diff --git a/tools/lib/api/fs/fs.c b/tools/lib/api/fs/fs.c
index 3cc302d4c47b..d16911818d4d 100644
--- a/tools/lib/api/fs/fs.c
+++ b/tools/lib/api/fs/fs.c
@@ -376,12 +376,13 @@ int filename__write_int(const char *filename, int value)
 {
 	int fd = open(filename, O_WRONLY), err = -1;
 	char buf[64];
+	int len;
 
 	if (fd < 0)
 		return -errno;
 
-	sprintf(buf, "%d", value);
-	if (write(fd, buf, sizeof(buf)) == sizeof(buf))
+	len = sprintf(buf, "%d", value);
+	if (write(fd, buf, len) == len)
 		err = 0;
 
 	close(fd);