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 "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>



1.2.30                            2009-01-19                     RRDTHREADS(1)
