diff --git a/ext/posix/posix.c b/ext/posix/posix.c
index 7809e48bf50..e9bfd1da661 100644
--- a/ext/posix/posix.c
+++ b/ext/posix/posix.c
@@ -575,7 +575,12 @@ PHP_FUNCTION(posix_ttyname)
 			fd = Z_LVAL_PP(z_fd);
 	}
 #if HAVE_TTYNAME_R
+#ifdef _SC_TTY_NAME_MAX
 	buflen = sysconf(_SC_TTY_NAME_MAX);
+#else
+	/* Arbitrary buffer size for systems which don't extrospect their tty name lengths, way overkill */
+	buflen = 64;
+#endif
 	p = emalloc(buflen);
 
 	if (ttyname_r(fd, p, buflen)) {