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#include <stddef.h>

#pragma pack(push, 1) /* 1 byte */
struct test{
  int a;
  char b;
  int c;
};
#pragma pack(pop)
 
/* ... safely copy bytes to user space ... */
extern int copy_to_user(void *dest, void *src, size_t size);

void do_stuff(void *usr_buf) {
  struct test arg = {.a=1,.b= 2,.c= 3};

  /* ... perform operations on arg ... */

  /* copy arg to user space */
  copy_to_user(usr_buf, &arg, sizeof(arg));

  /* ... */
}