Intermediate Pointers and Basic Structures: A Review, Study notes of C programming

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C Lab 2

Intermediate Pointers & Basic Structures

Goals

● Review ○ Pointers ○ Referencing/Dereferencing ○ free ● realloc ● structs ● ArrayList

Getting Pointer/Reference

● To get pointer to something, use ‘&’

● ‘&’ allows to pass items by reference

● To dereference or get item pointed to use ‘*’

● ‘*’ is the opposite of ‘&’

Pass by Copy

Pass by copy: void plus(int num){ num++; }

void main(){ int num = 3; plus(num); printf(“%d\n”, num); } What does main print?

Void * and realloc

● “void ” may point to arbitrary types (i.e. int, char*, etc.)

● Can be casted to appropriate types

● realloc increases the size of memory allotted to pointer

● Preserves data pointed to by original pointer

● Original pointer is NULL, if space is found elsewhere

Realloc and Equivalent

ptr = malloc(2); ptr = realloc(ptr, 1000);

ptr = malloc(2); ptr2 = malloc(1000); memcpy(ptr2, ptr, 2); free(ptr); ptr = ptr2; ptr2 = NULL;

Why not: ptr = malloc(2); realloc(ptr, 1000);

Structs

typedef struct arraylist { int *buffer; int buffersize; int length; } arraylist;

Editing Struct Fields

● You may declare structs on the stack

● You may access/edit fields of struct using ‘.’

● Think about why this works (Hint: pointers)

Editing Struct Fields

● You may declare structs on the heap

● Now you access/edit fields using ‘->’

● This syntax is more helpful visually

Editing Struct Fields

arraylist *a = (arraylist *)malloc(sizeof(arraylist)); a->buffer = NULL; a->buffer_size = 0; a->length = 0;

Memory Management

● Do not free what you did not malloc!!! ● Do not free address consecutively!!!

int num = 3; free(&num); // :,O

int *num = malloc(4) free(num); //yaaaayyy free(num); //staaahp

Memory Takeaways

● Only free what has been malloc’d

● Only free malloc’d memory once

● For more on stack vs. heap: http://gribblelab.org/CBootcamp/7_Memory_Stack_vs_Heap.html#sec-