Example of rdrand eax or rax in C.

hwrand.c

#include <stdio.h>
#include <stdint.h>

#ifdef __i386__
uint32_t hwrand()
{
	uint32_t eax;
	__asm__ volatile( ".byte 0x0f, 0xc7, 0xf0;" : "=a"( eax ) );
	return eax;
}
#else
uint64_t hwrand()
{
	uint64_t rax;
	__asm__ volatile( ".byte 0x48, 0x0f, 0xc7, 0xf0;" : "=a"( rax ) );
	return rax;
}
#endif

int
main()
{
#ifdef __i386__
	printf("%u\n", hwrand() );
#else
	printf("%lu\n", hwrand() );
#endif
	return 0;
}