Thread: Please explain to me why I am having this output

Hello I am the following piece of code that I am testing and cannot understand the output I'm getting:

Code:

#include<stdio.h>

//taken out of library I'm using to test my code
typedef struct
{
    unsigned int id;
    struct {
        unsigned char rtr;
        unsigned char ide;
        unsigned char length
    } header;
    unsigned char data[8];
} tCAN;
//--------------------------------


union{
    tCAN msg;
    
    struct{
        unsigned int dummy1;
        unsigned char dummy2;
        unsigned char dummy3;
        unsigned char dummy4;
        unsigned int tstamp;
        unsigned int sData;
    };
}in;






int main() {
char i;
    
    for(i=0;i<8;++i){
        in.msg.data[i]=i+1;
    }
    
    for(i=0;i<8;++i){
        printf("data%i: 0x%02x\n",i,in.msg.data[i]);
    }
    
    printf("\n");
    
    in.tstamp=0x12345678;
    in.sData=0xabcd;
    
    for(i=0;i<8;++i){
        printf("data%i: 0x%02x\n",i,in.msg.data[i]);
    }


}

what I am getting is this:

data0: 0x01
data1: 0x02
data2: 0x03
data3: 0x04 //this is as expected.
data4: 0x05
data5: 0x06
data6: 0x07
data7: 0x08


data0: 0x01 //why is data0 0x01
data1: 0x78
data2: 0x56
data3: 0x34
data4: 0x12
data5: 0xcd
data6: 0xab
data7: 0x00

expected output (maybe I'm wrong here):
data0: 0x78
data1: 0x56
data2: 0x34
data3: 0x12
data4: 0xcd
data5: 0xab
data6: 0x00
data7: 0x00

Anybody can suggest why I'm getting this output?

Originally Posted by Salem

Alignment.
All your chars in your tCan have zero padding.
In your anonymous struct in the union, the int following 3 chars has a padding byte.
Result, your data is skewed.

OK. thank you for your explanation.

How can I get round this?

Originally Posted by Salem

Get around what?
If you want to put ints directly into data, then use memcpy.
myint = 1234;
memcpy(&in.data[0],&myint,sizeof(myint));

Note there is no way to directly access data[0]..[3] as an integer because it is misaligned with respect to integer.

I'll give that a go... thank you