National Instruments Drums 320685D-01 ユーザーズマニュアル

Chapter 1

LabWindows/CVI Compiler

©

 National Instruments Corporation

1-15

LabWindows/CVI Programmer Reference Manual

LabWindows/CVI provides run-time error checking for pointers and arrays in dynamically 
allocated memory.

You can use the ANSI C library functions 

malloc

 or 

calloc

 to allocate dynamic memory. 

These functions return 

void *

 values that you must cast to some other type before the 

memory can be used. During program execution, LabWindows/CVI uses the first such cast 
on the return value of each call to these functions to determine the type of the object that will 
be stored in the dynamic memory. Subsequent casts to different types can disable checking on 
the dynamic data, as explained in the 

 discussion in this section.

You can use the 

realloc

 function to resize dynamically allocated memory. This function 

increases or decreases the size of the object associated with the dynamic memory. 
LabWindows/CVI adjusts the user protection information accordingly.

The LabWindows/CVI run-time error checking mechanism dynamically allocates data to 
keep track of pointers that you dynamically allocate in your program. When you no longer 
use the pointers, LabWindows/CVI uses garbage collection to deallocate its corresponding 
dynamic memory. 

A case exists where the garbage collection fails to retrieve all the memory it allocated. This 
occurs when you pass the return value of one function to another function, the return value is 
a pointer to dynamically allocated memory, and you do not assign the pointer to a variable in 
the argument expression. The following is an example:

MyFunc (1, 2, malloc(7));

This call passes the return value from 

malloc

 to 

MyFunc

 but does not assign it to a variable. 

If you make this call repeatedly in your program with run-time checking enabled, you lose a 
small amount of memory each time.

Change the code as follows to avoid this problem.

void *p;

MyFunc (1, 2, p = malloc(7));

The following code also works and uses better programming style.

void *p;

p = malloc(7);

MyFunc (1, 2, p);

00ProRef.book : 06chap01.fm  Page 15  Monday, March 9, 1998  3:23 PM