• Multithreaded programming with Pthreads (POSIX threads).
• Practicing synchronization, use of lock variables.
• Designing and performing experiments; applying probability and statistics
You can do this project in groups of two students each. You will use C and Linux.
Thread-safe Hash Table Library
In this project you will first develop a library (libhash.a) that will implement
thread-safe hash table data structure and its operations. A multi-threaded application using the library will be able to create one or more hash tables and access them
from multiple threads running concurrently. The library code will sit in a file called
hash.c and the respective header file (i.e., library interface) will be hash.h.
A hash table has N buckets (buckets 0 through N-1), i.e., N entries. A key i
and an associate value (data), i.e., a key-value pair, will be inserted into bucket j
= hash(i), where j is in range [0, N-1]. In this project, the hash function will be a
simple hash function, i.e., hash (i) = i mod N. The key type is integer and valid
keys are positive. A key value 0 is not valid. The value type will be void *. With
proper type casting, you can store an integer value in a field that is of type void
*. Multiple key-value pairs mapping to the same bucket will be added to a linked
list (chaining). In this way collisions will be resolved. Hence, for each bucket of the
hash table we will have a linked list, initially empty.
A hash table will be protected by multiple locks to reduce lock contention while
accessing the hash table from multiple threads. There will be one lock per M
consecutive buckets in the hash table. We call such M consecutive buckets as a
region. There will be N/M = K regions, hence K locks. The first M consecutive
buckets is the region 0 and is protected by lock 0; the next M buckets is region 1
and protected by lock 1, and so on. While doing an operation on the hash table (like
insert, delete, get) and accessing a bucket, the corresponding lock must be acquired.
N can be a value between 100 and 1000. M can be a value between 10 and 1000.
M should be ≤ N and N is a multiple of M. K can be a value between 1 and 100.
Your library will implement the following functions. It will define a HashTable
type as well, so that applications can use this data type.
• HashTable *hash init (int N, int K). Creates a hash table of N buckets
protected by K locks. Each bucket will have an associated linked list (chain)
initialized to empty list. Returns a pointer to the hash table created if success;
otherwise returns NULL.
• int hash insert (HashTable *hp, int k, void * v). Inserts key k and the
associated value v into the hash table hp. Value is a pointer that can point
to a structure containing data. The allocation and deallocation of memory
for the structure is left to the application using the library. If the data is a
simple integer, then it can be directly stored as the value (i.e., no need to
separately allocate memory for the integer and store a pointer to that as the
value). We are assuming void * and integer type has the same size. This
function returns 0 if success; otherwise returns -1. If key already presents,
does nothing and returns -1.
• int hash delete (HashTable *hp, int k). Removes key k and the associated
value v from the hash table hp. If success returns 0, otherwise returns -1.
• int hash update (HashTable *hp, int k, void *v). Updates the value of
key k to be v. If success returns 0, otherwise returns -1.
• int hash get (HashTable *hp, int k, void **vp). The value associated
with key k is retrieved into a pointer variable (of type void *) whose address
is vp. If success returns 0, otherwise -1.
• int hash destroy (HashTable *hp). Destroys the hash table and frees all
resources used by it.
A multi-threaded application, for example x.c, that will use your library will
first include the header file hash.h corresponding to your library. The application
may create many threads and each thread may do a lot of table operations. An
application will be compiled and linked with your library as follows:
gcc -Wall -o x -L. -lhash x.c
Develop a Multi-threaded Application
Implement a multi-threaded integer-count program. The program will take C
text files as input. Each input file may contain a large number of positive integers.
Repetitions are allowed. The program will count the number appearances of unique
integers. In other words, for each integer appearing in one of those C files, it will
find out how many times that integer occurs in those files. Your program will use a
separate thread for each file. The threads will use a shared hash table to do counting.
That means a key-value pair in the table will be an integer and its count. At the
end, the program will write the integers in ascending sorted order to an output file.
The output file will have a separate line for each unique integer. A line will contain
an integer and its count separated by a colon as in the following output example:
A sample invocation of the program is shown below:
integer-count 7 1.txt 2.txt 3.txt 4.txt 5.txt 6.txt 7.txt out.txt
There are 7 input files. The program will use 7 new threads. Each input file
may contain an unsorted sequence of positive integers (keys). Each line of input will
contain a single integer.
Experiments and Report
Assume we are wondering about the effect of the number of locks used (K) on the
performance of the hash table operations. If there is one lock used, there will be high
contention for it from multiple threads trying to access the hash table. Write an
application (test.c) and design some experiments to analyze the effect of number
of locks (K) on the performance. Performance metric can be, for example, the time
to execute a set of operations on the hash table for a set of threads. Try to find
out if you can see a performance increase while K is increased. Measure the time.
Make a lot of experiments with different parameter values: number of threads (T),
number of keys/operations (W), number of locks (K), table size (N). Try to see the
effect of number of threads (T) on the performance as well. When T is 1, there will
be no contention. Contention will increase when there are more keys be operated
with and when there are more threads used. Show your numeric results in tables or
graphs and try to interpret. Put your results, your interpretations, discussions into
Put your report.pdf file and your program files (hash.c, hash.h, integer-count.c,
test.c) and a Makefile and a README.txt file into a directory named with your ID
(for a group, a single file will be uploaded using the ID of one of the students). Then
tar and gzip the directory. For example a student with ID 21404312 will create a
directory named 21404312 and will put the files there. Then he will tar the directory
(package the directory) as follows:
tar cvf 21404312.tar 21404312
Then he will gzip the tar file as follows:
In this way he will obtain a file called 21404312.tar.gz. Then he will upload this file
Late submission will not be accepted (no exception). A late submission will get
0 automatically (you will not be able to argue it). Make sure you make a submission
one day before the deadline. You can then overwrite it.
Tips and Clarifications
• You need to learn how to use Pthreads mutex variables. There are links to
some resources in the References section of the course webpage. You can find
additional resources from Internet.
• A project 2 skeleton of code is posted in github:
You can clone it and start with it. At least, it will help you with the compilation of the library and programs.
• You will include the Makefile with your project submission. Make sure it
works in your environment (name your files accordingly). We will just type
make and your programs should compile.
• We may put clarifications to the homepage of the course (near the project
• You are suggested to test your hash table library first by developing simple
applications. Then you can start developing integer-count application.
• You can use mutex locks in the application code as well for integer-count
• You can not change the interface (hash.h).
• Our test programs will destroy the hash table after all threads finished using