rrdthreads - Provisions for linking the RRD library to use in
multi-threaded programs
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.
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.
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 "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 there exist thread-safe variants of
"rrd_update",
"rrd_create",
"rrd_dump",
"rrd_info",
"rrd_last", and
"rrd_fetch".
Peter Stamfest <peter@stamfest.at>