Memory allocation CSE 2451 Matt Boggus

sizeof The sizeof unary operator will return the number of bytes reserved for a variable or data type. Determine: – Returning the byte length of a data type – Number of bytes reserved for a structure (user defined type) – Byte length of an array

Returning the length of a data type /* How big is an int? Most machines size ints as 4 bytes */ main() { printf("%d \n", sizeof(int)); }

Number of bytes reserved for a structure /* On most machines a struct with two ints is the same size as two ints: 8 */ main(){ struct { int a; int b; } TwoInts; printf("%d \n", sizeof(TwoInts)); }

Length of an array main() { char String[20]; printf ("%d \n", sizeof String); printf ("%d \n", sizeof (String)); } /* As a unary operator and not a function, parenthesis are not necessary (if the argument is a variable), but aid readability */

Dynamic memory functions In the stdlib.h library: malloc() – Allocate a memory block free() – De-allocate memory calloc() – Allocate space for an array realloc() – Change the size of previously allocated memory Each function is used to initialize a pointer with memory from free store (a section of memory available to all programs)

malloc The function malloc() will allocate a block of memory that is size bytes large. If the requested memory can be allocated a pointer is returned to the beginning of the memory block. Note: the content of the received block of memory is not initialized. malloc() prototype: – void * malloc ( size_t size ); Parameters: – Size of the memory block in bytes. Return value: – If the request is successful then a pointer to the memory block is returned. – If the function failed to allocate the requested block of memory, a null pointer is returned. Example –

malloc usage int *ptr = (int*) malloc( sizeof (int) ); int *ptr = (int*) malloc( sizeof (*ptr) );

calloc calloc() prototype: – void * calloc ( size_t num, size_t size ); Parameters: – Number of elements (array) to allocate and the size of elements. Return value: – Will return a pointer to the memory block. If the request fails, a NULL pointer is returned. Example: – function-calloc function-calloc

Static vs. Dynamic memory Static arrays – size defined at compile time – Memory stored on the stack – Stack grows when entering new blocks (branches, loops, functions) – Stack shrinks when leaving blocks – Obeys scoping rules Dynamic array – size defined at run time – Memory stored on the heap – Stays available until removed In C – manually with function calls In Java – automatically with garbage collection Why have dynamic memory? – Input of unknown size – Data structures that require dynamic memory allocation Linked lists, trees, etc.

free The free function deallocates memory free( ptr ); – Frees the memory of whatever ptr is pointing at After freeing a pointer, it is a good idea to set it to point to 0 – This makes it a null pointer – Invalid dereferencing is easier to spot with NULL pointers (Some compilers support explicitly setting a pointer to NULL)