% svn diff --diff-cmd diff -x '-u -p' io.c
Index: io.c
===================================================================
--- io.c	(revision 22003)
+++ io.c	(working copy)
@@ -563,9 +563,11 @@ io_fflush(rb_io_t *fptr)
     rb_io_check_closed(fptr);
     if (fptr->wbuf_len == 0)
         return 0;
+fprintf(stderr, "%s:%d io_fflush before rb_thread_fd_writable\n", __FILE__, __LINE__);
     if (!rb_thread_fd_writable(fptr->fd)) {
         rb_io_check_closed(fptr);
     }
+fprintf(stderr, "%s:%d io_fflush after rb_thread_fd_writable\n", __FILE__, __LINE__);
   retry:
     if (fptr->wbuf_len == 0)
         return 0;
@@ -778,6 +780,7 @@ static long
 io_binwrite(VALUE str, rb_io_t *fptr, int nosync)
 {
     long len, n, r, offset = 0;
+fprintf(stderr, "%s:%d io_binwrite start\n", __FILE__, __LINE__);
 
     len = RSTRING_LEN(str);
     if ((n = len) <= 0) return n;
@@ -808,9 +811,11 @@ io_binwrite(VALUE str, rb_io_t *fptr, in
             return len;
         /* avoid context switch between "a" and "\n" in STDERR.puts "a".
            [ruby-dev:25080] */
+fprintf(stderr, "%s:%d before rb_thread_fd_writable start\n", __FILE__, __LINE__);
 	if (fptr->stdio_file != stderr && !rb_thread_fd_writable(fptr->fd)) {
 	    rb_io_check_closed(fptr);
 	}
+fprintf(stderr, "%s:%d after rb_thread_fd_writable start\n", __FILE__, __LINE__);
 	arg.fptr = fptr;
 	arg.str = str;
       retry:
@@ -886,6 +891,7 @@ do_writeconv(VALUE str, rb_io_t *fptr)
 static long
 io_fwrite(VALUE str, rb_io_t *fptr, int nosync)
 {
+fprintf(stderr, "%s:%d io_fwrite start\n", __FILE__, __LINE__);
     str = do_writeconv(str, fptr);
     return io_binwrite(str, fptr, nosync);
 }
@@ -896,6 +902,7 @@ io_write(VALUE io, VALUE str, int nosync
     rb_io_t *fptr;
     long n;
     VALUE tmp;
+fprintf(stderr, "%s:%d io_write start\n", __FILE__, __LINE__);
 
     rb_secure(4);
     io = GetWriteIO(io);
@@ -909,7 +916,9 @@ io_write(VALUE io, VALUE str, int nosync
     if (RSTRING_LEN(str) == 0) return INT2FIX(0);
 
     GetOpenFile(io, fptr);
+fprintf(stderr, "%s:%d io_write: before rb_io_check_writable\n", __FILE__, __LINE__);
     rb_io_check_writable(fptr);
+fprintf(stderr, "%s:%d io_write: after rb_io_check_writable\n", __FILE__, __LINE__);
 
     n = io_fwrite(str, fptr, nosync);
     if (n == -1L) rb_sys_fail_path(fptr->pathv);