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3.4.2 Routines for dynamic memory allocation

To support using any of the memory functions, you need to implement sbrk(). malloc(), calloc(), and realloc() all call sbrk() at there lowest level. caddr_t is defined elsewhere as char *. RAMSIZE is presently a compile time option. All this does is move a pointer to heap memory and check for the upper limit. Example libc support code. sbrk() returns a pointer to the previous value before more memory was allocated. 

/* _end is set in the linker command file *
extern caddr_t _end;/

/* just in case, most boards have at least some memory */
#ifndef RAMSIZE
#  define RAMSIZE             (caddr_t)0x100000
#endif

/*
 * sbrk -- changes heap size size. Get nbytes more
 *         RAM. We just increment a pointer in what's
 *         left of memory on the board.
 */
caddr_t
sbrk(nbytes)
     int nbytes;
{
  static caddr_t heap_ptr = NULL;
  caddr_t        base;

  if (heap_ptr == NULL) {
    heap_ptr = (caddr_t)&_end;
  }

  if ((RAMSIZE - heap_ptr) >= 0) {
    base = heap_ptr;
    heap_ptr += nbytes;
    return (base);
  } else {
    errno = ENOMEM;
    return ((caddr_t)-1);
  }
}