diff --git a/Makefile b/Makefile
index 17817e2..526feef 100644
--- a/Makefile
+++ b/Makefile
@@ -16,7 +16,7 @@
 
 .POSIX:
 
-CFLAGS += -std=c11 -pedantic -Wall -Wextra
+CFLAGS += -std=c89 -pedantic -Wall -Wextra
 CFLAGS += -O3 -flto
 CFLAGS += -D_XOPEN_SOURCE=700
 CFLAGS += -march=native
diff --git a/eval.c b/eval.c
index 19cdd48..45fb7dc 100644
--- a/eval.c
+++ b/eval.c
@@ -28,21 +28,25 @@
 
 int main(void)
 {
-	FILE *rfp = fopen(RESNAME, "rb");
+	FILE *rfp, *sfp;
+	unsigned solved = 0, errors = 0, i, j;
+	char res[NCELLS], sol[NCELLS];
+	int allcells;
+	double pc;
+
+	rfp = fopen(RESNAME, "rb");
 	if (!rfp) {
 		fputs("Failed to open results file\n", stderr);
 		return EXIT_FAILURE;
 	}
 
-	FILE *sfp = fopen(SOLNAME, "rb");
+	sfp = fopen(SOLNAME, "rb");
 	if (!sfp) {
 		fputs("Failed to open solutions file\n", stderr);
 		return EXIT_FAILURE;
 	}
 
-	unsigned solved = 0, errors = 0;
-	char res[NCELLS], sol[NCELLS];
-	for (unsigned i = 0; i < NPUZZ; ++i) {
+	for (i = 0; i < NPUZZ; ++i) {
 		if (fread(&res, sizeof(char), NCELLS, rfp) != NCELLS) {
 			fprintf(stderr, "Failed to read result #%u\n", i);
 			return EXIT_FAILURE;
@@ -52,10 +56,10 @@ int main(void)
 			return EXIT_FAILURE;
 		}
 
-		bool allcells = true;
-		for (unsigned j = 0; j < NCELLS; ++j) {
+		allcells = 1;
+		for (j = 0; j < NCELLS; ++j) {
 			if (res[j] != sol[j]) {
-				allcells = false;
+				allcells = 0;
 				if (res[j] != '0')
 					++errors;
 			}
@@ -64,7 +68,7 @@ int main(void)
 			++solved;
 	}
 
-	double pc = 1e2 * (double)solved / (double)NPUZZ;
+	pc = 1e2 * (double)solved / (double)NPUZZ;
 	printf("%u/%u (%.2f%%) solved, %u errors\n", solved, NPUZZ, pc, errors);
 
 	fclose(rfp);
diff --git a/genlut.c b/genlut.c
index 968203f..4f91eb5 100644
--- a/genlut.c
+++ b/genlut.c
@@ -25,10 +25,11 @@
 
 static void prlut(const char *name, unsigned lut[NCELLS][NDIGITS - 1])
 {
+	unsigned i, j;
 	printf("const unsigned %s[NCELLS][NDIGITS - 1] = {\n", name);
-	for (unsigned i = 0; i < NCELLS; ++i) {
-		printf("\t[%2u] = { ", i);
-		for (unsigned j = 0; j < NDIGITS - 1; ++j) {
+	for (i = 0; i < NCELLS; ++i) {
+		fputs("\t{ ", stdout);
+		for (j = 0; j < NDIGITS - 1; ++j) {
 			printf("%2u", lut[i][j]);
 			if (j != NDIGITS - 2)
 				putchar(',');
@@ -44,14 +45,14 @@ int main(void)
 	unsigned rowidx_lut[NCELLS][NDIGITS - 1];
 	unsigned colidx_lut[NCELLS][NDIGITS - 1];
 	unsigned segidx_lut[NCELLS][NDIGITS - 1];
+	unsigned rowi, coli, segi, r, c, i, j, sr, sc;
 
 	/* Populate tables. */
-	unsigned rowi, coli, segi;
-	for (unsigned r = 0; r < NDIGITS; ++r) {
-		for (unsigned c = 0; c < NDIGITS; ++c) {
+	for (r = 0; r < NDIGITS; ++r) {
+		for (c = 0; c < NDIGITS; ++c) {
 			/* Calculate row and column indices. */
 			rowi = coli = 0;
-			for (unsigned i = 0; i < NDIGITS; ++i) {
+			for (i = 0; i < NDIGITS; ++i) {
 				if (i != c)
 					rowidx_lut[r * NDIGITS + c][rowi++] = r * NDIGITS + i;
 				if (i != r)
@@ -60,10 +61,10 @@ int main(void)
 
 			/* Calculate segment indices. */
 			segi = 0;
-			const unsigned sr = SEGLEN * (r / SEGLEN);
-			const unsigned sc = SEGLEN * (c / SEGLEN);
-			for (unsigned i = sr; i < sr + SEGLEN; ++i) {
-				for (unsigned j = sc; j < sc + SEGLEN; ++j) {
+			sr = SEGLEN * (r / SEGLEN);
+			sc = SEGLEN * (c / SEGLEN);
+			for (i = sr; i < sr + SEGLEN; ++i) {
+				for (j = sc; j < sc + SEGLEN; ++j) {
 					if (i == r && j == c)
 						continue;
 					segidx_lut[r * NDIGITS + c][segi++] = i * NDIGITS + j;
diff --git a/main.c b/main.c
index 78e0884..afeb0a9 100644
--- a/main.c
+++ b/main.c
@@ -41,12 +41,12 @@ static char *obuf;
 
 void *threadproc(void *arg)
 {
-	unsigned start = CELLPT * (uintptr_t)arg;
+	unsigned start = CELLPT * (uintptr_t)arg, o;
 	const char *libuf = ibuf + start;
 	char *lobuf = obuf + start;
-
 	struct sudoku sud;
-	for (unsigned o = 0; o < CELLPT; o += NCELLS) {
+
+	for (o = 0; o < CELLPT; o += NCELLS) {
 		load(&sud, libuf + o);
 		solve(&sud);
 		save(&sud, lobuf + o);
@@ -58,6 +58,8 @@ void *threadproc(void *arg)
 int main(void)
 {
 	int fd;
+	pthread_t pool[NTHREADS];
+	uintptr_t i;
 
 	fd = open(IFNAME, O_RDONLY);
 	if (fd == -1) {
@@ -83,8 +85,6 @@ int main(void)
 	}
 	close(fd);
 
-	pthread_t pool[NTHREADS];
-	uintptr_t i;
 	for (i = 0; i < NTHREADS; ++i) {
 		if (pthread_create(&pool[i], NULL, threadproc, (void *)i) != 0)
 			fprintf(stderr, "Failed to create thread #%lu\n", i);
diff --git a/solve.c b/solve.c
index c1fc7f0..2399ba2 100644
--- a/solve.c
+++ b/solve.c
@@ -23,7 +23,8 @@
 
 static void setpval(struct sudoku *sud, unsigned i, uint16_t pval)
 {
-	for (unsigned val = 0; val < NDIGITS; ++val) {
+	unsigned val;
+	for (val = 0; val < NDIGITS; ++val) {
 		if (pval & 1) {
 			update(sud, i, val);
 			return;
@@ -35,11 +36,11 @@ static void setpval(struct sudoku *sud, unsigned i, uint16_t pval)
 int solve(struct sudoku *sud)
 {
 	unsigned n, i, j, val;
-	bool match;
+	int match;
 	uint16_t valmask, pvals;
 
 	for (n = 0;; ++n) {
-		match = false;
+		match = 0;
 
 		for (i = 0; i < NCELLS; ++i) {
 			if (DET(sud->cells[i]))
@@ -54,7 +55,7 @@ int solve(struct sudoku *sud)
 				if (pvals & 1) {
 					if (pvals == 1) {
 						update(sud, i, val);
-						match = true;
+						match = 1;
 						goto next_cell;
 					}
 					break;
@@ -71,7 +72,7 @@ int solve(struct sudoku *sud)
 				valmask |= sud->cells[rowidx_lut[i][j]];
 			if ((pvals = sud->cells[i] & ~valmask)) {
 				setpval(sud, i, pvals);
-				match = true;
+				match = 1;
 				continue;
 			}
 
@@ -84,7 +85,7 @@ int solve(struct sudoku *sud)
 				valmask |= sud->cells[colidx_lut[i][j]];
 			if ((pvals = sud->cells[i] & ~valmask)) {
 				setpval(sud, i, pvals);
-				match = true;
+				match = 1;
 				continue;
 			}
 
@@ -97,7 +98,7 @@ int solve(struct sudoku *sud)
 				valmask |= sud->cells[segidx_lut[i][j]];
 			if ((pvals = sud->cells[i] & ~valmask)) {
 				setpval(sud, i, pvals);
-				match = true;
+				match = 1;
 				continue;
 			}
 
diff --git a/sud.c b/sud.c
index 70d41bf..26113d0 100644
--- a/sud.c
+++ b/sud.c
@@ -16,8 +16,8 @@
  * <https://www.gnu.org/licenses/>.
  */
 
-#include "lut.h"
 #include "sud.h"
+#include "lut.h"
 
 #include <stdio.h>
 #include <stdlib.h>
@@ -28,28 +28,31 @@
 
 static void init(struct sudoku *sud)
 {
-	for (unsigned i = 0; i < NCELLS; ++i)
+	unsigned i;
+	for (i = 0; i < NCELLS; ++i)
 		sud->cells[i] = CELLINIT;
 }
 
-bool load(struct sudoku *sud, const char *ptr)
+int load(struct sudoku *sud, const char *ptr)
 {
+	unsigned i;
 
 	init(sud);
 
-	for (unsigned i = 0; i < NCELLS; ++i) {
+	for (i = 0; i < NCELLS; ++i) {
 		if (ptr[i] == '0')
 			continue;
 		if (update(sud, i, ptr[i] - '1') != OK)
-			return false;
+			return 0;
 	}
 
-	return true;
+	return 1;
 }
 
 void save(struct sudoku *sud, char *ptr)
 {
-	for (unsigned i = 0; i < NCELLS; ++i) {
+	unsigned i;
+	for (i = 0; i < NCELLS; ++i) {
 		if (DET(sud->cells[i]))
 			*ptr++ = '1' + VAL(sud->cells[i]);
 		else
@@ -60,10 +63,11 @@ void save(struct sudoku *sud, char *ptr)
 enum update_res update(struct sudoku *sud, unsigned i, unsigned val)
 {
 	const uint16_t clearmask = ~(1 << val);
+	unsigned j;
 
 	/* Update possible values of cells in same row, column and
 	 * segment. */
-	for (unsigned j = 0; j < NGROUP; ++j) {
+	for (j = 0; j < NGROUP; ++j) {
 		sud->cells[rowidx_lut[i][j]] &= clearmask;
 		sud->cells[colidx_lut[i][j]] &= clearmask;
 		sud->cells[segidx_lut[i][j]] &= clearmask;
@@ -79,7 +83,9 @@ enum update_res update(struct sudoku *sud, unsigned i, unsigned val)
 
 void print(const struct sudoku *sud)
 {
-	for (unsigned r = 0; r < NDIGITS; ++r) {
+	unsigned r, c;
+
+	for (r = 0; r < NDIGITS; ++r) {
 		/*
 		 * Print horizontal divider if on a segment boundary (but not
 		 * at the start).
@@ -87,7 +93,7 @@ void print(const struct sudoku *sud)
 		if (r != 0 && r % SEGLEN == 0)
 			puts("------+-------+------");
 
-		for (unsigned c = 0; c < NDIGITS; ++c) {
+		for (c = 0; c < NDIGITS; ++c) {
 			/*
 			 * Print vertical divider if on a segment boundary (but
 			 * not at the start).
@@ -106,17 +112,19 @@ void print(const struct sudoku *sud)
 
 static void zerocounts(unsigned counts[NDIGITS])
 {
-	for (unsigned i = 0; i < NDIGITS; ++i)
+	unsigned i;
+	for (i = 0; i < NDIGITS; ++i)
 		counts[i] = 0;
 }
 
-static bool checkcounts(unsigned counts[NDIGITS])
+static int checkcounts(unsigned counts[NDIGITS])
 {
-	for (unsigned i = 0; i < NDIGITS; ++i) {
+	unsigned i;
+	for (i = 0; i < NDIGITS; ++i) {
 		if (counts[i] != 1)
-			return false;
+			return 0;
 	}
-	return true;
+	return 1;
 }
 
 enum check_res check(const struct sudoku *sud)
@@ -165,13 +173,14 @@ enum check_res check(const struct sudoku *sud)
 	return SOLVED;
 }
 
-bool filled(const struct sudoku *sud)
+int filled(const struct sudoku *sud)
 {
-	for (unsigned r = 0; r < NDIGITS; ++r) {
-		for (unsigned c = 0; c < NDIGITS; ++c) {
+	unsigned r, c;
+	for (r = 0; r < NDIGITS; ++r) {
+		for (c = 0; c < NDIGITS; ++c) {
 			if (!(DET(sud->cells[IDX(r, c)])))
-				return false;
+				return 0;
 		}
 	}
-	return true;
+	return 1;
 }
diff --git a/sud.h b/sud.h
index d11f411..08b28d3 100644
--- a/sud.h
+++ b/sud.h
@@ -19,7 +19,6 @@
 #ifndef SUD_H
 #define SUD_H
 
-#include <stdbool.h>
 #include <stdint.h>
 
 #define SEGLEN 3
@@ -48,7 +47,7 @@ enum check_res { INCOMPLETE, INCORRECT, SOLVED };
  * Read `NCELLS` values from the given pointer and load them into the
  * sudoku.
  */
-bool load(struct sudoku *sud, const char *ptr);
+int load(struct sudoku *sud, const char *ptr);
 
 /**
  * Write the sudoku to the given pointer (`NCELLS` bytes).
@@ -75,6 +74,6 @@ enum check_res check(const struct sudoku *sud);
 /**
  * Determine whether all the sudoku's cells have been determined.
  */
-bool filled(const struct sudoku *sud);
+int filled(const struct sudoku *sud);
 
 #endif