CSc 352: Assignment 10 solution


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The purpose of this assignment is to expand Assignment 9 to function as a simple make program,
using simple system commands to check file dates and to run programs.
General Requirements
1. Your C code should adhere to the coding standards for this class as listed in the
Documents section on the Resources tab for Piazza. This includes protecting against
buffer overflows whenever you read strings.
2. Your programs should indicate if they executed without any problems via their exit
status, i.e., the value returned by the program when it terminates:
Execution Exit Status
Normal, no problems 0
Error or problem encountered 1
3. Under bash you can check the exit status of a command or program cmd by typing the
command “echo $?” immediately after the execution of cmd. A program can exit with
status n by executing “exit(n)” anywhere in the program, or by having main() execute
the statement “return(n)”.
4. Remember your code will be graded on lectura using a grading script. You should test
your code on lectura using the diff command to compare your output to that of the
example executable.
5. To get full points your code should compile without warnings or errors when the -Wall
flag is set in gcc
6. Anytime you input a string you must protect against a buffer overflow. Review slides 82
– 87 of the basic_C deck.
7. You must check the return values to system calls that might fail due to not being able to
allocate memory. (e.g. Check that malloc/calloc don’t return NULL) getline() is an
exception to this rule.
8. Your code must run without errors using valgrind.
9. You must break your code up into at least two source (.c) and one header (.h) files. Your
Makefile should create the executable file in such away that only the files that need to are
10. NEW REQUIREMENT: Your Makefile must include a phony target “clean” that
removes all the object files in the current directory as well as the executable
Example executables of the programs will be made available. You should copy and run these
programs on lectura to test your program’s output and to answer questions you might have about
how the program is supposed to operate. Our class has a home directory on lectura which is:
You all have access to this directory. The example programs will always be in the appropriate
assignments/assg#/prob# subdirectory of this directory. They will have the same name as the
assigned program with “ex” added to the start and the capitalization changed to maintain
camelback. So, for example, if the assigned program is theBigProgram, then the example
executable will be named exTheBigProgram. You should use the appropriate UNIX
commands to copy these executables to your own directory.
Your programs will be graded by a script. This will include a timeout for all test cases. There
must be a timeout or programs that don’t terminate will cause the grading script to never finish.
This time out will never be less than 10 times the time it takes the example executable to
complete with that test input and will usually be much longer than that. If your program takes an
exceedingly long time to complete compared to the example code, you may want to think about
how to clean up your implementation.
You will be required to include a Makefile with each program. Running the command:
make progName
should create the executable file progName, where progName is the program name listed for the
problem. The gcc commands in your Makefile that create the object files must include the -Wall
flag. Other than that, the command may have any flags you desire.
Your Makefile should also include a phony target “clean” that removes all the object files in the
current directory as well as the executable file created as indicated above. Note this command
should not produce error messages even if there are no files to remove.
Submission Instructions
Your solutions are to be turned in on the host Since the assignment will
be graded by a script, it is important you have the directory structure and the names of the files
exact. Remember that UNIX is case sensitive, so make sure the capitalization is also correct. For
all our assignments the directory structure should be as follows: The root directory will be named
assg#, where # is the number of the current assignment. Inside that directory should be a
subdirectory for each problem. These directories will be named prob# where # is the number of
the problem within the assignment. Inside these directories should be any files required by the
problem descriptions.
To submit your solutions, go to the directory containing your assg8 directory and use the
following command:
turnin cs352f18-assg10 assg10
prob1: mymake2
This problem involves extending the mymake program from Assignment 9 to have it implement
the core functionality of the make utility. You will also make the program treat arguments the
same way that make does.
In Assignment 9 you implemented code to construct and traverse a graph structure reflecting the
dependencies specified in an input file. This assignment involves extending the traversal
algorithm to check for file existence and last-modified timestamp associated with files, and
executing the command associated with a target when it is necessary to rebuild that target.
You will also alter how arguments are handled. mymake required 2 arguments. For mymake2
the arguments will be optional and they can come in either order. To indicate which is which, a -f
option flag will proceed the file name.
 Invocation: Your program will be compiled to an executable named mymake2 (see
under Makefile below). It will be invoked as follows:
mymake2 [-f aMakefile] [aTarget]
aMakeFile is is a file specifying dependencies and rules according to the format given
here. If no file name is specified, a default file called myMakefile should be used. It may
not be clear in the browser, but there is white space between the “-f” and the name of the
file. aTarget is the name of a target appearing in the makefile. If no target is specified, the
first target defined in the makefile should be used. This is exactly the same behaivor as
make. Note that the arguments may appear in either order. The name of the makefile
must follow a -f option.
 Behavior: An invocation “mymake2 of your program should behave as follows:
(i) read in the targets and dependencies specified in the makefile and construct the
corresponding data dependency graph; (you have already written the code to do this in
assignment 9)
(ii) traverse this graph using a postorder traversal, starting at the node corresponding to
the target specified (or the first target defined in the makefile if no target is specifed),
recursively (re)build any of the files that the target depends on, and finally (re)build the
target itself, as specified in the section labeled “Processing a Target in a Dependency”.
(This recursive rebuilding amounts to a post-order traversal.)
Important: Your program should free all dynamically allocated memory before exiting.
 Output: The output produced by your program is the sequence of commands that are
executed during the post-order traversal of the dependency graph. (Note that each
command that is executed is also printed out to stdout just before the command is
executed.) If no commands are executed, your program should print the line:
target is up to date.
Where target is the name of the target being built.
 Unix Issues: You can get the time when a file was last modified using the stat(2) library
call: see man 2 stat or man fstat for details. Note that it is your responsibility to allocate a
variable of type struct stat and pass a pointer to this variable to stat(), which will then fill
in the various fields of the struct with information about the file.
 You can execute a shell command using the library call system(3). See man system for
details. Note that you have to declare a variable of type struct stat and pass a pointer
to this variable into the stat() library call; see man 2 stat or man fstat for details.
 Files: You should structure your program so that conceptually distinct pieces of the
program reside in distinct files. For instance, the code for reading the makefile
specifications might be in a different file from the fuctions dealing with graphs. (for
example, finding a node, adding a node, traversing the graph, etc.) You should include at
least 2 different files of source code and one header file. You can include more if you feel
so inclined.
 Errors: Error messages should be sensible and informative (use perror where necessary)
and should be sent to stderr.
The following are all fatal errors and should cause the program to exit immediately with
exit status 1.
o A file name does not follow a -f argument.
o -f appears as an argument more than once.
o More than one target is specified in the arguments.
o Too many arguments are specified.
o The input file (either specified or default) cannot be opened for reading.
o The input file is in an illegal format.
o The specified target is not defined in the input file.
o A file the target recursively depends on does not exist and has no rule defining it
as a target.
o A command that is executed is unsuccessful (returns a value other than 0).
Assumptions and Restrictions
This program obviously builds on your solution to assignment 9. While you may want to
alter your code from assignment 9, the work must still be your own.
Dependency Graphs
A dependency graph is a data structure that captures the dependencies between different files as
specified in a make file. To keep this discussion specific, we will focus here on rules in
makefile2 format; however, the concepts are not specific to this assignment and generalize
readily to the full make utility.
1. Structure
A dependency graph is a directed graph satisfying the following:
Each node represents a target or something a target depends on as given in the input mymake2
file. Thus, for each rule of the form
A : … B …
\t cmd1
\t cmd2
there is a node named A and a node named B in the dependency graph and there is edge from
node A to node B if A “depends on” B.
The sequence of commands (cmd1 and cmd2 in the above example) is associated with the
dependency graph node for the target for the rule.
The following example illustrates the notion of dependency graphs. Consider the following
mymake2 file:
spellcheck.o : utils.h spellcheck.h spellcheck.c
\t gcc -Wall -c spellcheck.c
hash.o : hash.c utils.h hash.h
\t gcc -Wall hash.c
spellcheck : hash.o spellcheck.o
\t gcc *.o -o spellcheck
The corresponding dependency graph is as follows:
The ordering on the children of each node in a dependency graph is significant: it reflects the
left-to-right ordering on the dependencies specified as part of a rule. For example, the first rule in
the example above gives the dependencies of the target “spellcheck.o” in the following left-toright order:
The children of the node corresponding to this target in the dependency graph shown above
reflect this ordering.
2. Processing a Target in a Dependency Graph
A target aTarget in a dependency graph is processed as follows:
 Suppose that there is a rule with target aTarget:
aTarget : aDep1 … aDepn
In this case, we carry out the following actions:
1. Recursively process each of the targets aDep1 . . .aDepn (in that order);
2. If (a) aTarget does not exist, or (b) aTarget exists but one (or more) of the files
aDepi that it depends on is newer than aTarget, i.e., has a more recent lastmodified-time, then the command for the rule for aTarget is executed.
At each point of this recursive traversal, your program should print out to stdout
any command to be executed immediately before executing it.
 If there is no rule with target aTarget, then the file aTarget must exist; if it does not, it is
an error.
In other words you should be able to use your code from Assignment 9 to traverse the graph, but
instead of printing the name of the node, if that node represents a target, you process it using step
2 above. If it was not a target (i.e. it was a dependency for a target but no command for it was
given), then you need to make sure a file with that name exists.
Further Notes On Processing Commands
 Circular Dependencies
To match the way make checks for dates you need to detect circular dependencies and
treat them differently from cases where a node has already been visited, but not in a
cycle. It is actually not hard to detect cycles. To do so you need to add another mark (or
use different values for the same mark) to indicate when a node is “completed”. When
you first enter your post order traversal function (POT), you should mark the node as
“visited”. At the very end of the POT you should mark the node as “completed”. When
looping through a node’s dependencies, if you encounter a dependency that has been
“visited” but not “completed”, then you have detected a cycle. In this case an error
message should be printed out and the dependency in question should be ignored. NOTE
that even though you print a message to standard error, the exit status is still 0 in
this case. This is an exception to our usual rule. We make this exception to match the
behavior of make. The algorithm for this is sketched out below.
 When to grab modify dates
When you grab the modify date will affect whether a command is executed since any
command can modify a file and change the date. We will try to mimic make in the way
we get the dates. In POT, grab the node’s modify date after you mark it as visited, but
before you go through its dependencies. Then, if the node’s commands are run, grab the
modify date again after the commands are finished.
 What to do when no file is found
First, we will simplify our check. If stat() does not return a 0, we will assume it is
because the file isn’t found. There are actually many other reasons stat() might not return
a 0, but for simplicity we will ignore them. If the file isn’t found and it is not a target, then
an error message is printed and execution halts (after freeing memory). If the node is a
target, even if it contains no commands that build the file (or even no commands at all),
this is NOT an error. This seems strange to me, but it is how make works. If the file does
not exist, then its commands will be run when that point of the code is reached. If after a
target’s commands are run the file still does not exist, then any target which has it as a
dependency has its commands run.
Here is an algorithm that incorporates what is written above (Note that this depends on some
flags being set prior. For instance all the visited flags should initially be false.):
if n.visited then return
n.visited = true
set n.fileDate and n.doesExist
if not n.doesExist
if n not a target then
exit with error
n.mustBuild = true
for each dependency d of n do
if not d.completed (cycle found)
print err message (but don’t change program return status)
else if not n.mustBuild
if not d.doesExist or (d.fileDate > n.fileDate)
n.mustBuild = true
if n.mustBuild
run all of n’s commands (if any fail, exit with error)
set n.fileDate and n.doesExist
n.completed = true
Note this is just an algorithm to give what the program should do, not an implementation. You
may choose to implement it using different variables. For example, I use one integer field to
record both visited and completed.

3. Unix Issues
You can get the time when a file was last modified using the stat(2) library call: see man 2
stat or man fstat for details.
Note that it is your responsibility to allocate a variable of type struct stat and pass a pointer
to this variable to stat(), which will then fill in the various fields of the struct with information
about the file.
You can execute a shell command using the library call system(3). See man system for details.
4. Note on Testing
If you read the man page for 2 stat() then you will see that the modify date is stored in a field of
time time_t. What is actually saved is a time in seconds that have elapsed since Jan 1, 1970. The
good news is that this means that comparing dates is merely a matter of comparing two numbers
(the larger number is more recent). The bad news is that at a resolution of 1 second, mymake2
may not be able to see the more recent file if both were updated within the same second. Once
solution is to read at the bottom of the man page how to get the update time in “nano” seconds (I
put it in quotes because I’m sure lectura is not that accurate . . . milliseconds maybe). You are not
required to do this for this project though. So instead you may want to make your test files work
so that at least one second passes before the next targets commands are run. For example, your
myMakefile might include lines that look like:
A : B C
touch A
sleep 1
The sleep will cause at least one second difference in time between the time A is updated and when the
next targets commands are checked. Note: do NOT put a sleep in your program code. This will slow it
down. We will create test files that guarantee if a file is updated then its file date (to the second) will be
more recent than other files.