Dynamic Memory & ADTs in C
Optional Textbook Readings: CP:AMA 17.1, 17.2, 17.3, 17.4
The primary goal of this section is to be able to use dynamic memory.
CS 136 Winter 2022 10: Dynamic Memory & ADTs 1
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The heap is the final section in the C memory model.
It can be thought of as a big “pile” (or “pool”) of memory that is
available to a program.
Memory is allocated from the heap upon request.
This memory is “borrowed”, and can be “returned” (“freed”) back to the heap when it is no longer needed (deallocation).
Returned memory may be reused (“recycled”) for a future allocation. If too much memory has already been allocated, attempts to borrow
additional memory fail.
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Advantages of heap-allocated memory Dynamic: The allocation size can be determined at run time (while
the program is running).
Resizable: A heap allocation can be “resized”.
Duration: Heap allocations persist until they are “freed”. A function can create a data structure (one or more allocations) that continues to exist after the function returns.
Safety: If memory runs out, it can be detected and handled properly (unlike stack overflows).
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Read-Only Data
Global Data
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Unfortunately, there is also a data structure known as a heap, and the two are unrelated.
To avoid confusion, prominent computer scientist campaigned to use the name “free store” or the “memory pool”, but the name “heap” has stuck.
A similar problem arises with “the stack” region of memory because there is also a Stack ADT. However, their behaviour is very similar so it is far less confusing.
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The malloc (memory allocation) function obtains memory from the heap. It is provided in
// malloc(s) requests s bytes of contiguous memory from the heap
// and returns a pointer to a block of s bytes, or
// NULL if not enough contiguous memory is available
// time: O(1) [close enough for this course]
For example, for an array of 100 ints:
int *my_array = malloc(100 * sizeof(int));
or a single struct posn:
struct posn *my_posn = malloc(sizeof(struct posn));
These two examples illustrate the most common use of dynamic memory: allocating space for arrays and structures.
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Uninitialized
The heap memory provided by malloc is uninitialized. struct posn *my_posn = malloc(sizeof(struct posn));
printf(“mystery values: (%d, %d)\n”, my_posn->x,
my_posn->y);
There is also a calloc function which essentially calls malloc and then “initializes” the memory by filling it with zeros. calloc is O(n), where n is the size of the block.
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Dynamic Arrays
With pointer notation, the syntax for accessing a heap array is nearly identical to the syntax for a stack (or global) array.
int *my_array = malloc(n * sizeof(int));
// initialize the array
for (int i = 0; i < n; ++i) {
my_array[i] = 0;
We could use malloc(n * 4), but malloc(n * sizeof(int)) is clearer and better style.
Always use sizeof with malloc to improve communication.
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Strictly speaking, the type of the malloc parameter is size_t, which is a special type produced by the sizeof operator.
size_t and int are different types of integers.
Seashell is mostly forgiving, but in other C environments using
an int when C expects a size_t may generate a warning. The proper printf format specifier to print a size_t is %zd.
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The declaration for the malloc function is: void *malloc(size_t s);
The return type is a (void *) (void pointer), a special pointer that can point at any type.
int *my_array = malloc(100 * sizeof(int));
struct posn *my_posn = malloc(sizeof(struct posn));
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For this course, we assume malloc is O(1).
In practice, the constant time may be quite large.
In addition, the running time for malloc depends on the state of the heap.
After multiple allocations and deallocations, the heap can become fragmented, which may affect the performace of malloc.
This is one of the disadvantages of heap memory.
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example: visualizing the heap
int main(void) {
int *arr1 = malloc(10 * sizeof(int));
int *arr2 = malloc(5 * sizeof(int));
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Out of memory
An unsuccessful call to malloc returns NULL.
In practice it’s good style to check every malloc return value and
gracefully handle a NULL instead of crashing.
int *my_array = malloc(n * sizeof(int));
if (my_array == NULL) {
// handle out of memory
In the “real world” you should always perform this check, but in this course, you do not have to check for a NULL return value unless instructed otherwise.
In these notes, we omit this check to save space.
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Every block of memory obtained through malloc must eventually be freed (when the memory is no longer in use).
// free(p) returns memory at p back to the heap
// requires: p must be from a previous malloc
// effects: the memory at p is invalid
// time: O(1)
In the Seashell environment, every block must be freed.
int *my_array = malloc(n * sizeof(int));
free(my_array);
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Dangling pointers
Once a block of memory is freed, accessing that memory is invalid and may cause errors (or unpredictable results).
Consider this example:
int *my_array = malloc(10 * sizeof(int));
free(my_array);
The memory my_array is pointing at has been freed and is now invalid, but my_array is still pointing at it.
free(my_array) does not modify the pointer my_array.
A pointer to a freed allocation is known as a “dangling pointer” . It is
often good style to assign NULL to a dangling pointer.
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Invalid behaviour with free
It is invalid to free memory that was not returned by a malloc or that has already been freed. free(NULL) is okay and simply ignored.
int *my_array = malloc(10 * sizeof(int));
free(my_array + 1);
free(my_array);
int i = my_array[0];
my_array[0] = 42;
free(my_array);
my_array = NULL;
free(my_array); CS 136 Winter 2022
// INVALID
// VALID (my_array is now dangling)
// INVALID
// INVALID
// INVALID
// GOOD STYLE (no longer dangling)
// IGNORED
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Memory leaks
A memory leak occurs when allocated memory is not eventually
Programs that leak memory may suffer degraded performance or eventually crash.
int *my_array = NULL;
my_array = malloc(10 * sizeof(int));
my_array = malloc(10 * sizeof(int)); // Memory Leak!
In this example, the address from the original malloc has been overwritten.
That memory is now “lost” (or leaked) and so it can never be freed.
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Garbage collection
Many modern languages (including Racket) have a garbage collector.
A garbage collector detects when memory is no longer in use and automatically frees memory and returns it to the heap.
One disadvantage of a garbage collector is that it can be slow and affect performance, which is a concern in high performance computing.
C does not have a garbage collector, so you will have to ensure your programs have no memory leaks.
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Merge Sort
In merge sort, the array is split (in half) into two separate arrays. The two arrays are sorted and then they are merged back together into the original array.
This is another example of a divide and conquer algorithm.
The arrays are divided into two smaller problems, which are then sorted (conquered). The results are combined to solve the original problem.
To simplify our implementation, we use a merge helper function.
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// merge(dest, src1, len1, src2, len2) modifies dest to contain // the elements from both src1 and src2 in sorted order
// requires: length of dest is at least (len1 + len2)
// src1 and src2 are sorted [not asserted]
// effects: modifies dest
// time: O(n), where n is len1 + len2
void merge(int dest[], const int src1[], int len1,
const int src2[], int len2) {
int pos1 = 0;
int pos2 = 0;
for (int i = 0; i < len1 + len2; ++i) {
if (pos1 == len1 || (pos2 < len2 && src2[pos2] < src1[pos1])) { dest[i] = src2[pos2];
dest[i] = src1[pos1];
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void merge_sort(int a[], int len) {
if (len <= 1) return;
int llen = len / 2;
int rlen = len - llen;
int *left = malloc(llen * sizeof(int));
int *right = malloc(rlen * sizeof(int));
for (int i = 0; i < llen; ++i) left[i] = a[i];
for (int i = 0; i < rlen; ++i) right[i] = a[i + llen];
merge_sort(left, llen);
merge_sort(right, rlen);
merge(a, left, llen, right, rlen);
free(left);
free(right);
Merge sort is O(n log n), even in the worst case.
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A significant advantage of dynamic memory is that a function can obtain memory that persists after the function has returned.
// build_array(len) returns a new array initialized with // values a[0] = 0, a[1] = 1, ... a[len-1] = len-1
// requires: len > 0
// effects: allocates memory (caller must free)
int *build_array(int len) {
assert(len > 0);
int *a = malloc(len * sizeof(int));
for (int i = 0; i < len; ++i) {
a[i] = i; }
return a; // array exists beyond function return
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Dynamic memory side effect
Allocating (and deallocating) memory has a side effect: it modifies
the “state” of the heap.
A function that allocates persistent memory (i.e., not freed) has a side effect and must be documented.
The caller (client) is responsible for freeing the memory (communicate this).
// build_array(n) returns a new array...
// effects: allocates memory (caller must free)
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A function could also free memory it did not allocate. That would also be a side effect:
// process_and_destroy_array(a, len) ...
// requires: a is a heap-allocated array
// effects: frees a (a is now invalid)
This behaviour is rare outside of ADTs.
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example: strdup
The strdup function makes a duplicate (copy) of a string. // my_strdup(s) makes a duplicate of s
// effects: allocates memory (caller must free)
char *my_strdup(const char *s) {
char *newstr = malloc((strlen(s) + 1) * sizeof(char));
strcpy(newstr, s);
return newstr;
Recall that the strcpy(dest, src) copies the characters from
src to dest, and that the dest array must be large enough.
When allocating memory for strings, don’t forget to include space for
the null terminator.
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Resizing arrays
Because malloc requires the size of the block of memory to be allocated, it does not seem to solve the problem:
“What if we do not know the length of an array at allocation time?”
To solve this problem, we can resize an array by: • creating a new array
• copying the items from the old to the new array • freeing the old array
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example: resizing an array
As we will see shortly, this is not how it is done in practice, but this is an illustrative example.
// my_array has a length of 100
int *my_array = malloc(100 * sizeof(int));
// oops, my_array now needs to have a length of 101
int *old = my_array;
my_array = malloc(101 * sizeof(int));
for (int i = 0; i < 100; ++i) {
my_array[i] = old[i];
free(old);
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To make resizing arrays easier, there is a realloc function.
// realloc(p, newsize) resizes the memory block at p
// to be newsize and returns a pointer to the
// new location, or NULL if unsuccessful
// requires: p must be from a previous malloc/realloc
// or NULL (then realloc behaves like malloc)
// effects: the memory at p is invalid (freed)
// time: O(n), where n is min(newsize, oldsize)
Similar to our previous example, realloc preserves the contents from the old array location.
int *my_array = malloc(100 * sizeof(int));
my_array = realloc(my_array, 101 * sizeof(int));
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The pointer returned by realloc may actually be the original pointer, depending on the circumstances.
Regardless, after realloc only the new returned pointer can be used.
Assume that the address passed to realloc was freed and is now invalid.
Always think of realloc as a malloc, a “copy”, then a free. Typically, realloc is used to request a larger size and the
additional memory is uninitialized.
If the size is smaller, the extraneous memory is discarded.
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Be careful using realloc inside of a helper function.
// repeat(s) modifies s by repeating it ("abc" => “abcabc”)
// and returns the new s
// requires: s is a heap-allocated string
// effects: re-allocates memory (s is invalid)
char *repeat(char *s) {
int len = strlen(s);
s = realloc(s, (len * 2 + 1) * sizeof(char));
for (int i = 0; i < len; ++i) {
s[i + len] = s[i];
s[len * 2] = '\0';
return s; // this is ESSENTIAL
A common mistake is to make repeat a void function (not return the new address for s).
This causes a memory leak if the address of s changes.
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Although rare, in practice,
my_array = realloc(my_array, newsize);
could possibly cause a memory leak if an “out of memory”
condition occurs.
In C99, an unsuccessful realloc returns NULL and the original memory block is not freed.
// safer use of realloc
int *tmp = realloc(my_array, newsize);
if (tmp) {
my_array = tmp;
// handle out of memory condition
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String I/O: strings of unknown length
In Section 07 we saw how reading in strings can be susceptible to buffer overruns.
char str[81];
int retval = scanf("%s", str);
The target array is often oversized to ensure there is capacity to store the string. Unfortunately, regardless of the length of the array, a buffer overrun may occur.
To solve this problem we can continuously resize (realloc) an array while reading in only one character at a time.
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// read_str_slow() reads in a non-whitespace string from I/O // or returns NULL if unsuccessful
// effects: allocates memory (caller must free)
// time: O(n^2)
char *read_str_slow(void) {
char c = 0;
if (scanf(" %c", &c) != 1) return NULL; // ignore initial WS int len = 1;
char *str = malloc(len * sizeof(char));
str[0] = c;
while (1) {
if (scanf("%c", &c) != 1) break;
if (c == ' ' || c == '\n') break;
str = realloc(str, len * sizeof(char));
str[len - 1] = c;
str = realloc(str, (len + 1) * sizeof(char));
str[len] = '\0';
return str;
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Improving the efficiency
Unfortunately, the running time of read_str_slow is O(n2), where n is the length of the string.
This is because realloc is O(n) and occurs inside of the loop. A better approach might be to allocate more memory than
necessary and only call realloc when the array is “full”.
A popular strategy is to double the length of the array when it is full.
Similar to working with oversized arrays, we need to keep track of the “actual” length in addition to the allocated length.
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// time: O(n) [see analysis on next slide]
char *read_str(void) {
char c = 0;
if (scanf(" %c", &c) != 1) return NULL; // ignore initial WS int maxlen = 1;
int len = 1;
char *str = malloc(maxlen * sizeof(char));
str[0] = c;
while (1) {
if (scanf("%c", &c) != 1) break;
if (c == ' ' || c == '\n') break;
if (len == maxlen) {
maxlen *= 2;
str = realloc(str, maxlen * sizeof(char));
str[len - 1] = c;
str = realloc(str, (len + 1) * sizeof(char));
str[len] = '\0';
return str;
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With our “doubling” strategy, most iterations are O(1), unless it is necessary to resize (realloc) the array.
The resizing time for the first 32 iterations would be: 1,2,0,4,0,0,0,8,0,0,0,0,0,0,0,16,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,32
For n iterations, the total resizing time is at most: 1+2+4+...+n2 +n=2n−1=O(n).
By using this doubling strategy, the total run time for read_str is now O(n).
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Reading in an array of strings
In Section 09, we discussed how an array of strings is often stored as an array of pointers (of type char **).
The following example repeatedly calls read_str to generate an array of strings.
// read_aos(len) reads in all non-whitespace strings from I/O // and returns an array of those strings
// notes: modifies *len to store the length of the array
// returns NULL if there are no strings
// effects: allocates memory
// (caller must free each string and the array itself)
// modifies *len
// reads input
// time: O(n) where n is the length of all strings (combined)
char **read_aos(int *len);
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example: reading in an array of strings
char **read_aos(int *len) {
char **aos = NULL;
int maxlen = 0;
while (1) {
char *s = read_str();
if (s == NULL) break;
if (*len == maxlen) {
if (maxlen == 0) maxlen = 1;
maxlen *= 2;
aos = realloc(aos, maxlen * sizeof(char *));
aos[*len] = s;
*len += 1;
if (*len < maxlen) {
aos = realloc(aos, *len * sizeof(char *));
return aos; }
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The read_aos function uses the array doubling strategy on the array of pointers.
Each element in the array (e.g., aos[i]) is a pointer to a string.
To properly free the array of strings, each string must be freed in addition to the array itself.
int len = 0;
char **aos = read_aos(&len);
// free all the strings and the aos itself
for (int i = 0; i < len; ++i) {
free(aos[i]);
} free(aos);
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With dynamic memory, we now have the ability to implement an Abstract Data Type (ADT) in C.
In Section 06, the first ADT we saw was a simple stopwatch ADT . It demonstrated information hiding, which provides both security and flexibility .
It used an opaque structure, which meant that the client could not create a stopwatch.
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example: stopwatch ADT
This is the interface we used in Section 06. // stopwatch.h [INTERFACE]
struct stopwatch;
// stopwatch_create() creates a new stopwatch at time 0:00
// effects: allocates memory (client must call stopwatch_destroy) struct stopwatch *stopwatch_create(void);
// stopwatch_destroy(sw) frees memory for sw
// effects: sw is no longer valid
void stopwatch_destroy(struct stopwatch *sw);
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We can now complete our implementation. // stopwatch.c [IMPLEMENTATION]
struct stopwatch {
int seconds;
// requires: 0 <= seconds
struct stopwatch *stopwatch_create(void) {
struct stopwatch *sw = malloc(sizeof(struct stopwatch)); sw->seconds = 0;
return sw;
void stopwatch_destroy(struct stopwatch *sw) {
assert(sw);
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Implementing a Stack ADT
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