Why sometimes the Heap Pointer will be pointed to 0x000000?

in my function, several heap pointers are defined like this

when I comment the 111 line. the whole code works well. But when I add this line, the pointer Test_feature, ZeroCenter_Parameters, C1K,C1B will all, or sometimes partly, point to 0x000000.

eventually in 2 debug with the same code, it may different.

so the Question is:

In general, What will let the Heap pointer point to 0x00000? before free.

for my observation, the heap size is enough on my project (HEAP SIZE=0x8000, the same as stack size). I'm using STM32F746NG-Discovery Board so the RAM Space(at least 240KB for algorithm) is also enough. So it may be the problem of the IDE or Compiler?

My environment:

   KeilV523, STM32F746NG-Disco, Win10 Edu.

for more general situation. this code.

void heaptest()

free(a);

0x08004468 BD10      POP           {r4,pc}

   176:         a=(int*)malloc(1024*4);

   177:         

0x0800446A F44F5080  MOV           r0,&sharp0x1000

   174: {

   175:         int *a;

   176:         a=(int*)malloc(1024*4);

   177:         

0x0800446E B510      PUSH          {r4,lr}

0x08004470 F000F8B6  BL.W          malloc (0x080045E0)

   178:         int i=1024;

0x08004474 F44F6180  MOV           r1,&sharp0x400

   179:         while(i--)

   180:         {

   181:                 

0x08004478 E000      B             0x0800447C

   182:                 *a=i;

   183:                 //printf(''%d'',*a);

   184:                 a++;

   185:         }

   186: 

0x0800447A C002      STM           r0!,{r1}

0x0800447C 1E49      SUBS          r1,r1,&sharp1

0x0800447E D2FC      BCS           0x0800447A

   187:                 free(a);

   188: }

   189: int main(void)

   190: {

   191:   SCB_EnableICache();

   192:   SCB_EnableDCache();

0x08004480 E8BD4010  POP           {r4,lr}

0x08004484 F7FFBF08  B.W           free (0x08004298)

before the free operation, the pointer will be pointed to 0x00000. who did that?