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_thwhen 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 *_rfunctions 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_thfunction 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. |
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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 arent thread-safe. Take care in the following
situations or when using the following library functions:
| ||||||||||||
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 poisondirective. 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_contextstructure and modify rrd_error.c, rrd_thread_safe.c and rrd_non_thread_safe.c | ||||||||||||
o |
Do not use getoptor getopt_longin * _r(neither directly nor indirectly).
getoptuses 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_ras an example. | ||||||||||||
o |
Do not use the rrd_parsetimefunction! It uses lots of global variables. You may use it in functions not designed to be thread-safe, like in functions wrapping the _rversion of some operation (e.g., rrd_create, but not in rrd_create_r) | ||||||||||||
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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>