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Lab 1

61C Fall 2024

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Compiling a C Program

  • gcc is used to compile C programs
  • gcc ex1.c test_ex1.c

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​

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  • Can specify the name of the executable file with the -o flag
    • gcc -o ex1 ex1.c test_ex1.c

ex1.c

test_ex1.c

gcc

a.out

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Running a C Program

  • To run an executable located in the current directory, use ./<executable_name>
    • ./ex1
  • The dot refers to the current directory
  • If you want to run a file in a different directory, specify the path after the dot
    • ./path/to/file/ex1

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Variable Types and Sizes

  • char = 1 byte (8 bits)
  • short = usually 2 bytes (16 bits)
  • int = usually 4 bytes (32 bits)
    • unsigned int
  • float = 4 bytes (32 bits)
  • double = 8 bytes (64 bits)
  • long = at least 4 bytes (32 bits), can be longer
  • long long = at least 8 bytes (64 bits), can be longer

Guarantee: sizeof(long long) >= sizeof(long) >= sizeof(int) >= sizeof(short)

​

To know for sure what size your variable is, use uintN_t or intN_t types.

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Defining a Function

Specify return type, function name, and function parameters.

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

​

void nothing() { return; }

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Conditionals

if (condition) {� do this;�} else if (condition) {� do this;�} else {� do this;�}

If-else

Switch statements

switch (expression) {

case constant1: � do these;

break;� case constant2:

do these;

break;

default:

do these;�}

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Loops

While loop

For loop

while (condition) {

do this;�}

for (int i = 0; i < 10; i++) {

do this;

}

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Pointers

  • *
    • Type definition
      • e.g. int* x_ptr;
      • Add an * after a type makes the type a pointer type. In the example, we declare x_ptr variable as integer pointer type, and assign the address of variable x as its value.
      • You can also do int**, int***, int****, …
    • Dereference
      • e.g. *x_ptr
      • Add an * before a variable gets the value at address [variable]
  • &
    • Get the address of a variable (opposite of dereference)
      • &x

100

Address

Data

0xebafb32c

x_ptr

x

&x

int* x_ptr;

int x = 100;

x_ptr = &x;

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Address

Data

…

…

0x61c0

19

0x61c4

0x61c0

0x61c8

14

…

…

Memory

x

​

​

&x

​

​

y

Assigning

int x = 19;

int* y = &x;

might produce something like:

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Address

Data

…

…

0x61c0

19

0x61c4

0x61c0

0x61c8

19

…

…

Memory

y

​

​

​

​

​

​

Assigning

(int* y = &x;)

int z = *y;

might produce something like:

z

​

​

​

​

​

​

*y / x

​

​

​

​

​

​

&x

​

​

​

​

​

​

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Address

Data

…

…

0x61c0

20

0x61c4

0x61c0

0x61c8

19

…

…

Memory

y

​

​

​

​

​

​

Assigning

(int* y = &x;)

int z = *y;

*y = 20;

might produce something like:

z

​

​

​

​

​

​

*y / x

​

​

​

​

​

​

&x

​

​

​

​

​

​

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Address

Binding

Data

0x61c0

x

19

0x61c4

y

0x61c0

0x61c8

z

14

Name

Value

x

19

&x

0x61c0

y

0x61c0

*y

19

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Address

Binding

Data

0x61c0

x

19

0x61c4

y

0x61c0

0x61c8

z

14

Name

Value

x

19

&x

0x61c0

y

0x61c0

*y

19

&x gets the address of the variable x

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Address

Binding

Data

0x61c0

x

19

0x61c4

y

0x61c0

0x61c8

z

14

Name

Value

x

19

&x

0x61c0

y

0x61c0

*y

19

To create a pointer, we need to declare y to be a pointer type:

int* y = &x;

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Address

Binding

Data

0x61c0

x

19

0x61c4

y

0x61c0

0x61c8

z

14

Name

Value

x

19

&x

0x61c0

y

0x61c0

*y

19

*y follows the pointer in y, and gets the data stored at that address

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Pointers

0xebafb32c

What will this print?

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Pointers

0xebafb32c

0xebafb32c

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Pointers

0xebafb32c

0xebafb32c

What will this print?

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Pointers

0xebafb32c

0xebafb32c

Another address, ex: 0xebafb320

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Pointers

0xebafb32c

0xebafb32c

0xebafb320

What will this print?

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Pointers

0xebafb32c

0xebafb32c

0xebafb320

22

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Pointers

0xebafb32c

0xebafb32c

0xebafb320

22

What will this print?

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Pointers

0xebafb32c

0xebafb32c

0xebafb320

22

22

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Pointers

0xebafb32c

22

Address

Data

0xebafb330

0xebafb32c

&my_var

my_var_p

&my_var_p

my_var

*my_var_p

&x = address of x

*x = contents at x

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Pointers to Structs

  • Pass a pointer of a struct to a function so that you can edit the contents
  • Access the struct contents by:
    • (*student).major
    • student->major

Arrow operator

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Arrays

  • A block of memory: size is static
    • int arr[2];
    • int arr[] = {1, 2};
  • Accessing elements: array indexing
    • arr[1]
  • An array variable is a “pointer” to the first element.
    • You can use pointers to access arrays!
    • arr[0] is the same as *arr
    • Can use pointer arithmetic to move the pointer
      • Each operation automatically moves the size of one whole “type” that ptr points to
      • arr[1] is the same as *(arr + 1)
  • Arrays do not keep track of their own size

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Strings

  • An array of chars representing individual characters
    • Ends in a null terminator ‘\0’
  • strlen(char* string)
    • returns the length of the string (num chars until the null terminator)
  • strcpy(char* dest, char* src)
    • copies the string stored in src to the location dest, until and including the null terminator

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Structs

  • What do they allow us to do?
  • What is a structure tag?
  • What are the two ways to declare struct variables?
  • How do we access the members of a struct?

Structure Tag

Variable declarations

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Structs

  • What do they allow us to do?
  • What is a structure tag?
  • What are the two ways to declare struct variables?
  • How do we access the members of a struct?

Dot operator

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Structs

  • typedef
    • Lets you avoid rewriting struct every time you want to declare a new struct variable
    • Can no longer declare variables in the struct definition