diff --git a/ext/extconf.rb b/ext/extconf.rb
index 12eb097..db6a711 100644
--- a/ext/extconf.rb
+++ b/ext/extconf.rb
@@ -63,11 +63,6 @@ def sys(cmd)
 have_func('rb_gc_add_event_hook', ['ruby.h', 'node.h'])
 have_func('rb_postponed_job_register_one', 'ruby.h')
 
-# increase message size on linux
-if RUBY_PLATFORM =~ /linux/
-  $defs.push("-DBUF_SIZE=256")
-end
-
 # warnings save lives
 $CFLAGS << " -Wall "
 
diff --git a/ext/rbtrace.c b/ext/rbtrace.c
index a516170..e8f0f0f 100644
--- a/ext/rbtrace.c
+++ b/ext/rbtrace.c
@@ -79,7 +79,7 @@ timeofday_usec()
 #define MAX_TRACERS 100 // max method tracers
 #define MAX_EXPRS 10    // max expressions per tracer
 #ifndef BUF_SIZE        // msgq buffer size
-#define BUF_SIZE 120
+#define BUF_SIZE 1024
 #endif
 
 typedef struct {
@@ -1138,6 +1138,8 @@ Init_rbtrace()
   rbtrace_module = rb_define_module("RBTrace");
   VALUE output = rb_define_module_under(rbtrace_module, "OUT");
 
+  rb_const_set(rbtrace_module, rb_intern("BUF_SIZE"), INT2NUM(BUF_SIZE));
+
   rb_define_singleton_method(output, "write", send_write, 1);
 
   // hook into the gc
diff --git a/lib/rbtrace/core_ext.rb b/lib/rbtrace/core_ext.rb
index f90630d..d8e93f6 100644
--- a/lib/rbtrace/core_ext.rb
+++ b/lib/rbtrace/core_ext.rb
@@ -1,7 +1,3 @@
-class String
-  alias :bytesize :size
-end unless ''.respond_to?(:bytesize)
-
 module FFI::LastError
   Errnos = Errno::constants.map(&Errno.method(:const_get)).inject({}) do |hash, c|
     hash[ c.const_get(:Errno) ] = c
diff --git a/lib/rbtrace/msgq.rb b/lib/rbtrace/msgq.rb
index e97b98c..c585516 100644
--- a/lib/rbtrace/msgq.rb
+++ b/lib/rbtrace/msgq.rb
@@ -1,3 +1,4 @@
+require 'rbtrace'
 require 'ffi'
 
 module MsgQ
@@ -5,7 +6,7 @@ module MsgQ
   ffi_lib FFI::CURRENT_PROCESS
 
   class EventMsg < FFI::Struct
-    BUF_SIZE = RUBY_PLATFORM =~ /linux/ ? 256 : 120
+    BUF_SIZE = RBTrace::BUF_SIZE
     IPC_NOWAIT = 004000
 
     layout :mtype, :long,
diff --git a/lib/rbtrace/rbtracer.rb b/lib/rbtrace/rbtracer.rb
index 25da99a..a672d9b 100644
--- a/lib/rbtrace/rbtracer.rb
+++ b/lib/rbtrace/rbtracer.rb
@@ -323,7 +323,9 @@ def send_cmd(*cmd)
       msg = cmd.to_msgpack
       # A message is null-terminated, but bytesize gives the unterminated
       # length.
-      raise ArgumentError, 'command is too long' if msg.bytesize >= MsgQ::EventMsg::BUF_SIZE
+      if msg.bytesize >= RbTrace::BUF_SIZE
+        raise ArgumentError, "command is too long (#{msg.bytesize}B >= #{MsgQ::EventMsg::BUF_SIZE}B)"
+      end
       MsgQ::EventMsg.send_cmd(@qo, msg)
     rescue Errno::EINTR
       retry
diff --git a/test.sh b/test.sh
index 4edc983..49b618a 100755
--- a/test.sh
+++ b/test.sh
@@ -38,6 +38,7 @@ trace -m sleep Dir.chdir Dir.pwd Process.pid "String#gsub" "String#*"
 trace -m "Kernel#"
 trace -m "String#gsub(self,@test)" "String#*(self,__source__)" "String#multiply_vowels(self,self.length,num)"
 trace -e 'p(1 + 1)'
+trace -h
 trace --gc --slow=200
 trace --gc -m Dir.
 trace --slow=250