Homework 2

Computer Architecture I ShanghaiTech University
HW1 (in Autolab)    HW2 HW3

Task 1: Skip Lists in C


Implement a Skip List in C. See the header file skiplist.h for further details.

Your submission should only include the source code for the Skip List, skiplist.c. Any other file found will directly result in a grade of 0. We will NOT be using your copy of the header file or Makefile.

Compilation of the source code requires no errors or warnings and conformity to the C89 standard. The Makefile syntax is given below.

CFLAGS=-Wpedantic -Wall -Wextra -Werror -std=c89

skiplist.o: skiplist.c skiplist.h
  ${CC} ${CFLAGS} skiplist.c -o skiplist.o

The above compilation requires a main function. However, your submission should NOT include any main function.

We will test your program using the functions declared in the header file provided as well as some other functions defined by us. The output of the individual test cases will NOT be given in auto-grader and any attempt at retrieving the test cases or the program outputs will be seen as plagiarism.

We require that you will have one line with comments for every four non-blank lines. Comments have to be meaningful and in English.

No memory leaks are allowed. Memory leaks will be automatically detected and manually screened and will result in a decrease in your score after the deadline.

We will also impose a limit of 450 on your LoC (lines of code). Do not exceed this limit as we will deduct points for those who exceed this limit.

Consider Learning Before You Start

Skip List

A skip list (Pugh, 1990) is a randomized tree-like data structure based on linked lists. It consists of a level 0 list that is an ordinary sorted linked list, together with higher-level lists that contain a random sampling of the elements at lower levels. When inserted into the level i list, an element flips a coin that tells it with probability p to insert itself in the level i+1 list as well.

Searches in a skip list are done by starting in the highest-level list and searching forward for the last element whose key is smaller than the target; the search then continues in the same way on the next level down. The idea is that the higher-level lists act as express lanes to get us to our target value faster. To bound the expected running time of a search, it helps to look at this process backwards; the reversed search path starts at level 0 and continues going backwards until it reaches the first element that is also in a higher level; it then jumps to the next level up and repeats the process. On average, we hit 1 + 1/p nodes at each level before jumping back up; for constant p (e.g. 1/2), this gives us O(logn) steps for the search.

source: Yale CPSC223

There are various ways to implement a skip list. You can learn about the basics of skip list here. However, note that the implementation required in this homework differs from that on wikipedia

Implementation details

You are required to implement skiplists with following rules:


Refer to the submission section at the end.

Task 2: Libraries

For all tasks assume that skiplist.h will be provided in the working directory and your skiplist.c will be copied to the current working directory.

Task 2.1: Static Library


Write a shell script named staticlib.sh that when invoked, produces a static library named liblist.a from the source files skiplist.c and skiplist.h.

Then create a static-linked executable staticlist from liblist.a and test.c. Do not execute the program in the shell script.

Hint: use the command ar to create the static library. Then use the command ld to create the executable file.


Write a shell script named dynamiclib.sh that when invoked, produces a dynamic-linked library named liblist.so from skiplist.c.

Then generate an executable named dynamiclist using liblist.so as well as test.c. Do not execute the program in the shell script.


You should submit a compressed file named as hw2.tar to autolab.

The directory tree of your submission should look like the following :

├── skiplist.c
├── dynamiclib.sh
└── staticlib.sh

Note: Autolab grading results:

Note: your submission should NOT contain main() function

Test Environment

The test environment on autolab is Ubuntu 16.04 with gcc 5.x

Last Modified: Mar. 9th, 2018