Disable structure padding in C without using pragma

Question

How can I disable structure padding in C without using pragma?

Answer 1

If you really want structs without padding: Define replacement datatypes for short, int, long, etc., using structs or classes that are composed only of 8 bit bytes. Then compose your higher level structs using the replacement datatypes.

C++'s operator overloading is very convenient, but you could achieve the same effect in C using structs instead of classes. The below cast and assignment implementations assume the CPU can handle misaligned 32bit integers, but other implementations could accommodate stricter CPUs.

Here is sample code:

#include 
#include 

class packable_int { public:

  int8_t b[4];

  operator int32_t () const       { return *(int32_t*) b; }
  void operator =  ( int32_t n )  { *(int32_t*) b = n; }

};

struct SA {
  int8_t   c;
  int32_t  n;
} sa;

struct SB {
  int8_t        c;
  packable_int  n;
} sb;

int main () {
  printf ( "sizeof sa  %d\n", sizeof sa );    // sizeof sa  8               
  printf ( "sizeof sb  %d\n", sizeof sb );    // sizeof sb  5               
  return 0;
}