Tales - On bits of code: printb

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While browsing OpenBSD’s httpd’s code, I stumbled upon “weird” flags packed to a char *, used via a function printb_flags(). Turn’s out the way it works is quite fun; here’s a runnable excerpt:

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

#define SRVFLAG_BITS							\
	"\10\01INDEX\02NO_INDEX\03AUTO_INDEX\04NO_AUTO_INDEX"		\
	"\05ROOT\06LOCATION\07FCGI\10NO_FCGI\11LOG\12NO_LOG\13ERRDOCS"	\
	"\14SYSLOG\15NO_SYSLOG\16TLS\17ACCESS_LOG\20ERROR_LOG"		\
	"\21AUTH\22NO_AUTH\23BLOCK\24NO_BLOCK\25LOCATION_MATCH"		\
	"\26SERVER_MATCH\27SERVER_HSTS\30DEFAULT_TYPE\31PATH_REWRITE"	\
	"\32NO_PATH_REWRITE\34NO_BANNER\33GZIP_STATIC\37LOCATION_FOUND"	\
	"\40LOCATION_NOT_FOUND"

const char *
printb_flags(const uint32_t v, const char *bits)
{
	static char  buf[2][BUFSIZ];
	static int   idx = 0;
	int          i, any = 0;
	char         c, *p, *r;

	p = r = buf[++idx % 2];
	memset(p, 0, BUFSIZ);

	if (bits) {
		bits++;
		while ((i = *bits++)) {
			if (v & (1 << (i - 1))) {
				if (any) {
					*p++ = ',';
					*p++ = ' ';
				}
				any = 1;
				for (; (c = *bits) > 32; bits++) {
					if (c == '_')
						*p++ = ' ';
					else
						*p++ = tolower((unsigned char)c);
				}
			} else
				for (; *bits > 32; bits++)
					;
		}
	}

	return (r);
}

int
main(void) {
	printf("%s\n", printb_flags((1 << 0) | (1 << 3) | (1 << 30), SRVFLAG_BITS));
	return 0;
}

Below’s a commented version of the previous excerpt; I encourage you to think about it on your own though.

Solution:[+]

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

/*
 * Code is in httpd, but was imported from relayd. Slightly less interesting
 * variants exists in OpenBSD's ifconfig, some apple's netcat (apparently), etc.
 */

/*
 * recall that \nn is an octal-encoded number; \40 is 32d; that's why
 * \08 or \09 are missing as well.
 *
 * hence this is a clever way to compactly encode a fixed-size, potentially
 * sparse array.
 */
#define SRVFLAG_BITS							\
	"\10\01INDEX\02NO_INDEX\03AUTO_INDEX\04NO_AUTO_INDEX"		\
	"\05ROOT\06LOCATION\07FCGI\10NO_FCGI\11LOG\12NO_LOG\13ERRDOCS"	\
	"\14SYSLOG\15NO_SYSLOG\16TLS\17ACCESS_LOG\20ERROR_LOG"		\
	"\21AUTH\22NO_AUTH\23BLOCK\24NO_BLOCK\25LOCATION_MATCH"		\
	"\26SERVER_MATCH\27SERVER_HSTS\30DEFAULT_TYPE\31PATH_REWRITE"	\
	"\32NO_PATH_REWRITE\34NO_BANNER\33GZIP_STATIC\37LOCATION_FOUND"	\
	"\40LOCATION_NOT_FOUND"

/*
 * v is a bit flag: it's a "collection" of 32 bits; each bit corresponds
 * to a a flag. the flag is enabled when the corresponding bit is 1, disabled
 * otherwise.
 *
 * it's a rather standard way of implementing flags in C.
 */
const char *
printb_flags(const uint32_t v, const char *bits)
{
	/*
	 * 1/ why two buffers? (you may want to read the rest of the
	 * code before pondering about this) the code originates
	 * from relayd, where it's sometimes called as such:
	 *
	 *	fprintf(stderr, "\tflags: %s, relay flags: %s\n",
	 *		printb_flags(proto->flags, F_BITS),
	 *		printb_flags(rlay->rl_conf.flags, F_BITS));
	 *
	 * in such cases, one wants two output buffers "simultaneously".
	 *
	 * in the ifconfig variant (printb()), output is directly printed
	 * to stdout (putchar(3)) so there's no need for any buffer. See:
	 *	http://bxr.su/OpenBSD/sbin/ifconfig/ifconfig.c
	 *
	 * 2/ BUFSIZ (1024 on OpenBSD) bytes is sufficient: it's reasonably easy
	 * to manually check that all potential value of bits (e.g. SRVFLAG_BITS)
	 * are small enough for printb_flags() not to ever outgrow BUFSIZ.
	 */
	static char  buf[2][BUFSIZ];
	static int   idx = 0;
	int          i, any = 0;
	char         c, *p, *r;

	/*
	 * of the two buffers, select one; change at each call.
	 * r is the returned pointer; p will move through r, so as
	 * to load it with relevant data from bits.
	 */
	p = r = buf[++idx % 2];

	/*
	 * reset (empty / fill with \0) the selected output buffer.
	 */
	memset(p, 0, BUFSIZ);

	/* guess it's just some assertion? */
	if (bits) {
		/* skip the heading \10 */
		bits++;

		/* while end of string (\0) hasn't been reached */
		while ((i = *bits++)) {
			/* if the given flag has been selected for output */
			if (v & (1 << (i - 1))) {
				/*
				 * if a previous flag has already been printed to the output
				 * buffer, make sure flags gets separated with a ", ".
				 */
				if (any) {
					*p++ = ',';
					*p++ = ' ';
				}
				any = 1;

				/*
				 * copy current flag to the output buffer.
				 * note that bytes below (≤) 32 are flag numbers.
				 * bytes above are (always) ascii characters to be outputted.
				 */
				for (; (c = *bits) > 32; bits++) {
					if (c == '_')
						*p++ = ' ';
					else
						*p++ = tolower((unsigned char)c);
				}
			} else
				for (; *bits > 32; bits++)
					;
		}
	}

	return (r);
}

int
main(void) {
	printf("%s\n", printb_flags((1 << 0) | (1 << 3) | (1 << 30), SRVFLAG_BITS));
	return 0;
}

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