Welcome!

If you are taking CS241 you can submit this homework at this Google Form.

// First can you guess which lyrics have been transformed into this C-like system code?
char q[] = "Do you wanna build a C99 program?";
#define or "go debugging with gdb?"
static unsigned int i = sizeof(or) != strlen(or);
char* ptr = "lathe"; size_t come = fprintf(stdout,"%s door", ptr+2);
int away = ! (int) * "";

int* shared = mmap(NULL, sizeof(int*), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
munmap(shared,sizeof(int*));

if(!fork()) { execlp("man","man","-3","ftell", (char*)0); perror("failed"); }
if(!fork()) { execlp("make","make", "snowman", (char*)0); execlp("make","make", (char*)0)); }

exit(0);

So you want to master System Programming? And get a better grade than B?

int main(int argc, char** argv) {
 puts("Great! We have plenty of useful resources for you but it's up to you to");
 puts("be an active learner and learn how to solve problems and debug code.");
 puts("Bring your near-completed answers the problems below");
 puts(" to the first lab to show that you've been working on this");
 printf("A few \"don't knows\" or \"unsure\" is fine for lab 1"); 
 puts("Warning; your peers will be working hard for this class");
 puts("This is not CS225; you will be pushed much harder to");
 puts(" work things out on your own");
 fprintf(stdout,"the point is that this homework is a stepping stone to all future assignments");
 char p[] = "so you will want to clear up any confusions or misconceptions.";
 write(1, p, strlen(p) );
 char buffer[1024];
 sprintf(buffer,"For grading purposes this homework 0 will be graded as part of your lab %d work.", 1);
 write(1, buffer, strlen(buffer));
 printf("Press Return to continue\n");
 read(0, buffer, sizeof(buffer));
 return 0;
}

Watch the videos and write up your answers to the following questions.

http://cs-education.github.io/sys/

There is also the course wikibook -

https://github.com/angrave/SystemProgramming/wiki

Questions? Comments? Use Piazza, https://piazza.com/illinois/spring2017/cs241/home

The in-browser virtual machine runs entirely in Javascript and is fastest in Chrome. Note the VM and any code you write is reset when you reload the page So copy your code to a separate document. The post-video challenges (e.g. Haiku poem) are not part of homework 0.

Chapter 1

  • Hello World (System call style)
  • Write a program that uses write() to print out "Hi! My name is ".
  • Hello Standard Error Stream
  • Write a program that uses write() to print out a triangle of height n to Standard Error
    • n should be a variable and the triangle should look like this for n = 3 ```C *

    ```

  • Writing to files
  • Take your program from "Hello World" and have it write to a file
    • Make sure to to use some interesting flags and mode for open()
    • man 2 open is your friend
  • Not everything is a system call
  • Take your program from "Writing to files" and replace it with printf()
  • Name some differences from write() and printf()

Chapter 2

  • Not all bytes are 8 bits?
  • How many bits are there in a byte?
  • How many bytes is a char?
  • Tell me how many bytes the following are on your machine: int, double, float, long, long long
  • Follow the int pointer
  • On a machine with 8 byte integers: C int main(){ int data[8]; } If the address of data is 0x7fbd9d40, then what is the address of data+2?
  • What is data[3] equivalent to in C?
  • sizeof character arrays, incrementing pointers

Remember the type of a string constant "abc" is an array. - Why does this segfault? C char *ptr = "hello"; *ptr = 'J'; - What does sizeof("Hello\0World") return? - What does strlen("Hello\0World") return? - Give an example of X such that sizeof(X) is 3 - Give an example of Y such that sizeof(Y) might be 4 or 8 depending on the machine.

Chapter 3

  • Program arguments argc argv
  • Name me two ways to find the length of argv
  • What is argv[0]
  • Environment Variables
  • Where are the pointers to environment variables stored?
  • String searching (Strings are just char arrays)

  • On a machine where pointers are 8 bytes and with the following code: C char *ptr = "Hello"; char array[] = "Hello"; What is the results of sizeof(ptr) and sizeof(array)? Explain why.

  • Lifetime of automatic variables

  • What datastucture is managing the lifetime of automatic variables?

Chapter 4

  • Memory allocation using malloc, heap and time
  • If I want to use data after the lifetime of the function it was created in, then where should I put it and how do I put it there?
  • Fill in the blank. In a good C program: "For every malloc there is a ___".
  • Heap allocation Gotchas
  • Name one reason malloc can fail.
  • Name some differences between time() and ctime()
  • What is wrong with this code snippet? C free(ptr); free(ptr);
  • What is wrong with this code snippet? C free(ptr); printf("%s\n", ptr);
  • How can one avoid the previous 2 mistakes?
  • struct, typedefs and a linked list
  • Create a struct that represents a Person and typedef, so that "struct Person" can be replaced with a single word.
    • A person should contain the following information: name, age, friends (pointer to an array of pointers to People).
  • Now make two persons "Agent Smith" and "Sonny Moore" on the heap who are 128 and 256 years old respectively and are friends with each other.
  • Duplicating strings, memory allocation and deallocation of structures
  • Create functions to create and destroy a Person (Person's and their names should live on the heap).
    • create() should take a name and make a copy of the name and also an age. Use malloc to reserve sufficient memory. Be sure initialize all fields (why?).
    • destroy() should free up not only the memory of the person struct but also free all its attributes that are stored on the heap (the array if it exists and the string). Destroying one person however should not destroy any others.

Chapter 5

  • Reading characters, Trouble with gets
  • What functions can be used for getting characters for stdin and writing them to stdout?
  • Name one issue with gets()
  • Introducing sscanf and friends
  • Write code that parses a the string "Hello 5 World" and initializes 3 variables to ("Hello", 5, "World") respectively.
  • getline is useful
  • What does one need to define before using getline()?
  • Write a C program to print out the content of a file line by line using getline()

C Development (A web search is useful here)

  • What compiler flag is used to generate a debug build?
  • You modify the makefile to generate debug builds and type make again. Explain why this is insufficient to generate a new build.
  • Are tabs or spaces used in Makefiles?
  • What are the differences between heap and stack memory?
  • Are there other kinds of memory in a process?

Optional (Just for fun)

  • Convert your a song lyrics into System Programming and C code covered in this wiki book and share on Piazza.
  • Find, in your opinion the best and worst C code on the web and post the link to Piazza
  • Write a short C program with a deliberate subtle C bug and post it on Piazza to see if others can spot your bug