1. Chien-Chung Shen CIS/UD cshen@udel.edu
Chapter 14 Memory API
Chien-Chung Shen
CIS/UD

2. Crux: How to Allocate and Manage Memory
In Unix/C programs, understanding how to allocate and manage memory is critical in building robust and reliable software

3. Types of Memory
In running a C program, two types of memory are allocated
Stack (or automatic) memory – managed implicitly by compiler for programmer (allocation/deallocation)
void func() {
int x; // declares an integer on the stack
... }
Heap memory – explicitly managed by programmer
int *p = (int *) malloc(sizeof(int));
free(p);
The may cause of many bugs (segmentation faults)

4. malloc() int *x = malloc(10 * sizeof(int)); printf("%d\n", sizeof(x));
int y[10];
printf("%d\n", sizeof(y));
sizeof() is a compile-time operator (i.e., the actual size is known at compile time) [so that it is not a function call, as a function call would take place at run-time]
How big a pointer to an integer is (4 or 8); not how much memory was dynamically allocated
There is enough static information for compiler to know that 40 bytes have been allocated

5. malloc() Returns [a pointer to type void] (i.e., void *)
Cast the return type of malloc() to [a pointer to a specific type]
int *x = (int *) malloc(10 * sizeof(int));
Casting doesn’t really accomplish anything, other than tell the compiler and other programmers who might be reading your code: “yeah, I know what I’m doing.”
By casting the result of malloc(), the programmer is just giving some reassurance; the cast is not needed for the correctness

6. free()
void func() {
int *p = (int *) malloc(sizeof(int));
...
free(p); }
The size of the allocated region is not passed in by the user, and must be tracked by the memory-allocation library itself

7. Memory Bugs (Character) strings!!! Forget to allocate memory
char *src = "hello”;
char *dst; // oops! unallocated
strcpy(dst, src); // segfault and die
Not allocate enough memory
char *src = "hello";
char *dst = (char *) malloc(strlen(src));
strcpy(dst, src);
Use malloc(strlen(src) + 1) for extra end-of-string character
Forget to initialize allocated memory
Forget to free memory – memory leak
// too small!
// buffer overflow