CSCI 235 — More C practice

Exploration in C

We’re going to run through a few exercises here.

 CSCI 235 lab

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char** argv) {
    return (EXIT_SUCCESS);

Make sure you can compile it.

Testing your homework answers

First download a pretty simple C program.

C gives you many ways to “cheat” on tranforming float to/from int (should use uint32_t) and double to/from long (should use uint64_t). Modify your program to try out the following:

Java provides some safer operations for these transformations.

Evidently, it is useful to access the bits when implementing some numerical routines.

Using command line arguments and creating file

Write a program that, when run with a variable number of arguments, as shown below:
    yourprogname filename1 filename2 ...
will, for each i, check if a file named filenamei exists (use access()) and, if it does not, create filenamei and write a single line containing i to filenamei.

Use access, fopen, fprintf, and fclose.

Listing a directory

Look at the GNU example of a simple program to list a directory. Cut-and-past the program and compile it on your computer.

Modify the program so displays only “files” that that are regular files (not directories). Display both the file name and inode number for these files. Look at the documentation for readdir to see how your can obtain the inode number.

Sorting a directory

This part is a little more difficult.

Read the /var/tmp directory; and store copies of the struct dirent returned for each file into an array. Assume that there are no more than 1000 files in the array. You should be able to copy the struct dirent into the array with a single assignment statement.

Now try using qsort to sort the files of /var/tmp by inode number.

OK. That is a bit much, but look at the Microsoft page on using qsort to see some outragious use of type puning.

Polishing off the old lab

Were going to finish up last week and then do a few more explorations of C.

Retrieving the old lab work

Download the C file you created last week. It should have a working copy of the name2num program.

I suggest you use the command line this week. It will be easier for testing. Use nano (or a similar text editor) to modify your program and use make for compilation and gdb for debugging.

Small task two: Routing addresses

Many of you have already done this part. If so, go to the next section.

The output of netstat -nr contains the following lines:

Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface         UG        0 0          0 eth0   U         0 0          0 eth0

The destination and mask together define a matching rule. Suppose the IP number of a running computer is in dotted decimal and consequently 0x98124546 in raw bits. Also, assume netstat has a routing entry for destination (0x98124500) with the mask (0xffffff00). If you go into gdb and perform a bit-wise AND (&) of 0x98124546 ( and 0xffffff00 (, you will get 0x98124500 (, which matches a route entry’s getway. That’s how the operating system knows that packets to should be sent directly through the Ethernet interface eth0.

See the Wikipedia article on routing tables for more information and try out the following gdb command to verify my calculation.
    printf "%x",0x98124546 & 0xffffff00

The real task

Modify your program to write LOCAL after each IP number that is local to the computer science VLAN, with gateway 255.255.255 — usually written as

Small task three: Reading command line arguments

Modify your program so that, if it is started with two arguments, one for the network destination and the other for the network mask, it will use use the first argument for the gateway and the second for this task. This means that your program could be started with either of the following commands:

Small task four: Writing to a file

Modify your program so that it writes its output to the file /var/tmp/output.txt. This is really a very small change: It involves adding one statement (calling fopen) and modifying two others.