Interning strings with a prefix tree

intern.c

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

const char *
intern(const char *s)
{
	union tree {
		const char *str;
		union tree *sub;
	};

	static union tree root;

	union tree *node = &root;
	const char *p = s;

	do {
		if(!node->sub) {
			node->sub = calloc(1 << CHAR_BIT, sizeof *node->sub);
			if(!node->sub)
				return NULL;
		}
		node = &node->sub[(unsigned char)*p];
	} while(*p++);

	if(!node->str)
		node->str = strdup(s);

	return node->str;
}

int
main(int argc, char **argv)
{
	int i;
	const char *s;

	for(i = 1; i < argc; i++) {
		s = intern(argv[i]);
		if(!s) {
			perror(argv[0]);
			return 1;
		}
		printf("%p: %s\n", (void *)s, s);
	}
	return 0;
}