CSE 351: The Hardware/Software Interface
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Lab 1a: Pointers in C

Assigned
Friday, January 7, 2022
Due
Wednesday, January 19, 2022
Overview

Learning Objectives

Gain familiarity with pointers and pointer arithmetic.

Pointers are a critical part of C and necessary for understanding assembly code (Lab 2-3) and memory allocation (Lab 5).

Code for this lab
or in terminal:
wget https://courses.cs.washington.edu/courses/cse351/22wi/files/labs/lab1a.tar.gz

Unzip

Running tar xvf lab1a.tar.gz will extract the lab files to a directory called lab1a.

Instructions

pointer.c contains a skeleton for some programming puzzles, along with a comment block that describes exactly what the functions must do and what restrictions there are on their implementation. Your assignment is to complete each function skeleton according to the following rules:

only straightline code (i.e., no loops or conditionals) unless otherwise stated. Look for "Control Constructs" under ALLOWED in pointer.c comments.
a limited number of C arithmetic and logical operators (described in pointer.c comments)
the C bit shift operators (<< and >>) may be needed for some problems but will not be covered in lecture until next Wednesday
no constants larger than 8 bits (i.e., 0 - 255 inclusive) are allowed
feel free to use "(", ")", and "=" as much as you want
you are permitted to use casts for these functions
we will not grade comments, but commenting your code is good practice and helpful for debugging

You will start working with basic pointers and use them to compute the size of different data items in memory, modify the contents of an array, and complete a couple of pointer "puzzles" that deal with alignment and array addresses.

Using Pointers

This section describes the functions you will be completing in pointer.c found in the lab1a folder you downloaded. Refer to the file pointer.c itself for more complete details.

Pointer Arithmetic

The first three functions in pointer.c ask you to compute the size (how much memory a single one takes up, in bytes) of various data elements (ints, doubles, and pointers). You will accomplish this by noting that arrays of these data elements allocate contiguous space in memory so that one element follows the next.

Manipulating Data Using Pointers

The next two functions in pointer.c challenge you to manipulate data in new ways with your new knowledge of pointers.

The swapInts function in pointer.c asks you to swap the values that two given pointers point to, without changing the pointers themselves (i.e., they should still point to the same memory addresses).

The changeValue function in pointer.c asks you to change the value of an element of an array using only the starting address of the array. You will add the appropriate value to the pointer to create a new pointer to the data element to be modified.You are not permitted to use [] syntax to access or change elements in the array anywhere in the pointer.c file.

Pointers and Address Ranges

The next two functions in pointer.c ask you to determine whether pointers fall within certain address ranges, defined by aligned memory blocks or arrays.

For the first of these two functions, you will determine if the addresses stored by two pointers lie within the same block of 64-byte aligned memory. The following are some examples of parameters and returns for calls to this function.

    ptr1: 0x0
    ptr2: 0x3F
    return: 1
    ptr1: 0x0
    ptr2: 0x40
    return: 0
    ptr1: 0x3F
    ptr2: 0x40
    return: 0
    ptr1: 0x3CE
    ptr2: 0x3EF
    return: 1
    ptr1: 0x3CE
    ptr2: 0x404
    return: 0

For the last function you will determine if the address stored in ptr is pointing to a byte that makes up some part of an array element for the passed array. The byte does not need to be the first byte of the array element that it is pointing to. That description is a bit wordy, so here are some examples.

    intArray: 0x0
    size: 4
    ptr: 0x0
    return: 1
    intArray: 0x0
    size: 4
    ptr: 0xF
    return: 1
    intArray: 0x0
    size: 4
    ptr: 0x10
    return: 0
    intArray: 0x100
    size: 30
    ptr: 0x12A
    return: 1
    intArray: 0x100
    size: 30
    ptr: 0x50
    return: 0
    intArray: 0x100
    size: 30
    ptr: 0x18C
    return: 0

Please post on the discussion board if you are having trouble understanding any of these examples!

Byte Traversal

The next two questions in pointer.c have you reading and writing data by understanding the layout of the bytes.

In C strings do not have knowledge of how long they are. In order to find out we must calculate it for ourselves. All strings in C are arrays of characters that end with a null terminator character - '\0'. The stringLength function has you return the length of a string, given a pointer to its beginning. You are allowed to use loops for this one. Also note that the null terminator character does NOT count as part of the string length.

The endianExperiment function has you set the value a pointer points to to the number 351351. Remember that we work with little endian data storage, and what that means.

Selection Sort

The final part of the lab has you implement selectionSort. Selection sort works by effectively partitioning an array into a sorted section, followed by an unsorted section. It repeatedly finds (and selects) the minimum element in the unsorted section, and moves it to the end of the sorted section (swapInts might be useful for this). The pseudo code might look something like this:

// "arr" is an array
// "n" is the length of arr
for i = 0 to n - 1
  minIndex = i
  for  j = i + 1 to n - 1
    if arr[minIndex] > arr[j]
      minIndex = j
    end if
  end for
  Swap(arr[i], arr[minIndex])
end for

Note that you are allowed to use loops and if statements in this one.

Checking Your Work

We have included the following tools to help you check the correctness of your work:

ptest.c is a test harness for pointer.c. You can test your solutions like this:

$ make ptest
./ptest

This only checks if your solutions return the expected result.

We may test your solution on inputs that ptest does not check by default and we will review your solutions to ensure they meet the restrictions of the assignment.

dlc.py is a Python script that will check for compliance with the coding rules. The usage is:

$ ./dlc.py

When no issues are found, the output will be [], otherwise, the output inside the brackets will tell you (using shorthand notation) what issues were found function-by-function.

The dlc program enforces a stricter form of C declarations than is the case for C++ or that is enforced by gcc. In particular, in a block (what you enclose in curly braces) all your variable declarations must appear before any statement that is not a declaration. For example, dlc will complain about the following code:

int foo(int x) {
  int a = x;
  a *= 3;     /* Statement that is not a declaration */
  int b = a;  /* ERROR: Declaration not allowed here */
}

Instead, you must declare all your variables first, like this:

int foo(int x) {
  int a = x;
  int b;
  a *= 3;
  b = a;
}

The dlc program will also complain about binary constants such as 0b10001000, so avoid using them.

Lab 1a Synthesis Questions

Make sure your answers to these questions are included in the file lab1Asynthesis.txt!

In both lab0.c and pointer.c, we saw the effects of pointers and pointer arithmetic:

Briefly describe why pointer arithmetic is useful/beneficial (not just its definition). Additionally, what is the relationship between arrays and pointers? (hint: find a formula and a written explanation)  [3 pt]
Think about how you calculated the actual difference (in bytes) between two addresses in C without any compiler warnings. Briefly explain why each step was necessary.  [3 pt]
Notice that the parameters to the function swapInts were both pointers. Explain why this is necessary. What would happen if the parameters were integers?  [3 pt]
Submission

Be sure to run make and ./dlc.py on your code before submitting! Submissions that don't compile or can't be parsed by dlc will automatically receive a score of ZERO.

Please submit your completed pointer.c and lab1Asynthesis.txt files to the "Lab 1a" assignment on Gradescope.

After submitting, please wait until the autograder is done running and double-check that you passed the "File Check" and "Compilation and Execution Issues" tests. If either test returns a score of -1, be sure to read the output and fix any problems before resubmitting. Failure to do so will result in a programming score of 0 for the lab.

It is fine to submit multiple times up until the deadline, we will only grade your last submission. NOTE that if you do re-submit, you MUST RE-submit BOTH files again.