diff -rup /usr/src.orig/lib/libc/stdio/makebuf.c /usr/src/lib/libc/stdio/makebuf.c
--- /usr/src.orig/lib/libc/stdio/makebuf.c	2009-08-03 10:13:06.000000000 +0200
+++ /usr/src/lib/libc/stdio/makebuf.c	2011-02-19 19:09:56.000000000 +0100
@@ -59,6 +59,7 @@ __smakebuf(fp)
 	FILE *fp;
 {
 	void *p;
+	char *bmode;
 	int flags;
 	size_t size;
 	int couldbetty;
@@ -79,7 +80,8 @@ __smakebuf(fp)
 	flags |= __SMBF;
 	fp->_bf._base = fp->_p = p;
 	fp->_bf._size = size;
-	if (couldbetty && isatty(fp->_file))
+	if (((bmode = getenv("STDIO_IOLBF")) && bmode[0] != '\0') ||
+	    (couldbetty && isatty(fp->_file)))
 		flags |= __SLBF;
 	fp->_flags |= flags;
 }
diff -rup /usr/src.orig/lib/libc/stdio/setbuf.3 /usr/src/lib/libc/stdio/setbuf.3
--- /usr/src.orig/lib/libc/stdio/setbuf.3	2009-08-03 10:13:06.000000000 +0200
+++ /usr/src/lib/libc/stdio/setbuf.3	2011-02-19 19:09:13.000000000 +0100
@@ -79,7 +79,9 @@ and an optimally-sized buffer is obtaine
 If a stream refers to a terminal
 (as
 .Dv stdout
-normally does) it is line buffered.
+normally does), or the environment variable
+.Ev STDIO_IOLBF
+is set, it is line buffered.
 The standard error stream
 .Dv stderr
 is always unbuffered.
@@ -176,6 +178,12 @@ The
 function returns what the equivalent
 .Fn setvbuf
 would have returned.
+.Sh ENVIRONMENT
+.Bl -tag -width ".Ev STDIO_IOLBF"
+If the environment variable
+.Ev STDIO_IOLBF
+is set, output streams will be line-buffered by default
+even when not referring to a terminal.
 .Sh SEE ALSO
 .Xr fclose 3 ,
 .Xr fopen 3 ,
diff -rup /usr/src.orig/lib/libc/stdio/stdio.3 /usr/src/lib/libc/stdio/stdio.3
--- /usr/src.orig/lib/libc/stdio/stdio.3	2009-08-03 10:13:06.000000000 +0200
+++ /usr/src/lib/libc/stdio/stdio.3	2011-02-19 12:56:00.000000000 +0100
@@ -137,7 +137,8 @@ an interactive or
 .Dq terminal
 device, as determined by the
 .Xr isatty 3
-function.
+function (this can be overriden with an environment variable, see
+.Xr setvbuf 3  ) .
 In fact,
 .Em all
 freshly-opened streams that refer to terminal devices