Rockwell-automation 1771-DMC_DMC1_DMC4_DXPS Control Coprocessor User M Instrukcja Użytkownika Pobierz pdf (Strona 133)

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Appendix B

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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

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Rockwell-automation 1771-DMC_DMC1_DMC4_DXPS Control Coprocessor User M Instrukcja Użytkownika Strona 133