Dear all,
I have this ultra simple strcture
typedef struct { uint8_t Hora; uint16_t Data1; } EEpromPaqueteDatosS;
When I do sizeof(EEpromPaqueteDatosS) it returns 4!!! But if I do the sizeof of the same structure with only one variable, in other words, with uint8_t variable it returns 1 and with uint16_t it returns 2.. but if the structure have the two variables it returns 4!!
Also if I copy the structure to a uint8_t vector I can found a strange byte in the middle.. like this:
EEpromPaqueteDatosS EEpromPaqueteDatos; uint8 data[4];
EEpromPaqueteDatos.Hora = 0x10; EEpromPaqueteDatos.Data1= 0x1020;
When I copy the structure to data, data is like 0x10, 0x??, 0x20, 0x10.. why this extra byte!!!!
Can anybody help me with this???
Thanks
You could start by helping yourself in various ways.
1) Try to take your own writing seriously, e.g. by finishing sentences with an appropriate amount of punctuation. That usually means one full stop, not half a dozen exclamation or question marks.
2) Stop blaming your tools all that easily. You need to get to grips with the idea that lots of people have been using these tools intensely before you started, so when you thing you have a problem with them, it's quite extremely unlikely that the problem really is on the tools' end of things.
3) Get some proper training, or at the very least a good textbook. You're developing a habit of jumping to conclusions based on incorrect beliefs. Wherever those beliefs come from, you need to replace that source of information by a better one.
Hi,
Hans, you are right, but, don't think my phrase was as bad as you said.. but, I will try to ask the questions better.
By the other hand memory is not important today, but, in some cases 3 or 4 more bytes when all data is 10 bytes in cost sensitive applications can be a problem.
Thanks.
"By the other hand memory is not important today, but, in some cases 3 or 4 more bytes when all data is 10 bytes in cost sensitive applications can be a problem."
You are not likely to find a chip with only 10 bytes of data, so you will then not get 30% or 40% of filler bytes. So a single struct can be used without pack.
And if you need to store 1000 structures, you have enough that it is meaningful to see it as a database and create a "read_record()" and "wrtie_record()" that handles pack/unpack.
When you have to store 10 bytes every 1 hour and the program must write data for some month, 3 or 4 bytes is very important, is the difference berween store 40 days or 70 days using a memory lime M24C128... You can place a larger memory or two memories, but, when is a cost sensitive application, you cant...
In that case, as already discussed, you should not be writing structures - you should have read() and write() functions to access the storage as a "stream"...
If I write a data logger, where memory is important, I would probably even check if I need 8 + 16 bits or maybe just 5 + 12 bits.
I might even, depending on type of sample, see if I can have one bit of data represent if following record is having absolute or relative values. Storing temperatures as deltas can save a lot of memory space.
But trying to use raw C structs for binary storage on a memory media is just plain silly. It's a great example of walking the wrong path. Especially so if that memory is removeable and should potentially be read by another device.
The language don't have a standard for packing structures just because it isn't a good way to solve the kind of problem you are trying to solve.
Time to take two steps back and switch to a real solution. A solution that is not depending on compiler flags, pragmas or other tricks.
By the way - your 8-bit value is named "Hora". That would be "hour". If your logger do record one sample every hour, you don't have a need to store any hour value in the sample. So 3 bytes suddenly became 2 bytes. And if the 16-bit value can never be larger than what fits in 15 bits, you could use one bit as "flag" to store a hour jump in case the device has been turned off and there is a jump in hours since previous sample recorded.
In the end, your packed data would probably waste a lot of memory, if memory really are important.
This was an example to solve the problem, in my case the sturcture is more complex and hard to explain... Is curious that the people try to say absolute solutions without knowing at all the question like use "write" o r "read" like stream or other "solutions" posted here without know other things related to the device or its operation... The problem is solved and this discusion have no sense now, anyway, thaks.
had I started this from scratch I would have created the struct the way i have done ever sice my first exposure to an "alignment critical" processor
I always make structs this way: dwords words bytes
that way there never is an alignment issue
Erik
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