Linux & C Debugging Questions & Answers with Coding Examples

This set of Linux Debugging questions and answers focuses on malloc, calloc, free and realloc Calls.

1. This program will allocate the memory of ___ bytes for pointer “ptr”.

```
   #include<stdio.h>
```
```
   #include<stdlib.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       int *ptr;
```
```
       ptr = realloc(0,sizeof(int)*10);
```
```
       return 0;
```
```
   }
```

a) 0
b) 10
c) 40
d) none of the mentioned
View Answer

Answer: c
Explanation: If the first argument of realloc() is NULL, then it behaves just like malloc().

2. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<stdlib.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
	   char *ptr;
```
```
       free(ptr);
```
```
       return 0	   
```
```
   }
```

a) this program will print nothing after execution
b) segmentation fault
c) Aborted (core dumped)
d) none of the mentioned
View Answer

Answer: c
Explanation: This prgram is trying to free the memory which is not available in the heap segment.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
*** glibc detected *** ./san: free(): invalid pointer: 0x4a77cff4 ***
======= Backtrace: =========
/lib/libc.so.6[0x4a6489f2] ./san[0x80483c9] /lib/libc.so.6(__libc_start_main+0xf3)[0x4a5e96b3] ./san[0x8048321] ======= Memory map: ========
08048000-08049000 r-xp 00000000 fd:01 394194 /home/sanfoundry/san
08049000-0804a000 rw-p 00000000 fd:01 394194 /home/sanfoundry/san
09233000-09254000 rw-p 00000000 00:00 0 [heap] 4a5ab000-4a5cc000 r-xp 00000000 fd:01 785334 /lib/ld-2.14.90.so
4a5cc000-4a5cd000 r–p 00020000 fd:01 785334 /lib/ld-2.14.90.so
4a5cd000-4a5ce000 rw-p 00021000 fd:01 785334 /lib/ld-2.14.90.so
4a5d0000-4a77a000 r-xp 00000000 fd:01 789110 /lib/libc-2.14.90.so
4a77a000-4a77b000 —p 001aa000 fd:01 789110 /lib/libc-2.14.90.so
4a77b000-4a77d000 r–p 001aa000 fd:01 789110 /lib/libc-2.14.90.so
4a77d000-4a77e000 rw-p 001ac000 fd:01 789110 /lib/libc-2.14.90.so
4a77e000-4a781000 rw-p 00000000 00:00 0
4a7e0000-4a7fc000 r-xp 00000000 fd:01 789128 /lib/libgcc_s-4.6.2-20111027.so.1
4a7fc000-4a7fd000 rw-p 0001b000 fd:01 789128 /lib/libgcc_s-4.6.2-20111027.so.1
b7724000-b7725000 rw-p 00000000 00:00 0
b773d000-b773f000 rw-p 00000000 00:00 0
b773f000-b7740000 r-xp 00000000 00:00 0 [vdso] bfc83000-bfca4000 rw-p 00000000 00:00 0 [stack] Aborted (core dumped)
[root@localhost sanfoundry]#

3. What is the output of this program?

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```
   #include<stdio.h>
```
```
   #include<stdlib.h>
```
```
   struct st{
```
```
       int a;
```
```
       char b;
```
```
   };
```
```
 
```
```
   int main()
```
```
   {
```
```
       struct st *st_ptr;
```

       st_ptr = malloc(sizeof(struct st));

       printf("%d\n",sizeof(struct st));

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

a) 8
b) 5
c) 0
d) none of the mentioned
View Answer

Answer: a
Explanation: Maximum size of the data type is 4 byte(int) in the structure.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
8
[root@localhost sanfoundry]#

4. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<stdlib.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       char *ptr;
```

       ptr = (char *)malloc(sizeof(char)*11);

```
       ptr = "sanfoundry";
```
```
       printf("%s\n",*ptr);
```
```
       return 0;
```
```
   }
```

a) sanfoundry
b) segmentation fault
c) syntax error
d) none of the mentioned
View Answer

Answer: b
Explanation: value of a string can not be assign to a pointer.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
Segmentation fault (core dumped)
[root@localhost sanfoundry]#

5. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<stdlib.h>
```

   #include<string.h>

```
 
```
```
   int main()
```
```
   {
```
```
       char *ptr;
```
```
       memcpy(ptr,"sanfoundry",11);
```
```
       printf("%s\n",ptr);
```
```
       return 0;
```
```
   }
```

a) sanfoudry
b) segmentation fault
c) syntax error
d) none of the mentioned
View Answer

Answer: b
Explanation: Memory must be allocated to pointer “ptr”.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
Segmentation fault (core dumped)
[root@localhost sanfoundry]#

6. What is the output of this program?

```
   #include<stdio.h>
```
```
   #include<stdlib.h>
```

   #include<string.h>

```
 
```
```
   int main()
```
```
   {
```
```
       char *ptr;
```

       ptr = (char*)malloc(sizeof(char)*11);

```
       strcpy(ptr,"sanfoundry");
```
```
       printf("%d\n",*ptr);
```
```
       return 0;
```
```
   }
```

a) s
b) sanfoundry
c) 115
d) segmentation fault
View Answer

Answer: c
Explanation: This program will print the equivalent decimal value at location pointed by “ptr”.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
115
[root@localhost sanfoundry]#

7. Which one of the following in true about this program?

```
   #include<stdio.h>
```
```
   #include<stdlib.h>
```

   #include<string.h>

```
 
```
```
   int main()
```
```
   {
```
```
       char *ptr;
```
```
       printf("%p\n",ptr);
```

       ptr = (char *)malloc(sizeof(char));

```
       printf("%p\n",ptr);
```
```
       return 0;
```
```
   }
```

a) this program will give segmentation fault
b) this program will print two same values
c) this program has some syntax error
d) none of the mentioned
View Answer

Answer: d
Explanation: This program will print two different values.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
0x4a77cff4
0x980c008
[root@localhost sanfoundry]#

8. In this program the two printed memory locations has the difference of ___ bytes.

```
   #include<stdio.h>
```
```
   #include<stdlib.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       int *ptr;
```

       ptr = (int*)malloc(sizeof(int)*2);

```
       printf("%p\n",ptr);
```
```
       printf("%p\n",ptr+1);
```
```
       return 0;
```
```
   }
```

a) 1
b) 4
c) can not be determined
d) none of the mentioned
View Answer

Answer: b
Explanation: Pointer will increment by 4 bytes because it is the types of integer.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
0x9b4e008
0x9b4e00c
[root@localhost sanfoundry]#

9. What is the output of this program?

```
   #include<stdio.h>
```
```
   #inlcude<stdlib.h>
```
```
 
```
```
   int main()
```
```
   {
```
```
       int *ptr;
```
```
       double *ptr;
```
```
       printf("%d\n",sizeof(ptr));
```
```
       return 0;	   
```
```
   }
```

a) 4
b) 8
c) the compiler will give the error
d) segmentaion fault
View Answer

Answer: c
Explanation: Just see the output carefully.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
san.c: In function ‘main’:
san.c:8:10: error: conflicting types for ‘ptr’
san.c:7:7: note: previous declaration of ‘ptr’ was here
[root@localhost sanfoundry]#

10. What is the output of this program?

```
    #include<stdio.h>
```
```
    #include<stdlib.h>
```

    #include<string.h>

```
 
```
```
    int main()
```
```
    {
```
```
        int ptr;
```

        ptr = (int)malloc(sizeof(int)*10);

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

a) syntax error
b) segmentaion fault
c) run time error
d) none of the mentioned
View Answer

Answer: d
Explanation: The memory has been allocated but we can not access rest of the memory other than 4 bytes.
Output:
[root@localhost sanfoundry]# gcc -o san san.c
[root@localhost sanfoundry]# ./san
[root@localhost sanfoundry]#

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