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FreeBSD 11.0 - man page for libstdbuf (freebsd section 3)

LIBSTDBUF(3)						   BSD Library Functions Manual 					      LIBSTDBUF(3)

libstdbuf -- preloaded library to change standard streams initial buffering
The libstdbuf library is meant to be preloaded with the LD_PRELOAD environment variable to as to change the initial buffering of standard input, standard output and standard error streams. Although you may load and configure this library manually, an utility, stdbuf(1), can be used to run a command with the appropriate environ- ment variables.
Each stream can be configured independently through the following environment variables (values are defined below): _STDBUF_I Initial buffering definition for the standard input stream _STDBUF_O Initial buffering definition for the standard output stream _STDBUF_E Initial buffering definition for the standard error stream Each variable may take one of the following values: "0" unbuffered "L" line buffered "B" fully buffered with the default buffer size size fully buffered with a buffer of size bytes (suffixes 'k', 'M' and 'G' are accepted)
In the following example, the stdout stream of the awk(1) command will be fully buffered by default because it does not refer to a terminal. libstdbuf is used to force it to be line-buffered so vmstat(8)'s output will not stall until the full buffer fills. # vmstat 1 | LD_PRELOAD=/usr/lib/ \ STDBUF_1=L awk '$2 > 1 || $3 > 1' | cat -n See also the manpage of stdbuf(1) for a simpler way to do this.
The libstdbuf library first appeared in FreeBSD 8.4.
The original idea of the libstdbuf command comes from Padraig Brady who implemented it in the GNU coreutils. Jeremie Le Hen implemented it on FreeBSD.
April 28, 2012 BSD