Copy byte array to not aligned structure

Question

Hello trying to understand, why copying byte array to not aligned structure byte by byte i loose some data. Maybe there is a way how to solve it ?

Sample code bellow:

typedef struct {  //Not aligned data
    uint32_t val0;
    uint8_t  val1; 
    uint16_t val2;
    uint32_t val3;
}TestSt_t; 

TestSt_t testSt;
uint8_t testData[16] =  {
    0x11, 0x22, 0x33, 0x44,
    0x55,
    0x66, 0x77,
    0x88, 0x99, 0xAA, 0xBB
};
int main() {
    memcpy((uint8_t*)&testSt, (uint8_t*)&testData[0], sizeof(testSt));
}

It is lost in padding: sizeof(TestSt_t) is 12, not 11.

Evg
– Evg

2019-12-01 09:36:36 +00:00
Commented Dec 1, 2019 at 9:36 — Evg
– Evg, Commented Dec 1, 2019 at 9:36

tuket · Accepted Answer · 2019-12-01 09:40:31Z

2

The compiler is adding a byte of padding

typedef struct {  //Not aligned data
    uint32_t val0;
    uint8_t  val1; 
        uint8_t padding;
    uint16_t val2;
    uint32_t val3;
}TestSt_t;

So 0x66 is in that padding byte.

answered Dec 1, 2019 at 9:40

tuket

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2 Comments

Mislab Over a year ago

Is it possible to get a warning or error if compiler adds padding bytes ?

tuket Over a year ago

In GCC there is -Wpadded. I bet there must be something similar for VS: learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/…

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