Memory allocation

Memory allocation CSE 2451 Matt Boggus

sizeof • Thesizeofunary operatorwill 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 { inta; intb; } 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 • http://www.codingunit.com/c-reference-stdlib-h-function-malloc

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

calloc • calloc() prototype: • void * calloc ( size_tnum, 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: • http://www.codingunit.com/c-reference-stdlib-h-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)

Memory allocation