Intermediate Pointers and Basic Structures in C Lab 2
Explore the concepts of pointers, referencing/dereferencing, memory allocation, struct usage, and array manipulation in C programming. Learn how to pass by copy and reference, utilize void pointers and realloc function, and work with personalized data structures like structs.
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Presentation Transcript
C Lab 2 Intermediate Pointers & Basic Structures
Goals Review Pointers Referencing/Dereferencing free realloc structs ArrayList
Review: What are Pointers? A pointer is an address on either the stack or heap. EX: double * should address a double in memory. For the pointer to contain data, some other function must create the data it will point to. This is typically a call to malloc.
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?
Pass by Reference Pass by reference: void plus(int *num){ (*num)++; } void main(){ int num = 3; plus(&num); printf( %d\n , num); } What does main print now?
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 Personalized types, somewhat like classes May contain items of choice Often the basis of (data) structures
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 arraylist a; a.buffer = NULL; a.buffer_size = 0; a.length = 0;
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 You must free what you malloc (heap) Stack manages itself arraylist *a = (arraylist *)malloc(sizeof(arraylist)); free(a); //yaaaaaaaaay
Memory Management Do not free what you did not malloc!!! Do not free address consecutively!!! int *num = malloc(4) free(num); //yaaaayyy free(num); //staaahp int num = 3; free(&num); // :,O
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-4
Connect Thoughts Begin the lab exercise Where/When might realloc be useful? Where/When might free be useful?
