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#include <stddef.h>
#include <string.h>

struct test{
  int a;
  char b;
  int c;
};

/* ... 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;

  /* 
   * Initializes all bytes (including padding bytes)
   * of the struct to zero. 
   */
  memset(&arg, 0, sizeof(arg));

  arg.a = 1;
  arg.b = 2;
  arg.c = 3;

  /* ... Perform operations on arg ... */

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

  /* ... */
}