Converting a struct to char array using memcpy

Question

I am trying to convert the following struct to a char array so that I can send it via the serial port.

struct foo
{
    uint16_t voltage;
    char ID ;
    char TempByte;
    char RTCday[2];
    char RTCmonth[2];
    char RTCyear[2];
    char RTChour[2];
    char RTCmin[2];
    char Sepbyte;   
}dvar = { 500, 'X' , '>' , "18" , "10" , "15" , "20" , "15" , '#'};

I then convert it to a char array using the following:

char b[sizeof(struct foo)];
memcpy(b, &dvar, sizeof(struct foo));

However for some reason I get these trailing values in the char array

0x0A 0xFF

I initially thought it was getting the values because when i cast it to a char array it was effectively casting it to a string so I though the was the NULL '\0'

Any help will be appreciated.

Thanks

I don't see where is structure datadownload and what is the size of it — Gopi
– Gopi, Commented Oct 20, 2015 at 6:20
What do you mean by "trailing values"? How do you examine the contents of the array? And are you programming C or C++? The semantics could be different. On an unrelated note, have you though of using a union instead? — Some programmer dude
– Some programmer dude, Commented Oct 20, 2015 at 6:24
If you convert this to a char array, nobody is going to add a NULL terminator for you. You need to do that yourself. — fuz
– fuz, Commented Oct 20, 2015 at 6:26
Your memcpy is useless in the example, the cast happens with passing &dvar as the parameter of type void *. This would also work: char *b = &dvar; or better char *b = (char *)&dvar; — Melebius
– Melebius, Commented Oct 20, 2015 at 6:35

ThBB · Accepted Answer · 2015-10-20 06:29:22Z

4

On modern processors, sizeof(struct data download) needs to be aligned on 32bits boundaries. Your data structure size is 8 chars + 1 short (16 bits) integer. The compiler needs to add 2 chars to the size of the structure to be able to handle it correctly when assigning it. Since you're doing communication over a serial line and know exactly what you're sending, you might as well specify the exact number of bytes you're willing to send over your serial lines: 2 +/*1 short */ + 8 (8 bytes).

answered Oct 20, 2015 at 6:29

ThBB

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Comments

FrancTheTank · Accepted Answer · 2015-10-20 21:55:26Z

1

I have a sneaky suspicion you are using an 8bit microcontroller! You can debug by printing b[sizeof(foo)], and b[sizeof(foo)+1] These will be your two characters. If you noticed, you should not be referencing these, they are outside the bounds of your char array. eg n element array [0..(n-1)] (copied from your struct)

If you add an unused element to your struct(or increase the size of your final member) the char array can be terminated '\0' -compiler probably wants to do this.

Or do a pointer assignment as @Melebius has shown.

answered Oct 20, 2015 at 21:55

FrancTheTank

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