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GCC allows specifying declaration attributes using the keyword __attribute__((__packed__)). When this attribute is present, the compiler will not add padding bytes for memory alignment unless otherwise required by the _Alignas alignment specifier, and it will attempt to place fields at adjacent memory offsets when possibleunless an explicit alignment specifier for a structure member requires the introduction of padding bytes.

#include <stddef.h>

struct test {
  int a;
  char b;
  int c;
} __attribute__((__packed__));

/* 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};
  copy_to_user(usr_buf, &arg, sizeof(arg));
}

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