Big endian byte array to small endian struct elements in C, arm

Question

In my application microcontroller stm32f103 is receiving by USART fixed lenght messages, they contains gps velocity which is big endian data. But elements in structure are small endian. Is there any way without doing it manually to write it in correct way?

typedef struct {
    uint32_t test1;
    uint16_t test2;
}Mst;

uint8_t myArray[6] = { 0x01, 0x02, 0x03, 0x04, 0x05, 0x06 };

void main()
{
    Mst * myStruct_p = (Mst)myArray;
}

But after that myStruct_p->test1 equals 0x030201, but should be 0x010203, and myStruct_p->test2 equals 0x0605, but should be 0x0506.

Answer 1

As it is ARM-Cortex M3 we can use special processor instructions. ARM CMSIS have a very handy intrinsic functions __REV & __REV16 which actually compile to the single machine code instruction.

typedef union 
{
    struct 
    {
        uint32_t test1;
        uint16_t test2;
    };
    uint8_t bytes[6];
}Mst;

Mst mst = {.bytes = { 0x01, 0x02, 0x03, 0x04, 0x05, 0x06 }};

void main()
{
    mst.test1 = __REV(mst.test1);
    mst.test2 = __REV16(mst.test2);
}