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CSE 333

Section 1

Gitlab, Pointers, and C Strings

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If you haven’t yet, please clone your exercise git repo to your local machine!

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Logistics

Pre-quarter survey due yesterday (6/24) @ 11:59 PM
Exercise 1 due today (6/25) @ 11:59pm (solutions on Saturday)

Create ex1-submit tag in your exercise Gitlab repo before deadline

Exercise 2 released tomorrow (6/26), due Tuesday (6/30) @ 11:59 PM

Create ex2-submit tag in your exercise Gitlab repo before deadline

Homework 1 released on Tuesday (6/30), due ~2 weeks (7/9) ish

Will be in a separate homework Gitlab repo – lots of starter code to deal with

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Introductions

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TA Intro: FILL IN HOWEVER YOU WOULD LIKE!

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Icebreaker!

Please turn to the people next to you and share:

Name, pronouns, year
What are you excited to learn in CSE 333?
Favorite animal or pet that you wish you had?

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Computing Setup for 333: DEMO

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CSE Linux Environment

The CSE Linux environment (i.e., CSE lab machines, attu cluster) is where all of your work will be graded, so you should do all of your development and testing here!

e.g., uses gcc14 (not the latest)

You will need to be on the Husky OnNet VPN in order to access instructional machines (i.e., attu cluster) from off campus

Windows/MacOS: BIG-IP Edge Client, Linux: f5vpn, iOS/Android: F5 Access
New this quarter!

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Text Editor of Choice

Generally, whatever you used in CSE 351 (likely vim or emacs) will work
For CSE 333, we also encourage VS Code

✅ Remote SSH extension allows you to directly edit files on attu
✅ C/C++ extension will enable syntax highlighting
⛔ Use of Copilot to generate code violates 333’s generative AI policy! 🙅

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Gitlab Repositories

Sign-in using your CSE NetID @ https://gitlab.cs.washington.edu/

Non-majors should have received a temporary CSE NetID for the quarter

There should be a repo created for you titled: cse333-26su-ex-<netid>

This is your exercise repo; your homework repo will be created Friday (6/26)
Please let us know ASAP if you don’t have one!

If you haven’t already, please follow the setup instructions in our Gitlab Guide

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git Tagging Demo/Follow-Along

Clone your repo locally

git clone git@gitlab.cs.washington.edu:cse333-26su-students/cse333-26su-ex-<netid>.git

Go to the ex1 directory in the repo

cd cse333-26su-ex-<netid>
cd ex1

Edit your ex1 solution file into the ex1 directory

Something like: vim ex1.c

Add, commit, and push

git add ex1/ex1.c
git commit -m "add my ex1 solution"
git push

Create and push tag

git tag ex1-submit
git push --tags

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Git Repo Usage

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Try to use the command line interface (not Gitlab’s web interface)
Only push files used to build your code to the repo

No executables, object files, backup files, etc.
Don’t always use git add . to add all your local files

Commit and push when an individual chunk of work is tested and done

Don’t push after every edit
Don’t only push once when everything is done
Gives you stable checkpoint backups in case something goes wrong with your working copy

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Pointers in C

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Pointers

Data type that stores the address of (the lowest byte of) a datum

Can draw an arrow in memory diagrams from pointer to pointed to data, particularly if actual value (stored address) is unknown

Common uses:

Reference to data allocated elsewhere (e.g., malloc, literals, files)
Iterators (e.g., data structure traversal)
Data abstraction (e.g., head of linked list, function pointers)

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Pointer Syntax and Semantics

Declared as type* name; or type *name;

Doesn’t matter, just be consistent

Address-of operator & gets a variable’s address
Dereference operator * refers to the pointed-to datum

“Follows” the pointer/arrow in a memory diagram

Example code:

Example diagram:

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int* ar = (int*) malloc(3*sizeof(int)); // reference

int* p = &ar[1]; // iterator

*p = 3;

0x1b126b0

0x1b126b4

?

3

?

ar

p

Stack

Heap

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Output Parameters

Recall: The return statement in a function passes a single value back through the %rax register

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An output parameter is a C idiom that emulates “returning values” through parameters:

An output parameter is a pointer (i.e., the address of a location in memory)
The function with this parameter must dereference it to change the value stored at that location
The new value is “returned” by persisting after the function returns

Output parameters are the only way in C to achieve returning multiple values

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Output Parameter Example

Write a function to compute the sum of values and product of all values in an array. The function is given a pointer to the first element in an array, the length of the array, and two output parameters to return the product and sum.
void ProductAndSum(int* ar, int len, int* product, int* sum) {

int temp_sum = 0, temp_product = 1; // initializations

}

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for (int i = 0; i < len; i++) { // compute product & sum

temp_sum += ar[i];

temp_product *= ar[i];

}

*sum = temp_sum; // set output params

*product = temp_product;

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Exercise 1

Which parameters are output �parameters?

Draw a memory diagram of the beginning of this call to Division.

Fill in the blank spaces in our calls to Division and printf.

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void Division(int numerator,

int denominator,

int* quotient,

int* remainder) {

*quotient = numerator / denominator;

*remainder = numerator % denominator;

}

int main(int argc, char* argv[]) {

int quot, rem;

Division(22, 5, _____, _____);

printf("%d rem %d\n", _____, _____);

return EXIT_SUCCESS;

}

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Exercise 2

A prefix sum over an array is the running total of all numbers in the array up to and including the current number. For example, given the array �{1, 2, 3, 4}, the prefix sum would be {1, 3, 6, 10}.
Write a function PrefixSum to compute the prefix sum of an array given (1) a pointer to its first element, (2) the pointer to the first element of the output array, and (3) the length both arrays (assumed to be the same).

void PrefixSum(int* input, int* output, int length);
Are there any edge cases to look out for?

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C-Strings

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C-Strings

A string in C is declared as an array of characters that is terminated by a null character '\0'
When allocating space for a string, remember to add an extra element for the null character

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char str_name[size];

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Initialization Examples

Code:

Memory:

Notes:

The size 6 is optional, as it can be inferred from the initialization.
Both initialize the array in the declaration scope (e.g., on the stack if a local var), though the latter can be thought of as copying the contents from the string literal into the array.

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// list initialization

char str1[6] = {'H','e','l','l','o','\0'};

// string literal initialization

char str2[6] = "Hello";

index	0	1	2	3	4	5
value	'H'	'e'	'l'	'l'	'o'	'\0'

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Common String Literal Error (1/2)

Code:

Memory:

Notes:

By default, using a string literal will allocate and initialize the character array in read-only memory (Literals)

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index	0	1	2	3	4	5
value	'H'	'e'	'l'	'l'	'o'	'\0'

// pointer instead of an array

char* str3 = "Hello";

0x402037

str3

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Common String Literal Error (2/2)

Code:

Memory:

Notes:

By default, using a string literal will allocate and initialize the character array in read-only memory (Literals)
What would happen if we executed str3[0] = 'J';?

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index	0	1	2	3	4	5
value	'H'	'e'	'l'	'l'	'o'	'\0'

// pointer instead of an array

char* str3 = "Hello";

0x402037

str3

Segfault!

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Extra: Exercise 3

The following code has a bug. What’s the problem, and how would you fix it?

void Bar(char ch) {

ch = '3';

}

int main(int argc, char* argv[]) {

char fav_class[] = "CSE331";

Bar(fav_class[5]);

printf("%s\n", fav_class); // should print "CSE333"

return EXIT_SUCCESS;

}

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Extra: Exercise 4

Write strcpy, a standard library function that copies a string src into an output parameter called dest and returns a pointer to the destination string. You may assume that dest has sufficient space to store src.

char* strcpy(char* dest, char* src) {

}

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Function Pointers

Pointers can store addresses of functions

Functions are just instructions in read-only memory, their names are pointers to this memory.

Used when performing operations for a function to use

Like a comparator for a sorter to use in Java
Reduces redundancy

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int one() { return 1; }

int two() { return 2; }

int three() { return 3; }

int get(int (*func_name)()) {

return func_name();

}

int main(int argc, char* argv[]) {

int res1 = get(one);

int res2 = get(two);

int res3 = get(three);

printf("%d, %d, %d\n", res1, res2, res3);

return EXIT_SUCCESS;

}