uint24.c

#include <stdio.h>
#include <arpa/inet.h>

struct __attribute__((__packed__)) uint24 {
	uint8_t _;
	uint32_t value : 24;
};

int main(void)
{
	struct uint24 *s;
	unsigned char a[] = {
	    0x1, 0x2, 0x3,
	};
	unsigned char *c = a;

	s = (struct uint24 *)c;
	printf("%u\n", htons(s->value));
}