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Lecture 12 — more memory management, hw4, header files
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reminder: check remaining late days, no credit for late work
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midterm review 2 - 3 p.m. on Saturday, room TBD
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heap allocation review
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picture of memory
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void *malloc(size_t size)
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void* calloc (size_t num, size_t size);
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takes the number of things and the size of each thing as separate parameters
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initializes all the allocated memory with zeros
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use sizeof to allocate memory for non-char types
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void free(void *ptr)
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if we have
typedef struct {
char *str;
int len;
} String;
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exercise: what is the size of String?
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8 bytes for char* + 4 bytes for int = 12 bytes
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we allocate one like this
String *s = malloc(sizeof(*s));
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of an array of them like this
String *s = malloc(10 * sizeof(*s));
or this
String *s = calloc(10, sizeof(*s));
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be aware: with malloc, you will need to initialize members to null if you want to be able to check if they have been initialized
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in any case, free the memory like this
free(s);
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be aware: this does not allocate memory for the characters of str, just memory for the pointer itself
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exercise: where is the memory leak?
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int main (int argc, char **argv) {
int **matrix;
matrix = malloc (3 * sizeof (*matrix));
for (int i = 0; i < 3; i++) {
matrix[i] = malloc(5 * sizeof(*matrix[i]));
}
free(matrix);
}
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free is shallow, so while the memory allocated to matrix is freed, the memory allocated to the elements of matrix is not
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header files
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why would we want the ability to separate code into multiple files
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more readable, maintainable
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abstraction
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compilation
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header files (.h files) are how you share code between .c files
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struct definitions, function prototypes (declarations), #defines, and #includes
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no function definitions (leave that to the appropriate .c file)
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putting the #include target in " " searches the current directory (< > searches the standard library)
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can think of .h vs .c as public vs private
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.h in some ways provide an interface
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to avoid problems with double inclusion or mutually recursive inclusion, use #ifndef
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Any header file, such as some_header.h, should begin with
#ifndef SOME_HEADER_H
#define SOME_HEADER_H
// actual header code
#endif
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#ifdef and #ifndef are an example of conditional compilation
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the preprocessor uses or discards the enclosed code accordingly
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tests whether something has been #define-ed (or not)
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exercise: write a function debug_printf(char *format_str) that uses the preprocessor define DEBUG to determine whether it prints anything
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void debug_printf(char *output) {
#ifdef DEBUG
printf(output);
#endif
return;
}
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you can even set something to be defined as part of the gcc command with -D
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gcc -D Debug …
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homework 4
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Implement the T9 (text on 9 keys) system, without the predictive part
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so just the ability to encode words as a series of digits 2-9
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interactive command-line program
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729 -> ‘paw’
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729# -> ‘pax’
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729## -> ‘pay’
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core idea: represent this dictionary using a tree-based data structure called a trie
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the position of a node in the tree defines the key at that node
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organization
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trie data structure
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struct representing a node (recursive data structure)
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functions:
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create/init
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insert
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lookup
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destroy
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trie.h — declarations
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trie.c — implementation
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main
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#include “trie.h”
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probably shouldn’t define any structs, additional functions, etc.
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