun_t(void arg);
void fun_t(void arg)
{
sleep(1);
}
int main()
{
pthread_t pt;
void *res_t;
if(pthread_create(&pt,NULL,fun_t,NULL) != 0)
perror("pthread_create");
if(pthread_join(pt,&res_t) != 0)
perror("pthread_join");
printf("%sn",res_t);
return 0;
}
a) this process will pause
b) segmentation fault
c) run time error
d) none of the mentioned

b) segmentation fault
Explanation:
This program is trying to print the return value of the thread, but pthread_exit() function is not present in the thread.
Output:
[root@localhost google]# gcc -o san san.c -lpthread
[root@localhost google]# ./san
Segmentation fault (core dumped)
[root@localhost google]#

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80 Linux Debugging Questions and Answers: