CSCI 340, Project 2a (Fork/exec) and 2b (Signal handling, Unix Shell)

CSCI 340,
Spring 2013

Project 2a
(Fork/exec) and 2b (Signal handling, Unix Shell)


The purpose of this assignment is to become more
familiar with the concepts of process control and signaling. You’ll do this by
writing a simple Unix shell program that supports job control. This assignment
is based on a similar assignment developed by Anderson and Dahlin.


You may work in a group of two people in solving the
problems for this assignment. Your solutions will be submitted electronically
using the submit script discussed in class. Clarifications to the assignment
will be provided in class. Revisions will be posted to the CSCI 340 course Web

Note: This project will be graded on stono. Although you are
welcome to do testing and development on any platform you like, I do not have
the time to assist you in setting up other environments. You must test and do
final debugging on stono. The statement, “Well, it worked on my machine” will
not be considered in the grading process.


The provided file, shlab-starter.tar contains a template
for your program along with a number of useful helper functions. Get it from
the course Web page:

$ wget

Put the file
shlab-starter.tar in a properly named project 2a directory. Then type the
following commands in order:

$ tar
xvf shlab-starter.tar

$ make

The tar command will extract the tarfile and the make command will compile and link some test routines. Now, edit
the file, README and enter your team
member names at the top of the file.

Looking at the
file, tsh.c (tiny shell), you will
see that it contains a functional “skeleton” of a simple Unix shell. To help
you get started, the less interesting functions have already been implemented.
Your assignment is to complete the remaining empty functions listed below. As a
“sanity check”, the approximate number of lines of code for each of these
functions is listed in the reference solution (which includes lots of

eval: Main
routine that parses and interprets the command line. [70 lines]

builtin cmd:
Recognizes and interprets the built-in commands: quit, fg, bg, and jobs. [25

do bgfg:
Implements the bg and fg built-in commands. [50 lines]

waitfg: Waits
for a foreground job to complete. [20 lines]

sigchld handler:
Catches SIGCHILD signals. [80 lines]

sigint handler:
Catches SIGINT (ctrl-c) signals. [15 lines]

sigtstp handler:
Catches SIGTSTP (ctrl-z) signals. [15 lines]

Each time you
modify the file tsh.c, type make to recompile and link it. To run
your shell, type on the command line:

$ ./tsh

[type commands to your shell here]

General Overview of Unix Shells

A shell is an
interactive command-line interpreter that runs programs on behalf of the user.
A shell repeatedly prints a prompt, waits for a command line on stdin, and then carries out some action,
as directed by the contents of the command line.

The command line is a sequence of
ASCII text words delimited by whitespace. The first word in the command line is
either the name of a built-in command or the pathname of an executable file.
The remaining words are command-line arguments. If the first word is a built-in
command, the shell immediately executes the command in the current process.
Otherwise, the word is assumed to be the pathname of an executable program. In
this case, the shell forks a child process, then loads and runs the program in
the context of the child. The child processes created as a result of
interpreting a single command line are known collectively as a job. In general, a job can consist of
multiple child processes connected by Unix pipes.

If the command
line ends with an ampersand “&”, then the job runs in the background, which means that the shell
does not wait for the job to terminate before printing the prompt and awaiting
the next command line. Otherwise, the job runs in the foreground, which means that the shell waits for the job to
terminate before awaiting the next command line. Thus, at any point in time, at
most one job can be running in the foreground. However, any number of jobs can
run in the background. For example, typing the command line tsh
jobs causes the shell to execute the built-in jobs command. Typing the
command line tsh /bin/ls -l - d runs the ls program in the foreground. By convention, the shell ensures that
when the program

begins executing
its main routine int main(int argc, char *argv[]) the argc and argv arguments
have the following values:

argc == 3 ,

argv[0] == “/bin/ls”

argv[1]== “-l”

argv[2]== “-d”

Alternatively, typing the command line

/bin/ls -l -d & runs the ls program
in the background.

Unix shells
support the notion of job control,
which allows users to move jobs back and forth between background and
foreground, and to change the process state (running, stopped, or terminated)
of the processes in a job. Typing ctrl-c causes
a SIGINT signal to be delivered to each process in the foreground job. The
default action for SIGINT is to terminate the process. Similarly, typing ctrl-z causes a SIGTSTP signal to be
delivered to each process in the foreground job. The default action for SIGTSTP
is to place a process in the stopped state, where it remains until it is
awakened by the receipt of a SIGCONT signal. Unix shells also provide various
built-in commands that support job control. For example:

jobs: List
the running and stopped background jobs

bg <job:
Change a stopped background job to a running background job

fg <job:
Change a stopped or running background job to a running in the foreground

kill <job:
Terminate a job

Part 1: Fork/exec

In this phase of the project, you will learn about
the fork and exec system calls that you will use in the rest of the project.

Part 1-1: Reading

Read chapter 3 of Anderson and Dahlin, Operating Systems. Read this handout
before you write any code.

Part 1-2: Fibonacci

Update fib.c so that if invoked on the command line with some integer
argument n, it recursively computes
the nth Fibonacci number (n ≤ 13). For example

$ ./fib


$ ./fib


The “trick” is that each recursive
call must be made by a new process, so you will call fork() and then have the
new child process call doFib().

The parent must
wait for the child to complete and you need to figure out how to pass the
result of the child’s computation to its parent.

Part 1-3: Fork/Exec

The fork system
call creates a child process that is nearly identical to the parent. The exec call replaces the state of the
currently running process with a new state to start running a new program in
the current process.

Your job is to
create a prototype for the shell you will be creating later. This prototype
waits for a line of input. If the line is “quit”, it exits. Otherwise, it
parses the line and attempts to execute the program at the path specifed by the
first word with the arguments specifed by the remaining words. It waits for
that job to finish. Then it waits for the next line of input.

The prompt should be the string “psh

The command line typed by the user should consist of a name and zero or more arguments, all
separated by one or more spaces. If name is
a built-in command, then psh should
handle it immediately and wait for the next command line. Otherwise, psh should assume that name is the path of an executable file,
which it loads and runs in the context of a child process ( In this context,
the term job refers to this child
process). Your shell then waits for that job to finish. Then it waits for the
next line of input.

All commands and jobs are
executed in the foreground. In this phase you don’t have to worry about
background jobs. You also can assume that jobs execute until they exit; you
don’t need to worry about signal handling.

Your shell should implement one built-in command: quit. If the user types quit, your shell should exit.

For example, the following runs the ls program in the foreground:

/bin/ls -l - d

The file psh.c, provides framework for your shell, and util.h/util.c provide some helper functions. Read these files.

Update the file psh.c by implementing the functions eval(), which the main() function calls to process one line of input, and builtin cmd(), which your eval() function should call to parse and
process the built-in quit command.
(Later, you will extend the built-in command function to handle other built-in

Project 2a: Signal handling, Shell

Part 2a-1: Reading

Read chapter 3 of Anderson and Dahlin, Operating Systems. Examine the code for
the Signal() function in util.c.

Part 2a-2: Signal handling

Write a program in handle.c that first uses the getpid() system call to find its process
ID, then prints that ID, and finally loops continuously, printing “Still here\n”
once every second. Set up a signal handler so that if you hit ˆc (ctrl-c), the
program prints “Nice try.\n” to the screen and continues to loop.

Note: The printf() function is technically unsafe
to use in a signal handler. A safer way to print the message is to call

bytes; const int STDOUT = 1 ; bytes = write(STDOUT, "Nice try.\n",
10) ; if(bytes != 10) exit(-999);

Note: You should
use the nanosleep() library call
rather than the sleep() call so that
you can maintain your 1-second interval between “Still here” messages no matter
how quickly the user hits ˆc. You can terminate this program using kill -9. For
example, if the process ID is 4321

$ kill
-9 4321

Part 2a-3: Signal sending

Update the program from Part 2a-2 to catch the SIGUSR1
signal, print “exiting”, and exit with status equal to 1.

Now write a
program mykill.c that takes a process ID as an argument and that sends the
SIGUSR1 signal to the specifed process ID. For example

./handle 4321



Still here

$ ./mykill 4321




Part 2a-4: Signal mechanics

If you compile a C program with the
-S flag, the compiler produces the assembly language corresponding the the code
it would generate for the program. For example

$ gcc
-S handle.c $ cat handle.S ...

Also, in the gdb debugger, you can see the assembly code for
a function. For example

$ gdb

disassemble main

Dump of
assembler code for function main:

<main+0: push    %rbp

<main+1: mov %rsp,%rbp

<main+4: push    %r12

<main+6: push    %rbx ...

In gdb, you can put a breakpoint for a function

break main

1 at 0x10000097b: file handle.c, line 30.

( gdb )

and you can step to
the next C/C++ instruction or stepi to
the next assembly instruction


program: /Users/dahlin/Classes/439/labs/shlab/src/handle

1, main (argc=1, argv=0x7fff5fbff6e0) at handle.c:30

30    int pid
= getpid();


31    printf("%d\
n", pid);


31 printf("%d\ n", pid); (gdb)

31 printf("%d\ n", pid);

( gdb )

Finally, you can tell GDB to pass a particular signal to
your program

handle SIGUSR1 pass

Signal     Stop Print Pass to program Description

SIGUSR1    Yes Yes Yes User defined signal 1

handle SIGUSR1 nostop

Signal     Stop Print Pass to program Description

SIGUSR1    No Yes Yes User defined signal 1

( gdb )

In the file README, answer the following questions

1.    What
is the last assembly language instruction executed by the signal handler
function that you write?

2.    After
the instruction just identified executes, what is the next assembly language
instruction executed?

When the signal handler finishes running, it must
restore all of the registers from the interrupted thread to exactly their
values before the signal occurred. How is this done?

Project 2b: Shell

In this phase of the project,
you will implement your simple shell, tsh.
Your tsh shell should have the
following features:

The prompt should be the string “tsh”.

The command line typed by the user should consist of a name and zero or more arguments, all
separated by one or more spaces. If name is
a built-in command, then tsh should
handle it immediately and wait for the next command line. Otherwise, tsh should assume that name is the path of an executable file,
which it loads and runs in the context of an initial child process (In this
context, the term job refers to this
initial child process).

tsh need not
support pipes (|) or I/O redirection (<and

Typing ctrl-c (ctrl-z) should cause a SIGINT (SIGTSTP)
signal to be sent to the current foreground job, as well as any descendants of
that job (e.g., any child processes that it forked). If there is no foreground
job, then the signal should have no effect.

If the command line ends with an ampersand “&”,
then tsh should run the job in the
background. Otherwise, it should run the job in the foreground.

Each job can be identified by either a process ID (PID)
or a job ID (JID), which is a positive integer assigned by tsh. JIDs should be
denoted on the command line by the prefix “%”. For example, “%5” denotes JID 5,
and “5” denotes PID 5. All the routines needed for manipulating the job list
have been provided.

tsh should
support the following built-in commands:

The quit command
terminates the shell.

The jobs command
lists all background jobs.

The bg <jobcommand restarts <jobby sending it a SIGCONT signal, and then runs it in the
background. The <jobargument can be either a PID or a

–    The
fg <jobcommand restarts <jobby sending it a SIGCONT signal, and
then runs it in the foreground. The <jobargument can be either a PID or a

tsh should
“reap” all of its zombie children. If any job terminates because it receives a
signal that it didn’t catch, then tsh should
recognize this event and print a message with the job’s PID and a description
of the offending signal.

Checking Your Work

Some tools to help check your work have been provided.

. The Linux executable tshref
is the reference solution for the shell. Run this program to resolve any
questions you have about how your shell should behave. Your tsh shell should emit output that is identical to the reference solution (except
for PIDs, of course, which change from run to run).

Shell driver. The program executes a shell as a child process, sends it
commands and signals as directed by a trace file, and captures and displays the
output from the shell. Use the -h argument to find out the usage of

./ - h

./ [-hv] -t <trace -s <shellprog -a <args

-h  Print this message

-v  Be more verbose

<trace    Trace file

<shell    Shell program to test

<args     Shell arguments

-g  Generate output for autograder

16 trace files (trace01-16.txt) have been provided to be
used in conjunction with the shell driver to test the correctness of your
shell. The lower-numbered trace files do very simple tests, and the
higher-numbered tests do more complicated tests. You can run the shell driver
on your shell using trace file trace01.txt
(for instance) by typing:

./ -t trace01.txt -s ./tsh -a "-p"

(the -a “-p” argument tells your shell not to emit a
prompt), or

$ make

Similarly, to
compare your result with the reference shell, you can run the trace driver on
the reference shell by typing:

./ -t trace01.txt -s ./tshref -a "-p" or

$ make

For your reference, tshref.out gives the output of the
reference solution on all races. This might be more convenient for you than
manually running the shell driver on all trace files.

The neat thing
about the trace files is that they generate the same output you would have
gotten had you run your shell interactively (except for an initial comment that
identifies the trace). For example:

$ make

-t trace15.txt -s ./tsh -a "-p"


trace15.txt - Putting it all together



Command not found tsh ./myspin 10

Job [1]
(7858) terminated by signal 2 tsh ./myspin 3 & [1] (7860) ./myspin 3
& tsh ./myspin 4 & [2] (7862) ./myspin 4 & tsh jobs

(7860) Running ./myspin 3 & [2] (7862) Running ./myspin 4 & tsh fg

Job [1]
(7860) stopped by signal 20 tsh jobs

(7860) Stopped ./myspin 3 & [2] (7862) Running ./myspin 4 & tsh bg
%3 %3: No such job tsh bg %1

(7860) ./myspin 3 & tsh jobs

(7860) Running ./myspin 3 & [2] (7862) Running ./myspin 4 & tsh fg
%1 tsh quit


Your solution
shell will be tested for correctness on a Linux machine, using the same shell
driver and trace files that were included in your lab directory. Your shell
should produce identical output on
these traces as the reference shell, with only two exceptions:

The PIDs can (and likely will) be different

The output of the /bin/ps
commands in trace11.txt, trace12.txt, and trace13.txt will be different from run to run. However, the running
states of any myspin processes in the
output of the /bin/ps command should
be identical.


General hints

The waitpid,
kill, fork, execve, setpgid
, and sigprocmask
functions will come in very handy. The WUNTRACED and WNOHANG options to waitpid will also be useful.

Programs such as more,
less, vi
, and emacs do strange things
with the terminal settings. Don’t run these programs from your shell. Stick
with simple text-based programs such as /bin/ls,
/bin/ps, and /bin/echo.

Hints for part 2 b

Use the trace files to guide the development of your
shell. Starting with trace01.txt,
make sure that your shell produces the identical output as the reference shell.
Then move on to trace file trace02.txt,
and so on.

When you implement your signal handlers, be sure to
send SIGINT and SIGTSTP signals to the entire foreground process group, using
“-pid” instead of “pid” in the argument to the kill function. The
program tests for this error.

One of the tricky parts of the assignment is deciding
on the allocation of work between the waitfg
and sigchld handler functions.
The following approach is recommended:

In waitfg,
use a busy loop around the sleep function.

–    In
sigchld handler, use exactly one call
to waitpid.

While other solutions are
possible, such as calling waitpid in
both waitfg and sigchld handler, these can be very confusing. It is simpler to do
all reaping in the handler. Note that you probably can do something simpler for
the prototype psh you build in part
1. Then, be ready to change how this works when you get to part 3.

In eval, the
parent must use sigprocmask to block
SIGCHLD signals before it forks the child, and then unblock these signals,
again using sigprocmask after it adds
the child to the job list by calling addjob.
Since children inherit the blocked vectors of their parents, the child must be
sure to then unblock SIGCHLD signals before it execs the new program. The
parent needs to block the SIGCHLD signals in this way in order to avoid the
race condition where the child is reaped by sigchld
(and thus removed from the job list) before the parent calls addjob.

When you run your shell from the standard Unix shell,
your shell is running in the foreground process group. If your shell then
creates a child process, by default that child will also be a member of the
foreground process group. Since typing ctrl-c sends a SIGINT to every process
in the foreground group, typing ctrl-c will send a SIGINT to your shell, as
well as to every process that your shell created, which obviously isn’t

Here is the workaround: After the fork, but before the execve, the child process should call setpgid(0, 0), which puts the child in a
new process group whose group ID is identical to the child’s PID. This ensures
that there will be only one process, your shell, in the foreground process
group. When you type ctrl-c, the shell should catch the resulting SIGINT and
then forward it to the appropriate foreground job (or more precisely, the process
group that contains the foreground job).

Submission Instructions

Make sure you have included your names in the README

submit directory must contain source files, README, and

Use the submit script to submit your work

For example, if your work is in the directory,

submit csci340 proj2a assign2a

Good luck!
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