Checklist 03

Sample Question¶

Which of the following pipelines counts all the "Permission denied" errors encountered when recursively searching file names under the "/etc" directory?

• find /etc > /dev/null | wc -l
• find /etc 2>&1 > /dev/null | wc -l
• find /etc 2> /dev/null | wc -l

• find /etc > /dev/null 2>&1 | wc -l

Sample Question¶

• Given a list of integers called numbers, produce a new list called fives which consist only of the multiples of 5 by using filter and lambda.

  Then, do the same thing using list comprehensions.

• Write a program isitdown.py that uses os.system and curl to check if a series of websites are down (ie. respond with successful HTTP requests).

  $ ./isitdown.py google.com nd.edu fake.website example.com
  google.com UP
  nd.edu UP
  fake.website DOWN
  example.com UP
  

  Then, do the same thing using requests.

Sample Questions¶

1. Why is it important to specify all the dependencies of a target in a Makefile rule?

2. What is the difference between compiling and linking? What are some common errors that can occur in each phase?

3. Write a Makefile for a project that outputs a shared library, a static library, a dynamic executable, and a static executable.

Sample Question¶

Given the following code:

int main(int argc, char *argv[]) {  // | Label | Address | Value |
    int  n   = 16;                  // |       | 0xF     |       |
    int  a[] = {n};                 // | n     | 0xE     | 0x10  |
    int *p   = a;                   // |       | 0xD     |       |
    return 0;                       // |       | 0xC     |       |
}                                   // |       | 0xB     |       |

Assuming a little-endian 16-bit machine where each integer is 2 bytes, each character is 1 byte, and each pointer is 2 bytes, fill in the stack frame to the right by recording the location of the variables above and their values at the appropriate addresses.

The variable n is done for you; you need to add p and a to the stack frame.

Distinguish between the character 0 and the integer 0 by using hexadecimal for integers.

Sample Question¶

Given the following C program:

typedef struct
{
    char key;
    int  value;
} Pair;

int main(int argc, char* argv[])
{
    char    c = 'k';
    int     v = 2;

    Pair   p0 = {c, v};
    Pair  *p1 = malloc(sizeof(Pair));
    Pair  *p2 = calloc(v, sizeof(Pair));
    Pair **p3 = calloc(v, sizeof(Pair *));
    return 0;
}

How much memory was allocated on the stack, heap, and data for each of the variables in the main function?

Sample Questions¶

1. Given the following C program:

   typedef struct Node
   {
       int value; 
       struct Node *next; 
   } Node;
   
   typedef struct {
       Node *head;
   } List;
   
   void list_fprint(List *l, FILE *stream)
   {
       for (Node *c = l->head; c; c = c->next)
           fprintf(stream, "%d\n", c->value);
   }
   
   bool list_remove(List *l, int v)
   {
       // TODO
   }
   
   int main(int argc, char *argv[])
   {
       List l;
       char buffer[BUFSIZ];
   
       while (fgets(buffer, BUFSIZ, stdin))
       {
           Node *next  = calloc(1, sizeof(Node));
           next->value = atoi(buffer);
           next->next  = l.head;
           l.head      = next;
       }
   
       list_fprint(&l, stdout);
       list_remove(&l, 42);
       return 0;
   }
   

   Given the linked list code above, implement the list_remove function which removes all nodes that contain the specified value.  Return whether or not any node was removed.

2. Given the follow hash table (with linear probing) implementation at https://yld.me/raw/eRVG.c:

   /* hash_table.c */
   
   #include <stdbool.h>
   #include <stdio.h>
   #include <stdlib.h>
   
   /* Constants */
   
   #define DEFAULT_BUCKETS (1<<5)
   
   /* Structures */
   
   typedef struct
   {
       int    *data;
       size_t buckets;
   } Table;
   
   /* Functions */
   
   Table * table_create(size_t buckets)
   {
       Table *t = malloc(sizeof(Table));
       if (t)
       {
           t->buckets = (buckets ? buckets : DEFAULT_BUCKETS);
           t->data    = malloc(sizeof(int) * buckets);
       }
       return t;
   }
   
   void    table_delete(Table *t)
   {
       free(t);
   }
   
   bool    table_insert(Table *t, int value)
   {
       size_t hash = value;
   
       for (size_t offset = 0; offset < t->buckets; offset++)
       {
           size_t bucket = hash + offset;
           if (!t->data[bucket])
           {
               t->data[bucket] = value;
               return true;
           }
           if (t->data[bucket] == value)
           {
               return false;
           }
       }
   
       return false;
   }
   
   bool    table_search(Table *t, int value)
   {
       size_t hash = value;
   
       for (size_t offset = 0; offset < t->buckets; offset++)
       {
           size_t bucket = hash + offset;
           if (!t->data[bucket])
           {
               return false;
           }
           if (t->data[bucket] == value)
           {
               return true;
           }
       }
   
       return false;
   }
   
   /* Main Execution */
   
   int main(int argc, char *argv[])
   {
       int   d[] = {4, 6, 5, 5, 6, 0};
       Table  *t = table_create(0);
   
       for (int *p = d; *p; p++)
       {
           table_insert(t, p);
       }
   
       for (int i = 0; i < 10; i++)
       {
           printf("table_search(%d) = %s\n", i, table_search(t, i) ? "true" : "false");
       }
   
       table_delete(t);
       return EXIT_SUCCESS;
   }
   

   1. Compile and run the code using gdb. Where does the program segfault?

   2. Which line do you need to change to fix these errors?

   3. Compile and run the code using valgrind. What type of memory errors are there?

   4. Which line do you need to change to fix these errors?

   5. Compile and run the code using valgrind. What type of memory errors are there?

   6. Which line do you need to change to fix these errors?

   7. Compile and run the code using valgrind. What type of memory errors are there?

   8. Which line do you need to add to fix these errors?

   9. Both the table_insert and table_search do not compute the hash correctly. What is missing?

Sample Questions¶

How would you use system calls in C to accomplish the following tasks:

• Display the contents of a file?

• Copy the contents of a file?

• Check if a file is a regular file, directory, or symlink?

• Check if a file is a readable, writable, or executable?

• Check the size of the file?

• Check the last modification time?

• List the contents of a directory?

Sample Question¶

Modify http_client.c to implement a port scanner (ie. nc -z).

Sample Question¶

Given the following C code:

/* forking.c */

#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

int main(int argc, char *argv[])
{
    int status = 0;

    for (int i = 1; i < argc; i++)
    {
        pid_t pid = fork();

        if (pid < 0)
        {
            return EXIT_FAILURE;
        }

        if (pid == 0)
        {
            execlp(argv[i], "forkit", NULL);
        }
    }

    for (int i = 1; i < argc; i++)
    {
        int child_status;
        while (wait(&child_status) < 0)
        {
            status += child_status;
        }
    }

    return status;
}

Which of the following statements are true (select all that apply)?

1. The execlp always succeeds.
2. The execlp always fails.
3. The execlp may succeed depending on the arguments.
4. main will always return with success.
5. main will always return with failure.
6. main will may return with success depending on the arguments.
7. At most one child process may run concurrently with the parent.
8. Multiple child processes may run concurrently with the parent.
9. This program has fork bomb potential.
10. This program does not have fork bomb potential.
11. This program always terminates gracefully.
12. This program sometimes hangs indefinitely.