Davide Andrea Davide Andrea - 2 days ago 5
C Question

Array doubles in size if a struct defines both its uint16_t words and uint8_t bytes

I have an array each of whose elements could be either uint16_t or a pair of uint8_t.

Its elements are defined as a union of a uint16_t and a sub-array of 2 uint8_t.

Unfortunately, the compiler (MicroChip XC16) allocates twice as much memory as it should for the array.

typedef union {
uint16_t u16; // As uint16_t
uint8_t u8[2]; // As uint8_t
} my_array_t;

my_array_t my_array[1]; // 1 word array, for testing

my_array[0].u8[0] = 1;
my_array[0].u8[1] = 2;
uint8_t byte_0 = my_array[0].u8[0]; // Gets 0x01
uint8_t byte_1 = my_array[0].u8[1]; // Gets 0x02
uint16_t byte_0 = my_array[0].u16; // Gets 0x0201


The compiler allocates 4 bytes instead of 2 bytes as it should.

Workaround: if I change the struct to:

typedef union {
uint16_t u16; // As uint16_t
uint8_t u8[1]; // As uint8_t
} my_array_t;


The compiler allocates 2 bytes as it should, but then this is incorrect:

my_array[0].u8[1] = 2;


though it still works:

uint8_t byte_1 = my_array[0].u8[1]; // Gets 0x02


(except for the inconvenience that the debugger doesn't show its value).

Question: should I live with the workaround, or should I use a better solution?

Please refer to a previous discussion on this, where the above solution was suggested.




EDIT.

Per EOF's suggestion (below), I checked sizeof.

Before the workaround:

sizeof(my_array_t) // Is 4
sizeof(my_array[0]) // Is 4
sizeof(my_array[0].u8) // Is 2


After the workaround:

sizeof(my_array_t) // Is 2
sizeof(my_array[0]) // Is 2
sizeof(my_array[0].u8) // Is 2


That would indicate that it's a compiler bug.

Answer

Instead of an array of 2 bytes, use a structure of 2 bytes:

// Two bytes in a 16-bit word
typedef struct{
    uint8_t     lsb;    // As uint8_t, LSB
    uint8_t     msb;    // As uint8_t. MSB
} two_bytes_t;

typedef union {
   uint16_t u16;   // As uint16_t
   two_bytes_t  u8x2; // As 2 each of uint8_t
} my_array_t;


my_array_t my_array[1]; // 1 word array, for testing

my_array[0].u8x2.msb = 1;
my_array[0].u8x2.lsb = 2;

The XC16 compiler correctly allocates only 2 bytes for each element, and the debugger correctly shows the individual bytes.

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