1. UBC104 Embedded Systems
Functions & Pointers

2. Introduction to C - Arrays, Functions & Pointers
<return-type> <function-name> (<param-list>) {
<expr> }
Example:
int fname (int number, void* ptr) {
return 0;
Introduction to C - Arrays, Functions & Pointers

3. Introduction to C - Arrays, Functions & Pointers
Example Program I
void hello(int a, int b) { 2 } 4
int main(int argc, char** argv) { 6
hello(1,2); }
(gdb) disass hello
Dump of assembler code for function hello:
0x <hello+0>: push %ebp
0x <hello+1>: mov %esp,%ebp
0x <hello+3>: pop %ebp
0x <hello+4>: ret
Introduction to C - Arrays, Functions & Pointers

4. Introduction to C - Arrays, Functions & Pointers
Example Program II
(gdb) disass main
Dump of assembler code for function main:
0x <main+0>: push %ebp
0x a <main+1>: mov %esp,%ebp
0x c <main+3>: sub $0x8,%esp
0x f <main+6>: and $0xfffffff0,%esp
0x <main+9>: mov $0x0,%eax
0x <main+14>: sub %eax,%esp
0x <main+16>: movl $0x2,0x4(%esp)‏
0x <main+24>: movl $0x1,(%esp)‏
0x <main+31>: call 0x <hello>
0x d <main+36>: leave
0x e <main+37>: ret
Put parameter onto stack in reverse order
and call address of function
Introduction to C - Arrays, Functions & Pointers

5. Introduction to C - Arrays, Functions & Pointers
Example Program III
void hello(int *a, int *b) { 2 } 4
int main(int argc, char** argv) {
6 int i, j; 7
8 i= 1; j= 2;
hello(&i, &j); }
Introduction to C - Arrays, Functions & Pointers

6. Introduction to C - Arrays, Functions & Pointers
Parameters
Pass by value
Values of parameters are passed on the stack
Pass by reference
Addresses of parameter are passed on the stack
hello(1, 2);
i= 1; j= 2;
hello(&i,&j);
Introduction to C - Arrays, Functions & Pointers

7. Introduction to C - Arrays, Functions & Pointers
Parameters
Pass by value
Values of parameters are passed on the stack
Pass by reference
Addresses of parameter are passed on the stack
0x <main+16>: movl $0x2,0x4(%esp)‏
0x <main+24>: movl $0x1,(%esp)‏
0x <main+31>: call 0x <hello>
0x <main+30>: lea 0xfffffff8(%ebp),%eax
0x a <main+33>: mov %eax,0x4(%esp)‏
0x e <main+37>: lea 0xfffffffc(%ebp),%eax
0x <main+40>: mov %eax,(%esp)‏
0x <main+43>: call 0x <hello>
Introduction to C - Arrays, Functions & Pointers

8. Variable Parameter-List
<return-type> <function-name>( <param-list>,…) {
<expr> }
Example:
int foobar(int number, …) { }
foobar(3,“Foo”, “Bar”, ”!”);
Introduction to C - Arrays, Functions & Pointers

9. Variable argumentlist - Functions
#include <stdarg.h>
void va_start(va_list ap, last) :
 initializes the argument list ap
type va_arg(va_list ap, type) :
 returns the next argument in the list
void va_end(va_list ap) :
 terminates the use of the argument list ap
Introduction to C - Arrays, Functions & Pointers

10. Example: Variable Parameter-List
void foobar(int a, ...) {
va_list ap;
char *s;
va_start(ap, a);
s = va_arg(ap, char *);
printf("Parameter 1: %s\n", s);
va_end(ap); }
0x <main+16>: movl $0x80484a4,0xc(%esp)‏
0x <main+24>: movl $0x80484a6,0x8(%esp)‏
0x <main+32>: movl $0x80484aa,0x4(%esp)‏
0x <main+40>: movl $0x3,(%esp)‏
0x <main+47>: call 0x <foobar>
Introduction to C - Arrays, Functions & Pointers

11. Introduction to C - Arrays, Functions & Pointers
Pointers to Functions
<return-type> (*<function-name>) (<param-list>)‏
Declaration:
int (*foobar_ptr) (int number, void *ptr);
typedef int (*mainprt) (int argc, char** arg);
Example:
foobar_ptr= foobar;
foobar_ptr();
Introduction to C - Arrays, Functions & Pointers

12. Introduction to C - Arrays, Functions & Pointers
Pointer-to-Function
void hello(int a, int b) { }
int main(int argc, char** argv) {
void (*func)(int, int);
func= hello;
func(1,2);
Introduction to C - Arrays, Functions & Pointers

13. Call of Pointer-to-Function
(gdb) disass main
Dump of assembler code for function main:
0x <main+0>: push %ebp
0x a <main+1>: mov %esp,%ebp
0x c <main+3>: sub $0x18,%esp
0x f <main+6>: and $0xfffffff0,%esp
0x <main+9>: mov $0x0,%eax
0x <main+14>: sub %eax,%esp
0x <main+16>: movl $0x ,0xfffffffc(%ebp)‏
0x <main+23>: movl $0x2,0x4(%esp)‏
0x <main+31>: movl $0x1,(%esp)‏
0x f <main+38>: mov 0xfffffffc(%ebp),%eax
0x <main+41>: call *%eax
0x <main+43>: leave
0x <main+44>: ret
Introduction to C - Arrays, Functions & Pointers

14. Example: Pointers-to-Functions
struct flist {
void (*printstring) (char *s);
void (*printnumber) (int n); }
typedef struct flist flist;
void printer_pstring(char *s) {
flist f1, f2;
f1.printstring= printer_pstring;
f2.printstring= screen_pstring;
Introduction to C - Arrays, Functions & Pointers

15. Summary: Functions & Pointers
Pointers contain Addresses!
Addresses can reference data OR functions
Calls to functions are jumps to addresses
It does not matter where this address comes from
Introduction to C - Arrays, Functions & Pointers

16. Introduction to C - Arrays, Functions & Pointers
Recommended Reading
“A tutorial on pointers and arrays in C”
by Ted Jensen
Introduction to C - Arrays, Functions & Pointers