Is there any way, to not use the padding or to skip the padded bytes when writing to a file?

Question

I'm working with an ARM processor Cortex-m4, STM32L476RG Board.

I have read about the padding might be the reason to a bigger size of the struct, than what actually inside the struct.

But, I would really like to get the exact size of 68 bytes to the struct instead of 72.

Anyone have an idea on how to get the right sizee?

Consider the following struct:

struct data
{
  double Latitude;            // 00-07
  double Longtitude;          // 08-15
  float HorizontalAcc;        // 16-19
  float Altitude;             // 20-23
  float AltitudeAcc;          // 24-27
  float SpeedInMS;            // 28-31
  float SpeedAcc;             // 32-35
  float Heading;              // 36-39
  float HeadingAcc;           // 40-43
  int Data;                   // 44-47
  unsigned long PacketNumber; // 48-51
  int64_t y;                  // 52-59
  int64_t x;                  // 60-67
}; // size 8 + 8 + 7*4 + 4 + 4 + 8 + 8 = 68

The thing is that I need to write these data bytewise into a binary file, and the padding ( after PacketNumber ) ruining it.. Is there anyway, to not use the padding or maybe to skip the padded bytes when writing to a file?

I was looking for a solution, not a why... sad this is going to be a duplicate to a question there is only a why... as I explained in the question.. I know why, but I dont know how to get the right size — Noob14, May 29 '18 at 09:27
try reordering the datatypes by size. Ex: move the `int64_t` after the `double`, that should reduce/remove the padding — Jean-François Fabre, May 29 '18 at 09:39
read: https://stackoverflow.com/questions/867471/automated-field-re-ordering-in-c-structs-to-avoid-padding — Jean-François Fabre, May 29 '18 at 09:39
VisualC has a pragma to turn off padding. #pragma pack(1) Your compiler might have something similar. — Serve Laurijssen, May 29 '18 at 09:41
Not in a standard way, but ordering fields from strictest to most relaxed alignment usually minimises padding and moves all of it to the end of the struct. If this is not enough, and your compiler doesn't provide a way to turn packing on, you have to serialize your data manually field by field. — n. m. could be an AI, May 29 '18 at 09:51

score 0 · Accepted Answer · edited Aug 28 '18 at 09:51

0

#pragma pack(x)

is the solution and fixed my problem. In this case with either 4 or 8 bytes, I chose #pragma pack(4).

edited Aug 28 '18 at 09:51

Eric Aya

69,473
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answered Aug 28 '18 at 09:16

Noob14

165
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score 0 · Answer 2 · answered Aug 28 '18 at 10:14

gcc users can also use non-standard __attribute__ ((packed)) attribute for remove padding from the wanted struct:

struct data
{
  double Latitude;            // 00-07
  ...
  unsigned long PacketNumber; // 48-51
  int64_t y;                  // 52-59
  int64_t x;                  // 60-67
} __attribute__ ((packed));

Is there any way, to not use the padding or to skip the padded bytes when writing to a file?

2 Answers2