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RRDTHREADS(1)				     rrdtool				    RRDTHREADS(1)

NAME
       rrdthreads - Provisions for linking the RRD library to use in multi-threaded programs

SYNOPSIS
       Using librrd in multi-threaded programs requires some extra precautions, as the RRD
       library in its original form was not thread-safe at all. This document describes
       requirements and pitfalls on the way to use the multi-threaded version of librrd in your
       own programs. It also gives hints for future RRD development to keep the library thread-
       safe.

       Currently only some RRD operations are implemented in a thread-safe way. They all end in
       the usual ""_r"" suffix.

DESCRIPTION
       In order to use librrd in multi-threaded programs you must:

       o   Link with librrd_th instead of librrd (use "-lrrd_th" when linking)

       o   Use the ""_r"" functions instead of the normal API-functions

       o   Do not use any at-style time specifications. Parsing of such time specifications is
	   terribly non-thread-safe.

       o   Never use non *"_r" functions unless it is explicitly documented that the function is
	   tread-safe.

       o   Every thread SHOULD call "rrd_get_context()" before its first call to any "librrd_th"
	   function in order to set up thread specific data. This is not strictly required, but
	   it is the only way to test if memory allocation can be done by this function.
	   Otherwise the program may die with a SIGSEGV in a low-memory situation.

       o   Always call "rrd_error_clear()" before any call to the library. Otherwise the call
	   might fail due to some earlier error.

   NOTES FOR RRD CONTRIBUTORS
       Some precautions must be followed when developing RRD from now on:

       o   Only use thread-safe functions in library code. Many often used libc functions aren't
	   thread-safe. Take care in the following situations or when using the following library
	   functions:

	   o   Direct calls to "strerror()" must be avoided: use "rrd_strerror()" instead, it
	       provides a per-thread error message.

	   o   The "getpw*", "getgr*", "gethost*" function families (and some more "get*"
	       functions) are not thread-safe: use the *"_r" variants

	   o   Time functions: "asctime", "ctime", "gmtime", "localtime": use *"_r" variants

	   o   "strtok": use "strtok_r"

	   o   "tmpnam": use "tmpnam_r"

	   o   Many others (lookup documentation)

       o   A header file named rrd_is_thread_safe.h is provided that works with the GNU
	   C-preprocessor to "poison" some of the most common non-thread-safe functions using the
	   "#pragma GCC poison" directive. Just include this header in source files you want to
	   keep thread-safe.

       o   Do not introduce global variables!

	   If you really, really have to use a global variable you may add a new field to the
	   "rrd_context" structure and modify rrd_error.c, rrd_thread_safe.c and
	   rrd_non_thread_safe.c

       o   Do not use "getopt" or "getopt_long" in *"_r" (neither directly nor indirectly).

	   "getopt" uses global variables and behaves badly in a multi-threaded application when
	   called concurrently. Instead provide a *_r function taking all options as function
	   parameters. You may provide argc and **argv arguments for variable length argument
	   lists. See "rrd_update_r" as an example.

       o   Do not use the "rrd_parsetime" function!

	   It uses lots of global variables. You may use it in functions not designed to be
	   thread-safe, like in functions wrapping the "_r" version of some operation (e.g.,
	   "rrd_create", but not in "rrd_create_r")

   CURRENTLY IMPLEMENTED THREAD SAFE FUNCTIONS
       Currently there exist thread-safe variants of "rrd_update", "rrd_create", "rrd_dump",
       "rrd_info", "rrd_last", and "rrd_fetch".

AUTHOR
       Peter Stamfest <peter@stamfest.at>

1.4.8					    2013-05-23				    RRDTHREADS(1)
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