diff --git a/librb/src/unix.c b/librb/src/unix.c
index a929abfb..9cb84512 100644
--- a/librb/src/unix.c
+++ b/librb/src/unix.c
@@ -40,6 +40,11 @@
 #include <crt_externs.h>
 #endif
 
+#if defined(__FreeBSD__) || defined(__DragonFly__)
+#include <sys/types.h>
+#include <sys/sysctl.h>
+#endif
+
 #if defined(HAVE_SPAWN_H) && defined(HAVE_POSIX_SPAWN)
 #include <spawn.h>
 
@@ -159,6 +164,7 @@ const char *
 rb_path_to_self(void)
 {
 	static char path_buf[4096];
+	size_t path_len = sizeof(path_buf);
 #if defined(HAVE_GETEXECNAME)
 	char *s = getexecname();
 	if (s == NULL)
@@ -166,7 +172,12 @@ rb_path_to_self(void)
 	realpath(s, path_buf);
 	return path_buf;
 #elif defined(__linux__)
-	if (readlink("/proc/self/exe", path_buf, sizeof path_buf) != -1)
+	if (readlink("/proc/self/exe", path_buf, path_len) != -1)
+		return path_buf;
+	return NULL;
+#elif defined(__FreeBSD__) || defined(__DragonFly__)
+	int mib[4] = { CTL_KERN, KERN_PROC, KERN_PROC_PATHNAME, -1 };
+	if (sysctl(mib, 4, path_buf, &path_len, NULL, 0) == 0)
 		return path_buf;
 	return NULL;
 #elif defined(HAVE_DLINFO)