quest4tech.net

  

Explanations of C and C++ Terms

POINTERS

A pointer is a variable that holds an address and can be incremented. It can be returned from functions to point to strings

 TERM  EXPLANATION
 &  The address of
 *
 		  The value in the location pointed to by the address (which is in the  word following the *).
 char *pav;  Defines pav as a pointer to type char (1 byte).
 pav = av;  Makes pav point to av (the address of the array).
 av[i] = 0;  Assign zero to the object that pav points at.
 void *vptr;  Create a generic pointer.
 long *mem_ptr;  Create a pointer that is of type long (int).
 mem_ptr = (long *)saddr;   Put address into the pointer.
 *mem_ptr = sval;  Put data, sval, into the address given by mem_ptr.
 int *devaddr;  Defines devaddr as a pointer to an object type int.
 devaddr = 0xFF0A;  Address of a device.
 (char *)incoming_data  Gives start of the incoming data field.

 
Program example of checking the status word of a device using pointers.
   The example shows the use of a mask Hex. 0x4 to find out if bit 3 in the location pointed to by statusaddr is ON or OFF.  For other bits, replace Hex. 0x4 with their Hex. equivalent.

int status; // set up variable for status word
int *statusaddr; // set up pointer for address of status word  
statusaddr = &status; // put address of status word into pointer       
  // use status word to hold status of a device       
if (*statusaddr & 0x4) // is bit 3 set to a '1' in the data at address given by the pointer  
   printf("\nBit 3 is ON."); // bit 3 is a '1'  
else    
   printf("\nBit 3 is OFF."); // bit 3 is a '0'  

TERM EXPLANATION
char av[10] Reserves space for 10 chars (= 10 bytes),   from av[0] through to av[9].
extern Enables a variable to be declared without defining it.
For example:   extern int a;     instead of     int a = 1;
typedef Declares a new name for a type. For example:
typedef char* Pchar;
Pchar p1, p2;
char* p3 = p1;
static A static variable is one that is initialised only once, on its first pass. If no value is given, it is initialised to zero in its first pass.
void Self contained function with no need for input or output data.
de-reference De-reference a pointer with an *   i.e. refer to the value that it points to. The asterisk is the de-referencing operator.

 
TERM EXPLANATION
list.a The value of field a in the structure list.
list_ptr->a The value of field a in the structure at the address list_ptr.

 

Passing by value and passing by reference (i.e. using a pointer)

When passing a value, you pass a copy of that value. When you passing by reference, you pass the address, thereby giving the receiver direct access to the data at that address.

Return to Top

C & C++ index

quest4tech index

(c) Compiled by A J & B V Wood.

 Main Index

UK SPONSERS



USA SPONSERS