C语言指针一种容易错误使用的方法

/*------------------------------------------------------------------------------
HELLO.C

Copyright 1995-1999 Keil Software, Inc.
------------------------------------------------------------------------------*/

#include <REG52.H>                /* special function register declarations   */
                                  /* for the intended 8051 derivative         */

#include <stdio.h>                /* prototype declarations for I/O functions */
#include <stdlib.h>
#include <string.h>
#include <ABSACC.H>

#define ERR_CASE 1 //test pointer

#ifdef MONITOR51                         /* Debugging with Monitor-51 needs   */
char code reserve [3] _at_ 0x23;         /* space for serial interrupt if     */
#endif                                   /* Stop Exection with Serial Intr.   */
                                         /* is enabled                        */
#if ERR_CASE
void test_func(char *str)
{
 char *p;
 printf("%s/n",str);
 p=malloc(100);
 memset (p, '/0', sizeof (p));
 strncpy(p,"test string",100);
 str=p;
}
#else
void test_func(char **str)
{
#if 0
 char *p;
 p=malloc(100);
 memset (p, '/0', sizeof (p));
 strncpy(p,"test string",100);
 *str=p;
#else
 *str=malloc(100);
 memset (*str, '/0', sizeof (*str));
 strncpy(*str,"test string",100);
#endif
}
#endif
/*---------------------------------------------
The main C function.  Program execution starts
here after stack initialization.
------------------------------------------------*/
void main (void) {
 char *p_str="init string";
/*------------------------------------------------
Setup the serial port for 1200 baud at 16MHz.
------------------------------------------------*/
#ifndef MONITOR51
    SCON  = 0x50;          /* SCON: mode 1, 8-bit UART, enable rcvr      */
    TMOD |= 0x20;               /* TMOD: timer 1, mode 2, 8-bit reload        */
    TH1   = 221;                /* TH1:  reload value for 1200 baud @ 16MHz   */
    TR1   = 1;                  /* TR1:  timer 1 run                          */
    TI    = 1;                  /* TI:   set TI to send first char of UART    */
#endif

/*------------------------------------------------
Note that an embedded program never exits (because
there is no operating system to return to).  It
must loop and execute forever.
------------------------------------------------*/
  init_mempool (&XBYTE [0x2000], 0x1000);
/* initialize memory pool at xdata 0x2000
   for 4096 bytes */
#if ERR_CASE
 test_func(p_str);//把指向的内容传进去,只能读取传进的内容,不能改写;并不影响指针,退出时指针并不变化.
#else
 test_func(&p_str);//把指针传进去,对指针进行操作
#endif
 if(NULL!=p_str)
  printf ("%s/n",p_str);   /* Print "Hello World" */
  while (1) {
    P1 ^= 0x01;           /* Toggle P1.0 each time we print */

  }
}