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:
Link with
librrd_th
instead of
librrd
(use
-lrrd_th
when
linking)
Use the ``
_r
'' functions instead of the normal API-functions
Do not use any at-style time specifications. Parsing of such time
specifications is terribly non-thread-safe.
Never use non *
_r
functions unless it is explicitly documented that
the function is tread-safe.
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.
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:
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:
Direct calls to
strerror()
must be avoided: use
rrd_strerror()
instead, it provides a per-thread error message.
The
getpw*
,
getgr*
,
gethost*
function families (and some more
get*
functions) are not thread-safe: use the *
_r
variants
Time functions:
asctime
,
ctime
,
gmtime
,
localtime
: use
*
_r
variants
strtok
: use
strtok_r
tmpnam
: use
tmpnam_r
Many others (lookup documentation)
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.
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
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.
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
and
rrd_last
.
AUTHOR
Peter Stamfest <
mailto:peter@stamfest.at
peter@stamfest.at
>
