Rockwell Automation 1771-DMC_DMC1_DMC4_DXPS Control Coprocessor User Manual - Manual (Page 133)

API Library of Routines
Appendix B
Using Pointers
B-2
The C syntax section in this appendix provides, in C definition format, the
arguments for each function. In keeping with how functions are defined in
C, the syntax uses an asterisk (*) in the type declaration for an argument to
indicate that the function expects a pointer to the given type, for example:
unsigned DTL_DEF_AVAIL (num_avail)
unsigned *num_avail;
/*{a function definition would be here!} */
In actual practice, the pointer must point to some existing memory.
If the declaration above were used in an actual program, the pointer,
num_avail , would not point to anything.
To emphasize this and provide a guide for use, the C example section for
each function will declare a variable of the required type (which allocates
memory for the variable) and will then use the “address of” (&) operator to
pass the address.
main ()
{
unsigned num_avail; /* allocated memory for variable! */
CC_INIT ();
.
.
.
DTL_DEF_AVAIL (&num_avail);
if (num_avail >0) /* OK */
.
.
.
}
When a function requires a pointer to a type, ensure that you pass the
address of an area of memory that you have created.
char *pointer_only; /* NO */
char allocated_array [100]; /* YES! */
char *pointer_only = “string constant has an address \n”
/* YES */
When a function expects a pointer to any type or to a type that depends on
other arguments, the C syntax definition uses the pointer-to-void syntax.
void *ptr_to_some_unknown_type;
Your program should pass a pointer to a type that matches the data
being processed.
Using Pointers