CSC 8410 -- Creating and Running Processes in Unix

The goal of these exercises is to get more practice with creating and executing processes in Unix using the following system calls:

 
 fork, wait and exit
 execv and execl 

Exercises

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In your csc8410/forkexec Unix account, write code solutions for the exercises below.

• Exercise 1
• Exercise 2
• Exercise 3
• Exercise 4

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Exercise 1.

Write a program pfork.c that does the following:

• Takes two integers from the command line
• Forks a child process that prints out the difference of the two integers
• The parent process prints out the sum of the two integers.

Have your program print a message in case the command line is invalid:

 
 if(argc < 3)
  {
  printf("Invalid command line: please supply two integer values\n");
  exit(1);
   }

Here is all you need to know about printf. Use the atoi function to covert strings to their integer value.

(a) Sample output (assumming command line pfork 12 16):

  [Child 1235]: Difference of 12 and 16 is -4.
  [Parent 1234]: Sum of 12 and 16 is 28.
 
or 
  [Parent 1234]: Sum of 12 and 16 is 28.
  [Child 1235]: Difference of 12 and 16 is -4. 

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Exercise 2.

Step 1. At the shell prompt, try the command

  ps -f

A list of your own processes currently running will be displayed:

• The first column lists the owner of the process (your account name).
• The second column lists the ID of the process.
• The third column lists the ID of the parent process.

Very likely, your only process currently running is the bash shell. For a list all processes in the system, execute

 
  ps -e -f

Optionally, for more information on ps, use the Unix manual pages (type in man ps at the shell prompt).

Step 2. Copy the program from Exercise 1 into a new file pexec.c. Extend the code for the child to do the following:

• Print out the difference of the two integers, as before
• Execute the code for ps -f. To do this, initialize an array of strings cmd with the tokens in the command you'd like to execute, then use execv:

 
 char * cmd[] = {"/bin/ps", "-f", 0};  // in the variable declaration section
 ...
 execv(cmd[0], cmd);    // in the code section of the program

Sample output (assumming command line pexec 12 16):

 [Parent 1234]: Sum of 12 and 16 is 28.
 [Child 1235]: Difference of 12 and 16 is -4.
 
  UID PID PPID C STIME TTY TIME CMD
  mdamian 22686 22684 0 14:12:26 pts/5 0:01 -bin/bash
  mdamian 22826 22686 0 15:11:49 pts/5 0:00 pexec
  mdamian 22827 22826 0 15:11:49 pts/5 0:00 ps

Note: Make sure the parent waits for the child to finish.

Step 3. Answer the following question: Is it possible to have the child execute ps -f first, and then print out the difference value? Why or why not?

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Exercise 3.

This exercise asks you to experiment with various versions of execv.

• Instead of using an array of arguments such as cmd, you can pass the command tokens directly in a list using execl (trailing 'l' from 'execl' stands for 'list', trailing 'v' from 'execv' stands for vector):

  
 execl("/bin/ps", "/bin/ps", "-f", 0); 
 

Note that the argument "/bin/ps" appears twice: the first occurence accounts for cmd[0], while the rest of arguments account for all values in the array cmd, as used in execv. Try it.

• The system function execv requires the full path for the program to execute, starting at the root. For instance, we must use the entire path "/bin/ps", instead of just "ps". Unix provides a similar function, called execvp, that places a pointer to the process environment that contains the location of the file on the disk. When using execvp, you need not provide the entire path for the program to execute:

 
 char * cmd[] = {"ps", "-f", 0}; // in the variable declaration section
 ...
 execvp(cmd[0], cmd);  // in the code section of the program

Modify the code from Exercise 2 to include the lines of code above. Does it produce the same output?

• The equivalent for execl is execlp: just as execvp, it does not require the full path for the program to execute. Try it out.

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Exercise 4.

The word count program wc can take its input from a file whose name is specified in the command line. For instance, the Unix command

wc –L filename

will print out the number of lines in the file with name filename. Write a small program linecount.c that takes any number of file names as command line parameters. For each filename in the command line, the program should create a child process that does the following:

a) prints out the name of the file

b) uses execlp to execute the program “wc –L” on that file

The parent process should not exit until all processes have exited.

Sample output (user input is in italics, aa and bb are two files in the same directory as linecount):

bash$ ./linecount aa bb

aa

3 2 9 aa

bb

5 6 41 bb

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What to turn in:

Hand in a printout of the code for Exercise 4, along with a sample output. Leave the source code for all exercises in your directory csc8410/forkexec. Make sure you add informative comments to your code.