COMP 3000 Lab 3 2011

A few guidelines:

• Submit your solutions for both Part A and Part B via WebCT by Monday, October 24th at 11:30 PM.
• Please answer using a single text file. Do not submit doc, docx, pdf, or other formats. Also, please do not submit an archive (zip, tarball, rar) of multiple text files, just upload one. (Please don't just cut and paste your work into a text box on webct.)
• Show all your work. Short answers are not sufficient; you should list any websites or individuals you consult and should document any experiments you conducted. For any question that you could just answer without any external reference or experiment, write "(no work required)" after your answer.

Part A

1. Compile run-program.c with the command gcc -g run-program.c -o run-program and use it to answer the following questions.
1. Is the line "Is this line printed?" printed when you execute ./run-program /bin/ls -a /tmp? Why?
2. Change the program to use execve instead of execvp. What is the difference between the two system calls?
2. Linux signals is a simple form of IPC that is used for a variety of purposes.
1. What signal is generated when a program attempts to dereference an invalid pointer? Give a simple program that generates such a signal.
2. How would you send a signal to a process to have it pause execution? Resume?
3. What command can lower the priority of an already running process?
4. Create run-program-reverse.c that is the same as run-program.c except that it interprets its arguments in reverse order. Hence, ./run-program-reverse /tmp -a /bin/ls will do the same as ./run-program /bin/ls -a /tmp.
5. Create sanitize-env.c that does the same as run-program.c except it exec's the program with all environment variables stripped except the PATH and TERM variables (assuming they are defined). Be sure to document how you tested your program.

Part B

6. Create a simple program receive-usr1.c that uses sigaction in order to print "Ouch!" when you send the program the USR1 signal. Note that your program should not consume much CPU time while waiting (i.e., do not poll for the signal).
7. Create a simple program nocrash.c that accesses an invalid pointer but then prints "Oops, my mistake" rather than crash.
8. Create a program run-program-dots.c that works the same as run-program.c, except that it prints one dot every second while a given program runs. Note that it should stop printing dots once the exec'd program terminates. Your solution should use the fork(), sleep(), execve(), and sigaction() calls.
9. What is the difference between the PR and NI columns in the top command? Explain in the context of running a CPU-intensive program. (Hint: you should try running a CPU intensive program or two and observe these columns...)

Program Listings

/* run-program.c */

#include <unistd.h>
#include <stdio.h>

int main( int argc, char *argv[], char *envp[] ) {

   if( argc < 2 ) {
       printf( "Insufficient arguments.\n" );
       return -1;
   }
   execvp( argv[1], argv + 1 );
   printf( "Is this line printed?\n" );
   return 0;
}