8051 IAR C/C++ Compiler Reference Guide

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The difference between the alternative __xdata and __far memory allocation
functions and the standard functions is the pointer type of the return value and the
pointer type of the arguments. The functions malloc, calloc, and realloc return a
pointer to the allocated memory area and the free and realloc functions take a pointer
argument to a previously allocated area. These pointers must be a pointer of the same
type as the memory that the heap is located in, independent of the default memory and
pointer attributes.
Note: The corresponding functionality is also available in the DLIB runtime
environment.