diff --git a/tools/rb/fix-linux-stack.pl b/tools/rb/fix-linux-stack.pl
index b3c5213ee44..903cc15a81b 100755
--- a/tools/rb/fix-linux-stack.pl
+++ b/tools/rb/fix-linux-stack.pl
@@ -243,6 +243,7 @@ sub addr2line_pipe($) {
     return $pipe;
 }
 
+select STDOUT; $| = 1; # make STDOUT unbuffered
 while (<>) {
     my $line = $_;
     if ($line =~ /^([ \|0-9-]*)(.*) ?\[([^ ]*) \+(0x[0-9A-F]{1,8})\](.*)$/) {
diff --git a/tools/rb/fix-macosx-stack.pl b/tools/rb/fix-macosx-stack.pl
index decb17e5bf1..1c6477a0a0b 100755
--- a/tools/rb/fix-macosx-stack.pl
+++ b/tools/rb/fix-macosx-stack.pl
@@ -219,6 +219,7 @@ sub nm_lookup($$) {
            };
 }
 
+select STDOUT; $| = 1; # make STDOUT unbuffered
 while (<>) {
     my $line = $_;
     if ($line =~ /^([ \|0-9-]*)(.*) ?\[([^ ]*) \+(0x[0-9A-F]{1,8})\](.*)$/) {