1. Miscellaneous functions
Command Execution
The function system() int system(const char *s);
The string s contains a system command
system(“pwd”);
system(“ls >fname”);
system(“prog”);
Return value depends on system command
Learn that from man page

2. Miscellaneous functions
Command Execution Example:
int a, b; char command[MAXCMD];
sprintf(command, "prog %d %d > prog.out", a, b);
system(command);
In program compiled as prog, get a and b values through argc and argv[ ].
System call doesn't return data from the command, but by writing > prog.out creates an output file
The calling program can then input this file

3. Miscellaneous Functions
Storage Management - malloc()/free()
Review: Why is this incorrect?
for (p=head; p!=NULL; p=p->next)
free(p);
Review: Why must it be written like this?
for (p = head; p != NULL; p = q){ q = p->next; }

4. Miscellaneous Functions
Math functions
Need to use #include <math.h> in source code
And include the math library flag for gcc:
gcc source.c –lm
*Note: the –lm has to be at the end of the line

5. Pointers to Functions Function prototype with pointer to function
void qsort ( … , int (*comp) (void *, void *));
Function call passing a pointer to function
qsort( …, (int (*) (void *, void *))strcmp);
This is a cast to function pointer of strcmp
Within qsort(), function is called via a pointer
if ((*comp) (v[i], v[left]) < 0) …

6. Pointers to Functions Initialize a pointer to a function
/* function pointer *fooptr = cast of foo to func ptr */
int (*fooptr) (int *, int*) =
(int (*) (int *, int *)) foo;
Call the function foo via the pointer to it
(*fooptr) (to, from);

7. Miscellaneous Functions
Random number generator functions:<stdlib.h>
rand(void)
Produces a random number between 0 and RAND_MAX (at least on any implementation)
srand(unsigned int seed)
Sets the state or "seed" of the random number generator.
When two random number generators are initialized with the same seed, they produce the same sequence of random numbers.

8. qsort and binsearch Library function qsort prototype:
void qsort(void *base, size_t n, size_t size, int (*cmp) (const void *, const void *));
It sorts an array of data using quick sort algorithm
base a pointer to the table (an array of ??)
n the number of elements in the table
size the size of each element
What’s the last argument?
A pointer to a compare function for specific data type

9. qsort
Notice: qsort doesn’t understand the data type for the elements of the table it sorts
How can we tell that?
base (the pointer to the table) is a type void *
size (the size of each element) is provided
(if qsort knew the data type, code could use “sizeof”)
Last arg is a pointer to correct compare function
It doesn’t know anything about what it sorts

10. qsort Example /* a compare function for integers, we will
pass a pointer to this function to qsort */
int intcompare(int *i, int *j){
return (*i - *j); }

11. qsort Example main (){ int i;
int a[10] = {8, 2, 9, 6, 5, 1, 3, 7, 4, 0};
qsort (a, 10, sizeof(int),
(int (*) (void *, void *)) intcompare);
for (i = 0; i < 10; i++)
printf(“%d, ”, a[i]); }
/* prints “0, 1, 2, 3, 4, 5, 6, 7, 8, 9, ” */

12. bsearch Library function bsearch prototype:
void *bsearch(const void *key, const void *base,
size_t n, size_t size,
int (*cmp) (const void *, const void *));
It searches for element containing key in an array of data that is already sorted in ascending order
Returns:
pointer to element (if found) or
NULL (if not found)
Last argument is a pointer to compare function

13. Variable Length Argument Lists
Both printf and scanf have an argument (the format string) that defines the number and type of the remaining arguments in the list
C does not support multiple declarations of the same function each with different lists
How is it supported in C?
Look at stack frame after a function call!

14. Typical Stack frame Stack Pointer After Call Decreasing Addresses
Before Call A r g 1 A r g 2 A r g 3
Function’s
Automatic
Variables
Return Data
e.g.PC, other
Registers, etc
Code provides the location
of the last fixed argument in
call sequence to va_start
Address
0xffffffff
From fixed arguments, the code must determine the number of
additional arguments to access via offsets from stack pointer and va_arg can work its way back up the stack to get each argument

15. Variable Length Argument Lists
Use va_list data type and va_ macro package inside function with a variable length argument list to get args
va_start, va_arg, va_end macros are defined in /usr/include/stdarg.h
void foo (int n, ... ){ // Ellipsis is actual code!
va_list ap; /* variable name ap */
va_start(ap, n); /* n is last named arg */
Now ap points just before first unnamed arg

16. Variable Length Argument Lists
Each call to va_arg( ) moves pointer ap by one argument and returns value by type:
ival = va_arg(ap, int);
fval = va_arg(ap, float);
sval = va_arg(ap, char *);
Function must clean up before returning:
va_end(ap); }