Assignment: Sorting
You will write a simple sorting program. This program should be invoked as follows:
shell% ./fastsort -i inputfile -o outputfile
The above line means the users typed in the name of the sorting program ./fastsort and gave it two inputs: an input file to sort called inputfile and an output file to put the sorted results into called outputfile .
Input files are generated by a program we give you called generate.c (good name, huh?).
After running generate , you will have a file that needs to be sorted. It will be filled with binary data, of the following form: a series of 4KB (4096-byte) records, the first four bytes of which are an integer key, and the remaining 4092 bytes of which are integers that form the rest of the record.
Your goal: to build a sorting program called fastsort that takes in one of these generated files and sorts it based on the 4-byte key (the remainder of the record should of course be kept with the same key). The output is written to the specified output file.
Some Details
Using generate is easy. First you compile it as follows:
shell% gcc -o generate generate.c -Wall -Werror
Note: you will also need the header file sort.h to compile this program.
Then you run it:
shell% ./generate -s 0 -n 100 -o /tmp/outfile
There are three flags to generate . The -s flag specified a random number seed; this allows you to generate different files to test your sort on. The -n flag determines how many records to write to the output file, each of size 4KB. Finally, the -o flag determines the output file, which will be the input file for your sort.
The format of the file generated by the generate.c program is very simple: it is in binary form, and consists of those 4KB records as described above. A common header file sort.h has the detailed description.
Another useful tool is dump.c . This program can be used to dump the contents of a file generated by generate or by your sorting program.
Hints
In your sorting program, you should just use open() , read() , write() , and close() to access files. See the code in generate or dump for examples.
If you want to figure out how big in the input file is before reading it in, use the stat() or fstat() calls.
To sort the data, use any old sort that you'd like to use. An easy way to go is to use the library routine qsort() .
To exit, call exit() with a single argument. This argument to exit() is then available to the user to see if the program returned an error (i.e., return 1 by calling exit(1) ) or exited cleanly (i.e., returned 0 by calling exit(0) ).
The routine malloc() is useful for memory allocation. Make sure to exit cleanly if malloc fails!
If you don't know how to use these functions, use the man pages. For example, typing man qsort at the command line will give you a lot of information on how to use the library sorting routine.
Assumptions and Errors
32-bit integer range. You may assume that the keys are 32-bit integers, i.e. we will not test integers larger than 32 bits.
File length: May be pretty long! However, no need to implement a fancy two-pass sort or anything like that; the data set will fit into memory.
Invalid files: If the user specifies an input or output file that you cannot open (for whatever reason), the sort should EXACTLY print: Error: Cannot open file foo, with no extra spaces (if the file was named foo ) and then exit.
Too few or many arguments passed to program: If the user runs fastsort without any arguments or in some other way passes incorrect flags and such to fastsort, print Usage: fastsort -i inputfile -o outputfile and exit.
Important: On any error code, you should print the error to the screen using fprintf() , and send the error message to stderr (standard error) and not stdout (standard output). This is accomplished in your C code as follows:
fprintf(stderr, "whatever the error message is\n");
History and a Contest
This sorting assignment derives from a yearly competition to make the fastest disk-to-disk sort in the world. See the sort home page for details. To continue in this tradition, we will also be holding a sorting competition. Whoever turns in the fastest sorting program on the 100,000 record test wins. Read more about sorting, including perhaps the NOW-Sort paper, for some hints on how to make a sort run really fast. Or just use your common sense! Hint: you'll have to think a bit about hardware caches.
General Advice
Start small, and get things working incrementally. For example, first get a program that simply reads in the input file, one line at a time, and prints out what it reads in. Then, slowly add features and test them as you go.
Testing is critical. One great programmer I once knew said you have to write 5-10 lines of test code for every line of code you produce; testing your code to make sure it works is crucial. Write tests to see if your code handles all the cases you think it should. Be as comprehensive as you can be. Of course, when grading your projects, we will be. Thus, it is better if you find your bugs first, before we do.
Keep old versions around. Keep copies of older versions of your program around, as you may introduce bugs and not be able to easily undo them. A simple way to do this is to keep copies around, by explicitly making copies of the file at various points during development. For example, let's say you get a simple version of fastsort.c working (say that just reads in the file); type cp fastsort.c fastsort.v1.c to make a copy into the file fastsort.v1.c. More sophisticated developers use version control systems like CVS, but we'll not get into that here (yet).
Keep your source code in a private directory. An easy way to do this is to log into your account and first change directories into private/ and then make a directory therein (say p1, by typing mkdir p1 after you've typed cd private/ to change into the private directory). However, you can always check who can read the contents of your AFS directory by using the fs command. For example, by typing in fs listacl . you will see who can access files in your current directory. If you see that system:anyuser can read (r) files, your directory contents are readable by anybody. To fix this, you would type fs setacl . system:anyuser "" in the directory you wish to make private. The dot "." referred to in both of these examples is just shorthand for the current working directory.