diff --git a/README b/README
index c8afd69..7a95fd2 100644
--- a/README
+++ b/README
@@ -1,4 +1,4 @@
-### Compile
+### Compile on Linux
 
 gcc -Wall -c utils.c
 gcc -Wall -c f3write.c
@@ -6,11 +6,20 @@ gcc -Wall -c f3read.c
 gcc -o f3write utils.o f3write.o -lm
 gcc -o f3read utils.o f3read.o
 
+### Compile on Mac
+
+gcc -Wall -DAPPLE_MAC -c utils.c
+gcc -Wall -DAPPLE_MAC -c f3write.c
+gcc -Wall -DAPPLE_MAC -c f3read.c
+gcc -o f3write utils.o f3write.o -lm
+gcc -o f3read utils.o f3read.o
+
 ### Use example
 
 ./f3write /media/5EBD-5C80/
 ./f3read /media/5EBD-5C80/
 
-Please replace "/media/5EBD-5C80/" with appropriate path.
+Please replace "/media/5EBD-5C80/" with the appropriate path.
+USB devices are mounted in "/Volumes" on Macs.
 
 ### For more information see http://oss.digirati.com.br/f3/
diff --git a/f3write.c b/f3write.c
index 6bbbd87..05d2283 100644
--- a/f3write.c
+++ b/f3write.c
@@ -218,13 +218,19 @@ static void measure(int fd, struct flow *fw)
 	}
 
 	if (fw->progress) {
-		double percent = (double)fw->total_written * 100 /
-			fw->total_size;
 		/* Instantaneous speed. */
 		double inst_speed =
 			(double)fw->blocks_per_delay * fw->block_size * 1000 /
 			fw->delay_ms;
 		const char *unit = adjust_unit(&inst_speed);
+		double percent;
+		/* The following shouldn't be necessary, but sometimes
+		 * the initial free space isn't exactly reported
+		 * by the kernel; this issue has been seen on Macs.
+		 */
+		if (fw->total_size < fw->total_written)
+			fw->total_size = fw->total_written;
+		percent = (double)fw->total_written * 100 / fw->total_size;
 		erase(fw->erase);
 		fw->erase = printf("%.2f%% -- %.2f %s/s",
 			percent, inst_speed, unit);
@@ -330,21 +336,19 @@ static int fill_fs(const char *path, int progress)
 	int i, fine;
 
 	free_space = get_freespace(path);
+	pr_freespace(free_space);
 	if (free_space <= 0) {
 		printf("No space!\n");
 		return 1;
 	}
-	pr_freespace(free_space);
 
 	init_flow(&fw, free_space, progress);
 	i = 0;
 	fine = 1;
-	while (fine && free_space > 0) {
-		size_t size = free_space >= GIGABYTES ? GIGABYTES : free_space;
-		fine = create_and_fill_file(path, i, size, &fw);
-		free_space -= size;
+	do {
+		fine = create_and_fill_file(path, i, GIGABYTES, &fw);
 		i++;
-	}
+	} while (fine);
 
 	/* Final report. */
 	pr_freespace(get_freespace(path));
diff --git a/utils.c b/utils.c
index 526f538..1a6ab18 100644
--- a/utils.c
+++ b/utils.c
@@ -14,7 +14,7 @@ const char *adjust_unit(double *ptr_bytes)
 	return units[i];
 }
 
-#ifndef __GLIBC__
+#ifndef APPLE_MAC
 
 #include <stdio.h>
 #include <stdint.h>
@@ -89,4 +89,4 @@ int lrand48_r(struct drand48_data *buffer, long int *result)
 	return __nrand48_r(buffer->__x, buffer, result);
 }
 
-#endif	/* __GLIBC__ */
+#endif	/* APPLE_MAC */
diff --git a/utils.h b/utils.h
index b28220d..eae9e43 100644
--- a/utils.h
+++ b/utils.h
@@ -3,7 +3,6 @@
 
 #include <stdio.h>
 #include <string.h>
-#include <features.h>
 #include <ctype.h>
 #include <assert.h>
 #include <sys/time.h>
@@ -52,7 +51,22 @@ static inline long delay_ms(const struct timeval *t1, const struct timeval *t2)
 		(t2->tv_usec - t1->tv_usec) / 1000;
 }
 
-#ifndef __GLIBC__
+#ifdef APPLE_MAC
+
+/* For PATH_MAX. */
+#include <limits.h>
+
+#include <fcntl.h>
+static inline int fdatasync(int fd)
+{
+	/* It isn't exactly the same thing, but it's the best available on
+	 * Macs, and it's enough to work.
+	 */
+	return fcntl(fd, F_FULLFSYNC);
+}
+
+/* Mac's kernel doesn't take advices from applications. */
+#define posix_fadvise(fd, offset, len, advice)	0
 
 /*
  * The following functions were copied from GNU Library C to make F3
@@ -79,6 +93,6 @@ extern int srand48_r(long int __seedval, struct drand48_data *__buffer)
 extern int lrand48_r(struct drand48_data *__restrict __buffer,
 	long int *__restrict __result) __attribute__ ((nonnull(1, 2)));
 
-#endif	/* __GLIBC__ */
+#endif	/* APPLE_MAC */
 
 #endif	/* HEADER_UTILS_H */