Summary

1And in Conclusion$\dots$¶

C does not automatically handle memory for you, so it’s up to you, the programmer, to allocate, use, and free memory correctly. In each program, an address space is set aside, separated into 2 dynamically changing regions and 2 ‘static’ regions.

• The Stack: Stores local variables inside of functions. Data on the stack is garbage collected immediately after the function in which it was defined returns. Each function call creates a stack frame that holds the function’s arguments and local variables. The stack grows downwards with nested function calls (LIFO structure), and shrinks upwards as functions return.

• The Heap: Stores memory manually allocated by the programmer with malloc, calloc, or realloc. Used for data that needs to persist after the function returns. Grows upwards in memory to ‘meet’ the stack. Memory on the heap is only freed when the programmer explicitly frees it. Careful heap management is necessary to avoid tricky bugs called Heisenbugs!

• Data (or Static): Stores data that is a fixed size, like global variables and string literals. Does not grow or shrink through function execution.

• Text (or Code): Is loaded at the start of the program and does not change after, contains executable instructions and any pre-processor macros.

There are a number of functions in C that can be used to dynamically allocate memory on the heap. The following are the ones we use in this class:

• malloc(size_t size) allocates a block of size bytes and returns the start of the block. The time it takes to search for a block is generally not dependent on size.

• calloc(size_t count, size_t size) allocates a block of count * size bytes, sets every value in the block to zero, then returns the start of the block.

• realloc(void *ptr, size_t size) “resizes” a previously-allocated block of memory to size bytes, returning the start of the resized block.

• free(void *ptr) deallocates a block of memory which starts at ptr that was previously allocated by the three previous functions.

Be careful when allocating buffers on the stack and heap! The heap is the biggest source of subtle bugs in C code.

Take CS 162 for more!

2Textbook Reading¶

K&R 7.8.5, 8.7

3Additional References¶

• Professor Emeritus Brian Harvey’s notes on C

• Professor Emeritus Paul Hilfinger’s notes on memory management

4Exercises¶

Check your knowledge!

4.1Conceptual Review¶