Assignment: Pointers

Contents

Overview

The main task of this assignment is to implement a shell command parser as a small library in C. The shell is the program that reads and interprets the commands you type at a command line terminal. Later in the semester, you will build a full shell. For now, we focus on the first step: splitting a command line into its meaningful parts.

The parser takes a command line, a single string, and converts it to a command array, an array of strings representing the command and its space-separated arguments. The string split function available in many programming languages’ standard libraries would accomplish most of the task, but you will essentially implement split at a low level. We prohibit the use of string manipulation functions from the C standard library to force you to confront how strings are implemented and manipulated in memory.

Goals

  • To become familiar with the byte-addressable memory model through pointers and arrays at the C programming language level of abstraction.
  • To understand the link between pointer arithmetic and array indexing and representation.
  • To practice principled reasoning about program execution, using assertions, testing, and structured debugging.
  • To practice using tools for memory safety checking and debugging.
  • To have at least one aha! moment in appreciating how storage abstractions in your favorite programming language relate to (and hide!) details of the byte-addressable memory model.

Advice

Programming with C pointers has a tendency go wrong in entertaining or painful ways unless you practice careful discipline in programming.

For each stage in the suggested workflow:

  1. Plan carefully before typing any code.
  2. Implement one step at a time (with useful assertions).
  3. Test each step extensively, including with tools for detecting pointer errors, before implementing the next.
  4. Commit each tested step before implementing more.

This careful process will save time by catching bugs early and saving working versions you can recover if things start going wrong later.

Do true pair programming.

We recommend working in pairs on this assignment. Assuming you work with a partner, do your programming together, sitting in front of one screen. Take turns driving. If both partners will use a copy of the code, use the Team Workflow.

Time Reports

According to self-reported times on this assignment from a previous offering:

  • 25% of students spent <= 7 hours.
  • 50% of students spent <= 9 hours.
  • 75% of students spent <= 12 hours.

Setup

Get your repository with cs240 start pointers, whether working alone or with a teammate.

Your repository initially contains the following files:

  • For the command library implementation:
    • command.c: a file where you will implement a command parsing library for simple shell commands
    • command.h: a C header file describing the API of the command parsing library in command.c
  • For testing, demonstration, and automation:
    • command_lines.c: a file where you will add test cases for your command parser to be used by all testing drivers
    • command_demo.c: a simple demonstration of how the command parsing library functions are used, also useful for incremental testing during development
    • command_show_test.c, command_free_test.c, command_parse_test.c: drivers for testing parts of the library
    • Makefile: recipes to compile the various parts

Compile all command-parsing code and test drivers with the command make. This produces several executables for different testing purposes during development. Run tests with individual test drivers. Some tests automatically report failures; others require manual inspection of results.

You must use a CS 240 computing environment for CS 240 code assignments.

Tasks

You will modify command.c to implement a small library for parsing shell commands with the interface declared in command.h and the functionality described by the Specification section below.

char** command_parse(char* line, int* status);
void   command_show(char** command);
void   command_free(char** command);

The core of your library is the command_parse function, which takes a command line – a string capturing the command typed in the terminal – and parses it to produce a command array – an array of strings capturing the individual words of the command line. You will implement the command_parse function, plus three other simple functions for displaying and freeing command arrays.

A few provided test drivers include logic to help automate testing of your parsing library. These drivers all use command line test cases from command_lines.c. You will add several additional command line test cases for robust testing of your command parsing library implementation.

Grading considers design, documentation, style, correctness, and performance.

The remainder of this document describes:

  1. Preparatory exercises to help you navigate and plan the assignment.
  2. The definition of command lines and command arrays.
  3. The specification of the functions you must implement.
  4. The suggested implementation workflow.
  5. Documentation of how to compile and test your implementation.
  6. Tools and strategies for debugging.
  7. The grading criteria.
  8. Additional tips.

Preparatory Exercises

As you read this document, complete these exercises to familiarize yourself with command lines, command arrays, and the recommended workflow.

Preparation is your ticket for assistance.

Review Parts 1, 2, 4, and 7 (the initial valgrind section) of the Pointer Exercises.

  • Part 1 develops basic understanding of pointer types and operations. A sample solution for Part 1 is here.
  • Part 2 develops basic pointer programming skills. A sample solution for Part 2 is included in this assignment repository as practice.c.
  • Parts 4 and 7 develop critical debugging and testing skills and familiarity with tools for this assignment.
  • The other parts are also useful for exploring memory representation and debugging. You are encouraged to explore them, but doing so is not required.

Read this assignment and answer these questions about it:

  1. Is the & character ever considered part of a command word?
  2. Can spaces appear at the start or end of command lines?
  3. Is a command line with no words valid?
  4. Is command_parse allowed to mutate the contents of a command line string it receives as an argument?
  5. What is the command to compile testing code for this assignment?
  6. What is the command to test your parsing code for this assignment?
  7. What tool should be used to check for memory safety violations?
  8. Why does step 6a come before step 6b in the workflow?

Command Structure

Software driven by a command line interface (CLI), such as most software used in CS 240, interprets textual commands typed by a user at a command prompt. To interpret a command, a system must extract the command’s parts to understand its structure. In this assignment, your command parsing library will convert between two representations of simple multi-word commands:

  • A command line is a string capturing the full text of a command, just as the user types it in a terminal.
  • A command array is an array of strings, where each string captures one meaningful token in the command, in order.

The remainder of this section defines the structure of commands and these two representations as used in this assignment.

Command Lines

A command line is a null-terminated string containing zero or more space-separated words, with an optional single background status indicator character, '&', after the final word.

  • Spaces may appear anywhere.
  • Word characters include all printable characters except space (' ') and ampersand ('&').
  • A word is a contiguous sequence of word characters delineated on each end by a non-word character or the beginning or end of the string.
  • Ampersand ('&') is a special status indicator character, not a word character.
    • In a valid command, ampersand ('&') may appear at most once, and only after the final word, followed only by zero or more spaces to the end of the string. Any other placement of ampersand is invalid.1
    • A valid command containing no ampersand is a foreground command.
    • A valid command containing one ampersand is a background command.

Command Line Examples:

Here are two typical command lines:

  • The command line "ls -l cs240-pointers" indicates a foreground command containing the words "ls", "-l", and "cs240-pointers".
  • The command line "emacs cs240-pointers/command.c &" indicates a background command containing the words "emacs" and "cs240-pointers/command.c".

The definition above allows any spacing. The following lines are all valid command lines indicating the foreground command containing the words "ls", "-l", and "cs240-pointers":

"ls -l cs240-pointers"
"ls       -l cs240-pointers  "
"       ls   -l    cs240-pointers   "

The definition above requires that, if present, ampersand ('&') must be the last non-space character of the command line. It may appear adjacent to or separate from the last word. Regardless of placement, ampersand ('&') is not a word character and is never part of any command word. For example, these valid command lines all indicate the background command containing the single word "emacs":

"emacs &"
"emacs&"
"   emacs&   "

The following are examples of invalid command lines, with ampersand misused:

"&uhoh"
"  & uh oh"
"uh & oh"
"uh& &oh"
"uh oh & &"

Command Arrays

The parser converts a command line into a command array, an array of strings representing the words of the command line, in order, terminated by a NULL element. Recall that a string in C is not a special type; it is just an array of chars terminated by a null character ('\0'). A command array is thus an array of pointers to arrays of chars and has the type char**. All arrays in this structure must be null-terminated, using the right notion of nullness for each array.

'\0' is not NULL. 'a' is not "a".

'\0' is the null character. As a character (char), it is one byte in size.
NULL is the null address. As an address (pointer value), it is one machine word in size.
Do not mix them up!

Literal character (char) values in C are given in single quotes: 'a'.
Literal string values are given in double quotes: "a".
Do not mix them up!

Comparing 'a' == "a" will yield a false result. 'a' is the one-byte ASCII encoding of the letter a, a value of type char. "a" is the address of the start of the one-character null-terminated string a in memory, an 8-byte value of type char*.

Here is an example command array for the command line string "ls -l cs240pointers":

Command Array:             Null-Terminated Strings
Index   Contents           (stored elsewhere in memory)
      +----------+
    0 |  ptr  *----------> "ls"
      +----------+
    1 |  ptr  *----------> "-l"
      +----------+
    2 |  ptr  *----------> "cs240-pointers"
      +----------+
    3 |   NULL   |
      +----------+

Here is the same array drawn another way and showing how the array is arranged in memory, with each element’s offset from the base address of the array. Addresses grow left to right, and are assumed to be 64 bits (8 bytes), so indices are related to offsets by a factor of 8.

Index:        0       1       2       3 
Offset:   +0      +8      +16     +24     +32
          +-------+-------+-------+-------+
Contents: |   *   |   *   |   *   | NULL  |
          +---|---+---|---+---|---+-------+
              |       |       |
              V       V       V
              "ls"    "-l"    "cs240-pointers"

Although we draw “strings” in the above pictures, this is an abstraction. Each string is actually represented by a '\0'-terminated array of 1-byte characters in memory. Since each element is one byte, the addresses of adjacent characters differ by 1 and the offset is identical to the index.

Index:      0     1     2
Offset:  +0    +1    +2    +3
         +-----+-----+-----+
         | 'l' | 's' |'\0' |
         +-----+-----+-----+

Note: Practice reading memory diagrams every which way. Pay attention to what direction addresses grow. We are intentionally using different conventions in different drawings to help you get used to thinking on your feet…

Specification

You must write three functions (plus any necessary helper functions) supporting command parsing in command.c according to the headers in command.h.

If you write helper functions, be sure to read about how C function declarations work. We give a plan for implementation and testing below.

Function Specifications

  • char** command_parse(char* line, int* status)

    Parse the command line string, line, and:

    The command_parse function must not mutate the memory representing the original command line string.

  • void command_show(char** command)

    Print the structure of a command array according to the following format to aid in debugging and data inspection. You will need to learn how to use printf.

    The output should use the following format, which makes clear exactly what strings the command array holds. For example, evaluating this C statement:

    command_show(command_parse("ls -l cs240-pointers "));

    Should print this text:

    {
  "ls",
  "-l",
  "cs240-pointers",
  NULL
}

    Note that the use of added quotes in the output format distinguishes a correct command array where all spaces are stripped away, such as above, from an incorrect command array that contains some spaces in words, as below:

    {
  "ls ",
  "-l ",
  "cs240-pointers   ",
  NULL
}

    More generally, the output of command_show is the following sequence of lines, each ending with a newline character ('\n'):

    1. An initial line containing only a single left curly brace {.
    2. One line for each word in the command array, in order, where:
      • The line starts with two spaces   ; followed by
      • a double quote "; followed by
      • the characters of the command word in sequence; followed by
      • a double quote "; followed by
      • a comma ,.
    3. A line containing two spaces    followed by the four uppercase letters NULL, corresponding to the null-pointer terminator at the end of the command array.
    4. A final line containing only a single right curly brace }.

    This is valid C array initializer syntax. In other words, command_show prints a valid C source code description of the command array it is given, as long as the command array words do not contain characters in need of escape: " ('"'), \ ('\\'), tab ('\t'), carriage return ('\r'), newline ('\n'), etc. We will not test command lines that contain such characters. You are welcome to extend your command_show implementation to properly escape them in its output, but this is not required.

  • void command_free(char** command)

    Free all parts of a command array structure previously created by command_parse and not previously freed, including the individual command word strings and the top-level array.

Memory Rules

Naturally, your code should be free of memory safety violations (as detected by the valgrind memory error detector):

  • No use of unallocated, uninitialized, or freed memory contents.
  • No out-of-bounds accesses.
  • free only pointers to allocated blocks returned by malloc, and only once per block.

In addition, your code must follow these stricter memory allocation rules:

  • Clients of the command library own and manage the memory representing command line strings.
    • Command library functions must never free nor mutate command line strings.
    • Clients may free or mutate command lines at any time outside a call to command library functions.
  • The command_parse function must allocate command array structures dynamically with malloc and return them to the client code that invokes the library functions. Once returned, these structures are owned by the client. You may assume that:
    • Clients will not mutate command array structures returned by command_parse.
    • Clients will not free any parts of command array structures returned by command_parse, except through command_free.
    • Clients will call command_free to free an allocated command array (returned by command_parse) at most once, at any time outside a command library function.
  • Only command_parse may allocate memory and it must allocate no more memory than is necessary to represent the final command array structure at the time it is returned. This rule precludes temporary allocations.
  • Assuming that the client code eventually calls command_free on every command array returned by command_parse, all memory allocated by the command library functions must be freed.

Coding Style Rules

Follow this general C style guide as a baseline, along with the following specific rules:

  • Do not use any C standard library string functions in command.c. You must define all string processing from scratch.
  • For full credit, eliminate array indexing syntax in this assignment. Use idiomatic pointer style instead.
  • Document in comments how your code implements the specifications provided, connecting your C implementation to the specification.
    • Our sample solution for command_parse is under 70 clean lines of code and 20–30 lines of comments.
  • Document pre- and post-conditions and use assertions to check assumptions where reasonable.
  • Use well-scoped helper functions where reasonable.
    • Clean solutions are possible with and without helper functions.
    • If using helper functions, match them to well-defined sub-problems with clear arguments and results.
    • Be sure to order function headers properly.
  • Prefer simple efficient code over complicated code.
    • Our sample solution for command_parse is under 70 clean lines of code and 20–30 lines of comments.
  • Ensure your code is free of compiler warnings and compiler errors.
  • Ensure your code is free of run-time errors, including memory errors such as segmentation faults, as well as more subtle memory safety violations and leaks as detected by valgrind.

Idiomatic Pointer Style

For full credit, your submitted code must use only pointers and pointer arithmetic, with no array indexing notation. We require this to get you to confront the real memory representation of arrays in this assignment. Outside this assignment, array indexing is clearer style in many cases.

A simple way to think with arrays but write with pointers is to use *(a + i) wherever you think a[i]. However, this simple transformation rarely matches an idiomatic pointer style.

An idiomatic loop over an array with array indexing typically uses an integer index variable incremented on each iteration. Keep the scope of the index variable (or “loop variable”) as small as possible for the task.

// pre:  a is the address of a null-terminated string.
// post: replaces all characters in a by 'Z'
for (int i = 0; a[i] != '\0'; i++) {
    a[i] = 'Z';
}

An idiomatic loop over an array with pointer arithmetic typically uses a cursor pointer that is incremented to point to the next element on each iteration. Keep the scope of the cursor pointer variable as small as possible for the task.

// pre:  a is the address of a null-terminated string.
// post: replaces all characters in a by 'Z'
for (char* p = a; *p != '\0'; p++) {
    *p = 'Z';
}

Your final code should contain zero array[index] operations.

Implementation Workflow

Follow this workflow to understand and implement command_parse and friends. You may find it easiest to write a solution that uses array indexing at first, and then transform it to follow the coding style rules later, once it works. After each useful working step, commit your changes!

  1. Finish the prep exercises before starting to build your command parser.

  2. Add several more valid command line test case strings in command_lines.c.

  3. Implement command_show. Test with valgrind ./command_show_test_actual.bin at this stage.

  4. Implement command_free. Test with valgrind ./command_free_test_actual.bin at this stage. (Update: not quite sure what to do here? It’s fine to implement command_parse first and come back.)

  5. Add several more valid and invalid command line test cases to command_lines.c to test all aspects of the specification extensively and adversarially.

  6. Implement and test command_parse in stages. Test each stage and commit a working version before continuing. The basic algorithm should proceed (and be developed) in this order:

    1. Count the number of words in line and detect use of &, returning NULL for invalid commands and marking the foreground/background status for valid commands.
    2. Allocate the top-level command array.
    3. For each word in line, allocate properly sized space to hold the word as a null-terminated string, copy the word into this space, and save it in the command array.

  7. Once everything is implemented:

    • Repeat all tests with and without valgrind.
    • Remove or comment out any diagnostic printing you added during development.

Test and debug wisely.

Programming with pointers in C is error-prone, even for experts. Haphazard guess-and-check or print-based debugging is inefficient and often ineffective. Wield the testing and debugging tools outlined below to find bugs sooner and faster.

Compiling and Testing

To compile all test drivers, run make.

You are writing a command parsing library, i.e., a collection of related useful functions. The library itself cannot be run directly; it is not a complete program. We provide several test driver programs that act as clients of the library, calling its functions. You will write several command line test cases to be used by these drivers.

Test Cases

Develop your own large suite of test cases by adding command line strings to COMMAND_LINES in command_lines.c. By large, we mean perhaps dozens. Be adversarial. Create test cases for as many corner cases as possible in both valid and invalid commands. We will not grade your test cases themselves, but preparing and using them will help you ensure your code is correct and efficient. All test drivers use this array as their source of test commands.

Test Drivers

Each provided test driver is useful for different testing tasks. All driver executable files end with .bin (for binary executable). Each driver has two variants:

  • The actual variant uses your command.c library. Run this variant to test your code.
  • The expected variant uses a complete and correct (but hidden) implementation of the command library. Use it to see expected behavior or compare your code’s behavior to expected behavior.

In general, always run these executables under valgrind to help catch memory errors early. The drivers are listed below with their purpose, methods for checking correctness, source code file, command to compile just that driver, and the names of the relevant executables.

  1. show:
    • Purpose: testing command_show
    • Correctness: manually inspect outputs, check valgrind for memory errors
    • Source: command_show_test.c
    • Compile: make show
    • Executables: ./command_show_test_actual.bin
  2. free:
    • Purpose: testing command_free
    • Correctness: check valgrind for memory errors and memory leaks
    • Source: command_free_test.c
    • Compile: make free
    • Executables: ./command_free_test_actual.bin and ./command_free_test_expected.bin
  3. demo:
    • Purpose: demonstration of library functions, testing command_parse before completion
      • This is a simple driver that is easy to modify and extend by changing the the function command_demo in the command_demo.c file. For example, you might invoke helper functions directly in addition to command_parse. If doing so, you must copy their function headers to command.h to make them visible from command_demo.c.
    • Correctness: manually inspect outputs, check valgrind for memory errors or memory leaks, compare total memory allocations reported by valgrind for the actual and expected versions, optionally add custom additional checks to the driver code
    • Source: command_demo.c
    • Compile: make demo
    • Executables: ./command_demo_actual.bin and ./command_demo_expected.bin
  4. parse:
    • Purpose: testing a complete command_parse implementation under valgrind
    • Correctness: check reports of mismatches between actual and expected behavior, check valgrind for memory errors and memory leaks
    • Source: command_parse_test.c
    • Compile: make parse
    • Executables: ./command_parse_test_actual.bin and ./command_parse_test_expected.bin
  5. adversarial:
    • Purpose: testing a complete command_parse implementation
      • This driver replaces malloc and free with correct–but more adversarial–versions that make buggy command_parse implementations more likely to crash or cause obvious data corruption. Basically, allocated blocks are pre-filled with '%' characters (byte value 0x25) and the immediately adjacent (out of bounds) chunks of memory are filled with '@' characters (byte value 0x40), while freed blocks are mostly filled with '$' characters (byte value 0x24). This makes use of values from uninitialized ('%' == 0x25), out of bounds ('@' == 0x40), and recently freed ('$' == 0x24) locations less likely to get “lucky” by finding '\0' and more likely to cause obvious corruption or segmentation faults.
      • It is reasonable to run this driver under valgrind, but valgrind may report fewer errors than for the other test drivers due to the extra manipulation that the driver is doing.
    • Correctness: check reports of mismatches between actual and expected behavior, check for crashes and obvious data corruption, check valgrind for memory errors and memory leaks
    • Source: command_parse_test.c
    • Compile: make adversarial
    • Executables: ./command_adversarial_test_actual.bin and ./command_adversarial_test_expected.bin

Assertions

Assertions are “executable documentation” or “executable specifications”: they document rules about how code should be used or how data should be structured, but they also make it easier to detect violations of these rules (a.k.a. bugs!). Use the assert(...) statement in C by including assert.h and asserting expected properties. For example, the provided code already includes code that asserts that the arguments to command_ functions are not NULL. Thus if a NULL argument is ever passed to these functions, an error message will be printed and execution will halt immediately. Detecting errors early like this (vs. crashing or corrupting data later wherever the code depends on this assumption) saves a lot of time. Add assertions to make the “rules” of your code clear wherever you make assumptions.

Debugging

valgrind and gdb are the tools of choice. (See the Pointer Practice exercises, which introduce their usage.) Avoid print-based debugging. Backtrack from symptoms to cause.

valgrind

Valgrind is an extended memory error checker. It helps catch pointer errors, misuse of malloc and free, and more. Run valgrind on your compiled program like this: valgrind ./command_demo_actual.bin. (Replace ./command_demo_actual.bin with any other executable to check others.) The valgrind tool will run your program and observe its execution to detect errors at run time. See the tools page for additional reference links.

Always run your code under valgrind during development and testing to help catch memory errors early.

gdb

Use GDB to help debug your programs when you need more information than valgrind offers. Review previous lab activities for tips. When debugging programs with pointers, pay special attention to the pointer values your program generates. Inspect them like other variables or use the address-of (&) and dereference (*) operators at the gdb prompt to help explore program state.

Documentation and quick reference for gdb:

Does GDB tell you that it cannot display something due to compiler optimizations? If so, turn off compiler optimizations by adding -O0 (that’s “space dash Capital-Oh zero”) at the end of the CFLAGS = ... line in your Makefile. The CFLAGS variable here determines what flags are passed to the compiler. -O0 enables level zero of optimization (i.e., none). Run make clean and then make to compile your code again without optimizations.

Backtracking

Did your program hit an explicit error? An assertion failure? A segfault? Debug by collecting evidence and backtracking methodically to find the cause of this symptom.

  • Localize the crash. Exactly what line of code, what operation in this line, what value(s) used by this operation manifested what error and crashed? This symptom does not necessarily indicate the cause, but it marks where to begin the search.
    • valgrind will provide most of this information for every memory error.
    • If more information is needed, use gdb to run the code until it crashes, then find the line that crashed (and the context in which it executed) with bt, backtrace, or where.
    • Inspect the most relevant line of code and the error report to determine what the error means. Was it a segfault? If it was an assertion failure, what was the assertion checking?
    • Inspect the arguments shown in the backtrace or print variables to determine what ill-formed pointer was dereferenced to cause a segmentation fault or what illegal values failed an assertion.
  • Trace backward through the dependences of this broken operation to the original logic error.
    • Invalid values: Did any of this operation’s arguments hold invalid values? Did this operation load any invalid values from memory?
      • What operations produced these invalid values?
        • This is easy to answer for invalid arguments.
        • For invalid values loaded from memory, consider what earlier operations (perhaps in completely separate function calls) may have saved (or failed to save) this value.
      • Continue tracing backwards, treating these producer operations as broken operations.
    • Invalid choices: Was it invalid to apply this operation here given correct arguments (or regardless of the arguments)?
      • What control flow decision led to execution of this operation?
      • Is the logic of the decision correct?
      • If so, continue tracing backwards treating the control flow decision as a broken operation.
    • Callers: If using helper functions, you may need to trace back beyond the function where the crash happened.
      • Get a backtrace to see where the current function was called. (Don’t just assume you know which function called this one. Get the truth.)
      • If you want to inspect local variables in functions that called this one, use up and down in gdb to step up and down the call stack, then print what you want to see.
  • Minimize the input. Try to write a small command line that captures the essence of the command line on which your command parser encounters an error. A smaller input is easier to reason about in full.

Submission

Before submitting, disable any diagnostic printing in command.c. Only command_show should print, as specified.

Submit: The course staff will collect your work directly from your hosted repository. To submit your work:

  1. Test your source code files one last time. Make sure that, at a minimum, submitted source code is free of syntax errors and any other static errors (such as static type errors or name/scope errors). In other words: the code does not need to complete the correct computation when invoked, but it must be a valid program. We will not grade files that do not pass this bar.

  2. Make sure you have committed your latest changes. (Replace FILES with the files you changed and MESSAGE with your commit message.)

    $ git add FILES
$ git commit -m "MESSAGE"

  3. Run the command cs240 sign to sign your work and respond to any assignment survey questions.

    $ cs240 sign

  4. Push your signature and your latest local commits to the hosted repository.

    $ git push

Confirm: All local changes have been submitted if the output of git status shows both:

  • Your branch is up to date with 'origin/master', meaning all local commits have been pushed
  • nothing to commit, meaning all local changes have been committed

Resubmit: If you realize you need to change something later, just repeat this process.

Grading

Your grade is derived as follows:

We compute the correctness, memory safety, and efficiency components of your grade by running your code on a private suite of test inputs under valgrind to detect memory safety violations, measure memory allocation, and detect memory leaks.

You should extend command_lines.c with your own large suite of test inputs and run test drivers under valgrind. We will not grade your tests or anything outside the required functions in command.c, but careful and extensive testing will help you check that your code meets the specification and runs free of memory safety violations and memory leaks. Take this seriously.

More Tips

printf

The standard way of printing output in C is the printf function from the stdio library. The printf function accepts a format string (a template) and additional arguments to format according to each of the format specifiers (holes in the template with directions about how to fill them given the right kind of value). Use this linked documentation/introduction to get started with printf. Here’s an example:

void display_line_info(char* command_line) {
  // Note: strlen computes the length of a string,
  // but you may not use it in this assignment.
  printf("Command line is \"%s\" (%d characters).\n",
         command_line, strlen(command_line));
}

Calling display_line_info("Hello world!") would print this output:

Command line is "Hello world!" (12 characters).

Some important things to know when using printf:

  • printf prints exactly what you tell it to, not more: you must use a newline character ('\n')if you want a new line to be printed.
  • Instead of figuring out how to build a larger string just to print it, either use the printf format string to do the building or use multiple printf calls with smaller individual strings.

  • Even when printing a preexisting string value, an explicit format string should generally be used:

    char* my_string = computed_somehow();
// Use this:
printf("%s", my_string);
// Not this:
printf(my_string);
  • Since displaying individual characters to the screen is expensive, standard output (stdout), the channel to which printf prints to display in the terminal, is buffered: individual pieces of output are gathered up into larger chunks and flushed to the display all at once. This improves efficiency. For most printing tasks, this implementation detail is entirely imperceptible and not necessary to understand. However, if you are debugging code that interleaves small printfs with other operations that could crash or cause valgrind errors (e.g. memory operations), there’s a chance you will be confused if you are not aware of it.
    • The newline character ('\n') typically flushes all buffer text. In other words, printing a newline causes any text that has already been printed–but has not yet appeared in the output–to appear in the output immediately, in the order it was printed.
    • Strings without a newline may not appear immediately–perhaps only when the next newline is printed.
    • To flush the buffer explicitly at any time, you can use fflush(stdout);. This is not needed for a working command.c implementation, but if you run into seemingly truncated output while debugging, well-placed fflush calls might help clarify things during debugging. In general, this is a good reason to use a debugger instead of print-based debugging.

C Function Headers and Declaration Order

In C, a function is allowed to be used only after (i.e., later in the file than) its declaration. This differs from Java, which allows you to refer to later methods. When declaring helper functions, you can do one of a few things to deal with this:

  1. Just declare your helper function before the functions that use it.

  2. Write a function header earlier in the file and the actual definition later in the file. The function header just describes the name and type of the function, much like an interface method in Java. For example:

     // A function header declares that such a function exists,
 // and will be implemented elsewhere.
 int helper(int x, int y);

 // Parameter names are optional in headers.
 int helper2(char*);

 void needsHelp() {
     // OK, because header precedes this point in file.
     helper(7, 8);
     helper2("hello");
 }

 int helper(int x, int y) {
     return x + y;
 }

 int helper2(char* str) {
     return 7;
 }

  3. If the functions would likely get used elsewhere, then put the header in a header file, a file ending in .h that contains only function headers (for related functions) and data type declarations. For example, if you added another general function (not just a helper function) for manipulating commands (not required in this assignment), it would be best to place a function header for it with the other function headers in command.h so that users of your command library can call it.

    Header files are included (essentially programmatically copy-pasted) by the #include directive you often see at the tops of C source files. Then these functions can be used and their implementations will later be found elsewhere if compiled correctly.

Extra Fun Extensions

This part is not required. Extra Fun indicates ideas for optional extensions or challenges to try if you are having fun after completing the main assignment. Instructors will be happy to discuss ideas or review your solutions, but Extra Fun means what it says: it’s for extra fun, not extra credit.

Before trying either of these extensions:

  1. Finish and submit your work for the required parts with a descriptive commit message.
  2. Make a copy of your command_parse function and rename the copy command_parse_standard.
  3. Replace entire the body of the command_parse function with return command_parse_standard(line, status);. Now, command_parse is just a wrapper for whatever version you are working on.

After completing a working extension, make sure to test again before submitting any work on the extensions.

Extra Fun Extend the Command Language with Quoting

Start by making a copy of command_parse_standard and renaming it command_parse_quotes, then changing the body of command_parse to call this new function.

Extend your command-line parser to support quoting with the single-quote (') or double-quote (") characters. Note:

  • After a starting quote character, all characters (including ampersand & and space) until the matching close quote are part of the current word.
  • The delimiting quote characters are not part of the word.
  • A quoted section immediately adjacent to a non-quoted word (with no intervening space) together form a single word.
  • A literal quote character may be used as part of a quoted or unquoted word by using the escape character \ to prefix it.

Here are some examples using single-quote:

  • Basic quoting example with the single-quote character:
    • Command line:
      • As a C string literal: "echo 'hello && world' si'ngle word' &"
      • When printed: echo 'hello && world' si'ngle word' &
    • Command array:
      • "echo"
      • "hello && world"
      • "single word"
      • NULL
    • Command status: background
  • With escape characters to allow quote characters inside quoted elements:
    • Command line:
      • As a C string literal: "echo 'Don\\'t unquote until here,' I\\'m sure."
      • When printed: echo 'Don\'t unquote until here,' I\'m sure.
    • Command array:
      • "echo"
      • "Don't unquote until here,"
      • "I'm"
      • "sure."
      • NULL
    • Command status: foreground

Extra Fun Replace Multi-Pass Loop Code with Single There-and-Back-Again2 Recursive Code

Define a small family of recursive helper functions that accomplish the processes of counting words, counting letters, allocating, copying, etc., without using any loops. Use only recursion. Replace the body of command_parse with a single call to the top-level recursive function you define.

There are many levels of progressively more sophisticated recursive formulations.

Your loop-based code probably made several repeated scans of the command-line string or its subsections in order to avoid additional allocations. In the most sophisticated recursive versions, it is possible to accomplish the entire parsing process with only a single pass over the command-line string, with exactly one recursive function call and one command-line character array access for each character of the command line (plus null terminator). Local variables at each layer of recursion can remember (as recursion progresses deeper) and later reuse relevant input characters and other information (as recursion returns from deeper levels). This is not entirely free: no extra heap space or command-line scans are required, but the call stack will store information about each recursive call in memory.

License

Creative Commons License
Pointers by Benjamin P. Wood at Wellesley College is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Source code and evaluation infrastructure for Pointers are available upon request for reuse by instructors in other courses or institutions.

  1. This formulation restricts placement of ampersand further than most shells, for simplicity. ↩

  2. There and Back Again. Olivier Danvy and Mayer Goldberg. Fundamenta Informaticae, vol. 66, pp. 397–413. IOS Press. 2005. ↩