1. CSCI206 - Computer Organization & Programming
C Arrays, String library, and Memory
zyBook: 6.2, 6.9, 6.10, 6.12, 6.13

2. Partner Activity
#include <stdio.h> char a[80]; int main(){ char b[80]; printf("a is at %p\n", &a[0]); printf("b is at %p\n", &b[0]); }
This declares two 80 byte arrays of characters (strings).
The output is:
$ ./mem
a is at 0x601060
b is at 0x7ffd5beafab0
Talk to your partner about what you think this might mean about how C variables are located in memory.
format as a pointer (memory location)
address of first character in array.
Try out address.c

3. Memory Management
Because C deals directly with memory, we have to understand the program’s view of memory
To keep things organized, memory is split into (at least) 4 segments:
Stack
Heap
Data
Code

4. Memory Map Dynamic (runtime) allocation
a is at 0x601080
b is at 0x7ffd23ea0ad0
Local (automatic) variables inside functions; activation records (recursion); unsafe to share between functions / modules
Dynamic (runtime) allocation
Dynamically allocated memory: malloc and free; safe to share between functions / modules
Global variables
Static (compile time) allocation
Your program’s compiled machine code

5. Global data memory allocation
#include <stdio.h> char a[80]; int main(){ //... }
Data used multiple places (different functions) in your program.
can be exported across modules (extern).
Minimize use to keep your program clean.

6. Local memory allocation
int main(){ char b[80]; //... }
Data used within a single function.
Unsafe to share with other functions!
Use whenever possible to isolate variables to the function they belong.

7. Arrays
dataType identifier[numElements];
Creates numElements of dataType sequentially in memory from some starting address.
Array index starts at 0 (first element is [0]).
e.g., char name[32];
name[0] = ‘A’;

8. sizeof
sizeof(identifier) returns the total number of bytes in memory for identifier.
e.g., char a;
printf(“%d\n”, sizeof(a)); // prints 1
char *b;
printf(“%d\n”, sizeof(b)); // prints 8
int nums[20];
printf(“%d\n”, sizeof(nums)); // prints 80
Try out sizeof.c

9. Array initialization int x[10]; // no intialization
Warning, some of these are compiler specific and may behave differently on different compilers. Check the manual! Verify assumptions!
int x[10]; // no intialization
int x[10] = {};// no initialization
int x[10] = {1,2,3,4,5,6,7,8,9,10};
int x[10] = {0}; // set first int to 0
int x[10] = {1}; // set first int to 1
Try out array-example.c

10. Alternate Initialization
string.h has a useful function for initialization.
void bzero(void *s, size_t n);
n bytes starting from s set to zero
void *memset(void *s, int c, size_t n);
n bytes starting from s are set to the value c.
memset(my_array, 42, sizeof(my_array));
// every byte in my_array set to 42

11. 2d arrays As simple as adding an extra [x] in code float rain[5][12];
C stores arrays in row major order
array[row][column]
this means rows are stored together

12. float rain[5][12]; (logically)…
rain[0][11]
rain[1][0]
rain[1][1]
rain[1][2]
rain[1][11]
...
rain[4][0]
rain[4][1]
rain[4][2]
rain[4][11] 5

13. float rain[5][12]; (logically) rain[1] is highlighted.…
rain[0][11]
rain[1][0]
rain[1][1]
rain[1][2]
rain[1][11]
...
rain[4][0]
rain[4][1]
rain[4][2]
rain[4][11] 5

14. float rain[5][12]; (physically)
N dimensional arrays are physically stored as a long 1 dimensional array in memory
C translates your N-dimensional indexes to the correct offset from the beginning of the array (memory address)
5*12
rain[0][0]
rain[0][1]
rain[0][2]
...
rain[0][11]
rain[1][0]
rain[1][1]
rain[4][11]

15. Strings Don’t compare strings using == #include <string.h>
char *str == char str[];
is *almost* the same thing… they can be used interchangeably.
Strings
char *str = “initial value”;
char str[] = “initial value”;
Don’t compare strings using ==
this will compare the address of the string.
#include <string.h>
strcmp - lexical order of strings, 0 if the same
strcat - concatenate strings
strstr - find a substring in a string
strcpy - copy a string
strtok - split a string using a delimiter

16. strtok example 1/2
#include <stdio.h>
#include <string.h>
int main() {
char message[] = "Lorem ipsum dolor sit amet, consectetur adipiscing elit.";
char* word;
/* get the first word from the message, separated by
* space character */
word = strtok(message, " ");
printf("1st word: %s\n", word);
1st word: Lorem
1st call to strtok, pass the string (buffer) and delimiter.
1st matching token is returned (delimiters are removed).

17. char message[] = "Lorem ipsum dolor sit amet, consectetur adipiscing elit.";
strtok example 2/2
/* get the second word from the message, NULL must be
* used to get tokens from the previous string now */
word = strtok(NULL, " ");
printf("2nd word: %s\n", word);
/* the following loop gets the rest of the words until the
* end of the message */
while ((word = strtok(NULL, " ")) != NULL)
printf("Next: %s\n", word); }
2nd word: ipsum
Next: dolor
Next: sit
Next: amet,
Next: consectetur
Next: adipiscing
Next: elit.
subsequent calls FOR THE SAME STRING, pass NULL as the string!
The delimiter CAN change.
YOU can only tokenize one string at a time!
When there are no more tokens, strtok returns NULL
Try out string.c

18. Strings - possible errors
char *string;
char *string = “initial value”;
char string[100] = “initial value”;
string = “new value”;

19. Strings - possible errors
char *string;
no memory is allocated.
char *string = “initial value”;
“initial value” is const, so as the variable string, can’t be changed.
char string[100] = “initial value”;
is 100 bytes enough?
string = “new value”;
don’t assign with strings (strncpy).
until you understand pointers.