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Forum: ARM programming with GCC/GNU tools GCC ARM Embedded -> Compiler bug! Dereferenzing of Pointers.


von Ma H. (Guest)


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GCC seems to have a problem dereferenzing pointers.
Example code:
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#define    MAX_A    4
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#define    MAX_AS  5
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typedef    char    str16[16];
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typedef    struct  stc_a*    ptr_stc_a;
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typedef    struct  stc_a  {
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                            U8        number_a;
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                            str16     text_a;
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                          } t_a, *ptr_t_a;
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typedef    t_a            arr_t_a[MAX_AS];
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typedef    arr_t_a*       ptr_arr_t_a;
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typedef    struct  stc_b*    ptr_stc_b;
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typedef    struct  stc_b    {
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                              U8              number_b;
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                              ptr_arr_t_a     arr_a;
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                            } t_b, *ptr_t_b;
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#define    PTR_ARR_A      (ptr_arr_t_a)&array_a
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const      arr_t_a        array_a = {
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                                      {0,"ZERO\0"},
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                                      {1,"ONE\0"},
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                                      {2,"TWO\0"},
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                                      {3,"THREE\0"},
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                                      {4,"FOUR\0"}
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                                    };
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          t_b            b        = { 0, PTR_ARR_A };
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          ptr_t_b        ptr_b;
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#define    CR      PUTCH0('\r')
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void test_stc(void)
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{
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  U8    i;
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  ptr_b = (ptr_t_b)&b;
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  CR;
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  my_prints("TEST\r");
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  my_printf("ptr_b            = %lp\r",ptr_b);
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  my_printf("&b               = %lp\r",&b);
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  my_printf("&array_a         = %lp\r",&array_a);
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  my_printf("sizeof(t_a)      = 0x%02X\r",sizeof(t_a));
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  my_printf("sizeof(arr_t_a)  = 0x%02X\r",sizeof(arr_t_a));
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  my_printf("sizeof(t_b)      = 0x%02X\r",sizeof(t_b));
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  CR;
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  my_prints("Addresses dereferencing ptr_b \r");
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  my_printf("ptr_b->number_b = %lp\r",&(ptr_b->number_b));
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  for(i=0;i<MAX_AS;i++)
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  {
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    my_printf("ptr_b->arr_a[%d] = %lp\r",i,&(ptr_b->arr_a[i]));
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  }
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  CR;
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  my_prints("Addresses const array_a in FLASH \r");
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  my_printf("&array_a = %lp\r",&(array_a));
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  for(i=0;i<MAX_AS;i++)
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  {
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    my_printf("&array_a[%d] = %lp\r",i,&(array_a[i]));
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  }
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}


Debug output (UART)
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MCU          : Spansion FM3 MB9BF516N ARM Cortex-M3
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Firmware     : v00.02
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CM3 CMSIS    : v04.30
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GNUC         : v05.02.01 (46712)
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Compiled     : Feb 29 2016 @ 13:28:05
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TEST
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ptr_b            = 0x1fff8038
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&b               = 0x1fff8038
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&array_a         = 0xba5c
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sizeof(t_a)      = 0x11
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sizeof(arr_t_a)  = 0x55
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sizeof(t_b)      = 0x08
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Addresses dereferencing ptr_b
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ptr_b->number_b            = 0x1fff8038
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ptr_b->arr_a[0]            = 0xba5c
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ptr_b->arr_a[1]            = 0xbab1
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ptr_b->arr_a[2]            = 0xbb06
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ptr_b->arr_a[3]            = 0xbb5b
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ptr_b->arr_a[4]            = 0xbbb0
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Addresses const array_a in FLASH
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&array_a                   = 0xba5c
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&array_a[0]                = 0xba5c
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&array_a[1]                = 0xba6d
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&array_a[2]                = 0xba7e
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&array_a[3]                = 0xba8f
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&array_a[4]                = 0xbaa0

As you can see accessing the array via "ptr_b->arr_a[x]"
the pointer is incremented with sizeof(arr_t_a) instead
sizeof(t_a)...

Ideas/Workarounds???

: Locked by Moderator
von Sebastian V. (sebi_s)


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This is not a bug in GCC but in your source code. If you create a 
pointer to an array you get exactly that. You either want a pointer to 
the first element of the array or you have to access it like this:
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my_printf("ptr_b->arr_a[%d] = %p\n", i, &((*ptr_b->arr_a)[i]));
or
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my_printf("ptr_b->arr_a[%d] = %lp\r",i,&(ptr_b->arr_a[0][i]));

BTW: You code is really awful.

von Jörg W. (dl8dtl) (Moderator)


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