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CS107 Assignment 2: String me along
Assignment 2: String me along

Due: Mon Oct 16 11:59 pm
Ontime bonus 5%. Grace period for late submissions until Wed Oct 18 11:59 pm

Assignment by Julie Zelenski and Chris Gregg

Learning goals

This assignment is designed to give you

  • practice with C-strings (both raw manipulation and using string library functions)
  • an opportunity to view Unix utility programs from an internal perspective, as implementer not just client
  • exposure to interacting with the Unix filesystem and shell environment variables

Overview

Watch video walkthrough!

This assignment consists of some code-study exercises and a small program to write. Two of the code excerpts come from the C standard library (atoi and strtok) and the third introduces you to the opendir and readdir functions.

The program you will write is a version of the Unix which command, a utility used to locate and identify executable programs to run. This is an especially apropos way to be introduced to C and Unix; not only does it continue a thread you began in assign0, but implementing the Unix operating system and its command-line tools were what motivated the creation of the C language in the first place! Implementing such a utility program is a very natural use of C, and you'll see how comfortably it fits in this role.

Get started

Check out the starter project from your cs107 repo using the command

git clone /afs/ir/class/cs107/repos/assign2/$USER assign2

The starter project contains code.c with the code for the code-reading exercises, C files tokenize.c, scan_token.c, and mywhich.c, and the supporting Makefile, custom_tests, and readme.txt files. In the samples subdirectory, you'll find our sample solutions.

1. Code study: atoi

In assign0, you used the atoi function to convert command line arguments from strings to integers. The function name comes from ascii to integer, and as you found out in assign0, it is not particularly robust -- it bails at the first non-digit character with no indication the conversion failed. atoi has largely been superseded by the more full-featured strtol (used in assign1), but we chose the atoi implementation as the easier one to study.

Below is an implementation of atoi that uses pointer increment to advance through the input string. Although it is probably cleaner to use array indexing, if you read much C code, you will see plenty of pointer arithmetic, so you should get used to reading and understanding it.

int atoi(const char *s)
{
    int n=0, neg=0;
    while (isspace(*s)) s++;
    switch (*s) {
        case '-': neg=1;
        case '+': s++;
    }
    /* Compute n as a negative number to avoid overflow on INT_MIN */
    while (isdigit(*s))
        n = 10*n - (*s++ - '0');
    return neg ? n : -n;
}

In your readme.txt file for assign2, answer the following questions:

  1. How is a single digit character converted to its numeric value?
  2. If the string begins with a leading minus, at what point does it advance s past that char? (Look closely! How the control flows is subtle and easily overlooked)
  3. The loop builds up the number as a negative value and later negates it. The comment indicates this choice avoids overflow on INT_MIN. Why does INT_MIN necessitate such a special case?
  4. Below are five invalid calls to atoi. For each call, work out what is returned and then verify that your understanding is correct by running the program in code.c. In your readme.txt, indicate what is returned for each call and explain why.
    atoi(" $5");
    atoi("12:34");
    atoi("-2147483649");
    int num = 50;
    atoi(&num);
    atoi(num);
    

2. Code study: strtok

A common string-handling need is to split a string into "tokens" which are separated by one or more delimiting characters. The strtok function is the C standard library function used to split strings. However, unlike functions you may have used in CS106B (e.g., stringSplit), a single call to strtok does not return a nicely formatted vector of the tokens. Rather, you must call strtok repeatedly, each time receiving a single token, and continue until strtok returns NULL to indicate there are no more tokens.

Start by reading the function's man page (man strtok) to understand its basic operation. Be sure to read the BUGS section of the man page where it critiques the awkward design of this function.

The first peculiar feature of strtok is that it destructively modifies the input string. Rather than construct a new substring for each token, it overwrites the token's ending delimiter in the input string with a null byte, thus re-purposing the existing sequence of characters from the input string to become the token substring without copying those characters to new memory.

Another odd design decision is that strtok keeps track of the current state of the tokenization process on behalf of future calls to the function. The first time you call strtok on a string to tokenize, you pass the input string as the first argument, but for subsequent calls to strtok you pass NULL as the first argument. strtok tracks the input being tokenized by internally maintaining a pointer to the beginning of the next token. This variable is declared with the static qualifier, the purpose of static will be explored in a question below.

The following is musl's implementation of the strtok function:

 1    char *strtok(char *s, const char *sep)
 2    {
 3         static char *p = NULL;
 4
 5         if (s == NULL && ((s = p) == NULL))
 6             return NULL;
 7         s += strspn(s, sep);
 8         if (!*s)
 9             return p = NULL;
10         p = s + strcspn(s, sep);
11         if (*p)
12             *p++ = '\0';
13         else 
14              p = NULL;
15         return s;
16     }

In your readme.txt file for assign2, answer the following questions:

  1. This is likely the first time you have seen the static qualifier applied to a local variable. The Wikipedia article on static variables provides an overview of the static qualifier, and the Scope and Example section demonstrates an example of a static local variable and design rationale . Why does strtok declare the local variable p as static?
  2. Changing the initialization to static char *p = s and re-compiling will produce a compiler error. What is the error message? Use your C reference or web search to research this error message and how it relates to static variables. In readme.txt, explain how static variables are initialized and how it differs from non-static local variables.
  3. The first time strtok is called, the input string is passed as the first argument. On subsequent calls to continue tokenizing the same string, NULL is passed as the first argument. Given this context, when will the test in line (5) evaluate to true?
  4. Read the man page for strspn and explain what happens on line (7) when the remaining part of the input string consists of only delimiter characters.
  5. Explain what line (12) accomplishes and what this does to the input string.

3. Write scan_token

With the goal of making an improved function that performs the same type of tokenization as strtok without its awkward design, you are to write the function scan_token. You will write and test this function in isolation now, and then will use the function later when writing the mywhich program. The required prototype for scan_token is:

bool scan_token(const char **p_input, const char *delimiters,
                char buf[], size_t buflen);

The function scans the input string to determine the extent of the token using the delimiters as separators and then writes the token characters to buf , making sure to terminate with a null char. The function returns true if a token was written to buf, and false otherwise.

Specific details of the function's operation:

  • The function separates the input into tokens in the same way that strtok does: it scans the input string to find the first character not contained in delimiters. This is the beginning of the token. It scans from there to find the first character contained in delimiters. This delimiter (or the end of the string if no delimiter was found) marks the end of the token.

  • Note that the parameter p_input is a char **. This is a pointer argument that is being passed by reference. The client passes a pointer to the pointer to the first char of the input string. The function will update the pointer held by p_input to point to the next character following the token that was just scanned.

  • buf is a fixed-length array to store the token and buflen is the length of the buffer. scan_token should not write past the end of buf. If a token does not fit in buf, the function should populate buf with buflen - 1 characters, and write a null byte in the last slot. If the token is longer than buflen, the function should update the pointer held by p_input to point to the next character following the buflen - 1 characters in the token. In other words, the next token scanned will start at the first character that would have overflowed buf.

Consider this sample use of scan_token:

    const char *input = "super-duper-awesome-magnificent";
    char buf[10];
    const char *remaining = input;

    while (scan_token(&remaining, "-", buf, sizeof(buf))) {
        printf("Next token: %s\n", buf);
    }

Running the above code produces this output:

    Next token: super
    Next token: duper
    Next token: awesome
    Next token: magnifice
    Next token: nt

Write your implementation of scan_token in the scan_token.c file. You can test it using our provided tokenize.c program. The tokenize program is integrated with sanitycheck.

In addition to teaching you the inner workings of computer systems, CS107 also provides strength+cardio training for your coding skills. A key piece of this training is learning the value of thoughtful and thorough testing--better for you to find the bugs than our autograder! The default tests supplied with sanitycheck are a start but these basic tests are not comprehensive and should be supplemented with your own tests. In order to encourage you to do the careful testing that we hope you would do anyway, for assign2 we require that you submit your sanitycheck custom_tests file with at least 5 thoughtful and varied tests of your own for the tokenize program. These tests should cover a variety of cases that validate that scan_token is working properly on ordinary cases as well on on inputs that are unusual or edge conditions.

4. Code study: opendir/readdir

Many Unix utilities read and write from the filesystem. The <dirent.h> header file provides functions used to access to the contents of directories. In particular, you will be using the opendir and readdir functions in your mywhich program to get information about the files in a given directory.

The <dirent.h> header defines two important data types:

  • DIR : a type representing a directory stream

  • struct dirent : a record of information for one file or directory (name, number, type, and so on). Read the man page for readdir (man readdir) to see the struct definition and its documentation.

The easiest way to see how to gather directory information is through an example:

#include <dirent.h>
#include <stdio.h>

void list_filenames(const char *dirpath)
{
    DIR *dp = opendir(dirpath);
    if (dp == NULL) return;

                   // loop through directory entries
    struct dirent *entry;
    while ((entry = readdir(dp)) != NULL) {
        printf("%s\n",entry->d_name);
    }

    closedir(dp);
}

Notes:

  • If you are used to C++, the struct dirent *entry; line might look a bit funny. In C, unless structs are typedef'd, you need to declare a variable using the struct tag. As with C++, structs access their members with dot or arrow notation, as in entry->d_name in the program above.

  • You must call the closedir function after you are done with a DIR pointer to release its resources. The opendir call both allocates dynamic memory and uses an entry in the file table. If you forget to close the DIR, those resources cannot be reclaimed. Run the code.c program under valgrind with and without the call to closedir(dp) and see what Valgrind has to say about this.

In your readme.txt file, answer the following questions:

  1. What does struct dirent define to be the maximum filename length?
  2. How many bytes of memory does Valgrind report are "lost" if a program does an opendir without a matching closedir?

Review and comment starter code

The file mywhich.c is given to you with an incomplete main function that sketches the expected behavior for the case when mywhich is invoked with no arguments. You are to first read and understand this code, work out how to change/extend it to suit your needs, and finally add comments to document your strategy.

Some questions you might consider for self-test: (do not submit answers)

  • What is the third argument to main? How do you determine the end of the envp array?
  • What is PATH_MAX? What is it used for?
  • If the user's environment does not contain a value for MYPATH, what does mywhich use instead?
  • Do you see anything unexpected or erroneous? We intend for our code to be bug-free; if you find otherwise, please let us know!

As per usual, the code we provide has been stripped of its comments and it will be your job to provide the missing documentation.

5. Implement the mywhich program

What does the which command do?

The which command searches for a command by name and reports where its matching executable file was found. Read its man page (man which) and try it out, e.g. which ls or which make or which vim. The response from which is the full path to the matching executable file or no output if not found.

It may not be obvious at first, but this search is intimately related to how commands are executed by the shell. When you run a command such as ls or vim, the shell searches for an executable program that matches that command name and then runs that program.

Where does it search for executables? You might imagine that it looks for an executable file named vim in every directory on the entire filesystem, but such an exhaustive search would be both incredibly inefficient and dangerously insecure. Instead, it searches only those directories that have been explicitly listed in the user's search path. The default search path includes directories such as /usr/local/bin/ and /usr/bin/ which house the executable files for the standard unix commands. (The name bin is a nod to the fact that executable files are encoded in binary).

The user can configure their search path by changing the value of their PATH environment variable. As you saw in lab2, environment variables track information like username (USER=zelenski) and the user's shell (SHELL=/bin/bash). The environment variable of particular interest for which is PATH=/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/sbin:/sbin:/usr/games. The value for PATH is a sequence of directories separated by colons; these are the directories searched when looking for an executable. When looking for a command, which searches the directories in the order they are listed in the search path and stops at the first directory that contains a matching executable. In order to match, the file's name must be an exact match and the file must be readable and executable by the user.

How does mywhich operate?

The mywhich program you are to write is similar in operation to the standard which with these differences:

  • mywhich uses the environment variable MYPATH for the search path. If no such environment variable exists, it falls back to PATH. (Standard which always uses PATH as the search path)
  • mywhich invoked with no arguments prints the list of directories searched. (standard which with no arguments does nothing)
  • mywhich treats each command-line argument prefixed with a + as a wildcard match. (standard which has no option for wildcard match)
  • mywhich does not support the -a flag. (standard which -a prints all exact matches)

When invoked with no arguments, mywhich prints the directories in the search path, one directory per line. This use case is a testing aid to verify that you are accessing the correct environment variable and can properly tokenize it.

myth> ./mywhich 
Directories in search path:
/usr/bin
/usr/local/bin
/usr/pubsw/bin
/bin

When invoked with one or more arguments, mywhich searches the directories in the search path for an exact match for each argument. The sample output below shows invoking mywhich to find three executables. Two of them were found, but no executable named submit was found in any directory in the user's MYPATH and thus nothing was printed for it.

myth> ./mywhich xemacs submit cp
/usr/bin/xemacs
/bin/cp

Any argument prefixed with + is handled as a wildcard search, instead of an exact match. A wildcard search prints all executables that contain that pattern from all directories in the search path. Let's say you vaguely remember there is a "fun" unix command, so use a wildcard search to find it:

myth> ./mywhich +fun
/usr/bin/funzip
/usr/bin/pdfunite

For testing purposes, you should test having run mywhich with different directories in the search path. Rather than muck with your actual PATH (which can create total chaos), we recommend that you change MYPATH, which only affects mywhich and nothing else. Use env to set the value of MYPATHwhen running mywhich like this:

    myth> env MYPATH=/tmp:tools ./mywhich submit
    tools/submit

Requirements for mywhich

  • Usage. The mywhich program is invoked with zero or more arguments. Any argument prefixed with + is handled as a wildcard search. All other arguments are searched for using exact match.

  • Assumptions. You may assume correct usage in all cases and that the user's MYPATH and PATH variables are well-formed sequences of one or more directories separated by colons. You do not need to detect or cope with situations where these assumptions do not hold and we will not test on any inputs that violate these assumptions, e.g. no usage of unsupported -a flag and no malformed values for MYPATH.

  • Operation. The user's MYPATH (or PATH if there is no MYPATH variable in the user's environment) defines the search path. The directories are searched in the order they are listed in the search path. For an exact match, the search stops at the first directory containing a readable, executable file matching the command name. For a wildcard search, it searches all directories and prints all matching executables.

  • Expected output. For each command name, it prints the full path to the first matching executable or nothing if no matching executable was found. The matched executables are listed in the order that the command names were specified on the command-line. For a wildcard search, it prints the full path for every matching executable in any of the directories in search path. The directories are searched in order of the search path, but the matching files from the directory may be printed in any order.

  • Restrictions. Your own code should manually search the environment using the envp argument to main. Your code is prohibited from using facilities such as getenv, env, and which to do this work on your behalf.

Advice/FAQ

Don't miss out on the good stuff in our companion document!
Go to advice/FAQ page

Grading

Have you read how assignments are graded? For this assignment, the anticipated point breakdown will be in the neighborhood of:

Readme questions (35 points)

  • readme.txt. (30 points) For the code reading questions, you will be graded on the understanding of the issues demonstrated by your answers and the correctness of your conclusions.
  • custom_tests. (5 points) These points reward your effort in identifying cases that provide comprehensive test coverage for the tokenize program. We are looking for at least 5 thoughtful tests that cover a variety of inputs, including edge conditions.

Functionality (65 points)

  • Sanity cases (25 points) Correct results on the default sanity check tests.
  • Comprehensive/stress cases (30 points) Correct results for additional test cases with broad, comprehensive coverage and larger, more complex inputs.
  • Clean compile (2 points) Compiles cleanly with no warnings.
  • Clean run under valgrind (6 points) Clean memory report(s) when run under valgrind. Memory errors (invalid read/write, use of freed memory, etc) are significant deductions. Memory leaks are a minor deduction. Every normal execution path is expected to run cleanly with no memory errors nor leaks reported. We will not test exceptional/error cases under Valgrind.

Code review (buckets together weighted to contribute ~15 points)

  • Use of pointers and memory. We expect you to show proficiency in handling pointers/memory, no unnecessary levels of indirection, correct use of pointee types and typecasts, and so on. For this program, you should not need and should not use dynamic memory (i.e. no malloc/strdup).
  • Program design. We expect your code to show thoughtful design and appropriate decomposition. Data should be logically structured and accessed. Control flow should be clear and direct. When you need the same code in more than one place, you should unify, not copy and paste.
  • Style and readability. We expect your code to be clean and readable. We will look for descriptive names, defined constants (not magic numbers!), and consistent layout. Be sure to use the most clear and direct C syntax and constructs available to you.
  • Documentation. You are to document both the code you wrote and what we provided. We expect program overview and per-function comments that explain the overall design along with sparing use of inline comments to draw attention to noteworthy details or shed light on a dense or obscure passage. The audience for the comments is your C-savvy peer.

On-time bonus (+5%)

The on-time bonus for this assignment is 5%. Submissions received by the due date earn the on-time bonus. The bonus is calculated as a percentage of the point score earned by the submission.

Finish and submit

Review the How to Submit page for instructions. Submissions received by the due date receive the on-time bonus. If you miss the due date, late work may be submitted during the grace period without penalty. No submissions will be accepted after the grace period ends, please plan accordingly!

How did it go for you? Review the post-task self-check.