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Explicit probes (XProbes) - static user space probes with natural data access.
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Explicit probes

Explicit probes (XProbes for short) is a framework for static user space probes. It consists of a shared library libxprobes and its header files, and xprobes control utility. XProbes enables you to define static probe sites in the source code, and later attach probe modules to them. Probes may be attached both at application startup, and while the application is running. xprobes control utility provides commands for dynamic loading/unloading of probe modules, listing available sites and probes, and module enabling/disabling.

Among XProbes features are:

  • Easy to use.

    On application side single macro is used to defined probe sites.

    On probe module side two macros are used in a pair.

    Probes can be written in the same language as the application itself (we mean C and C++ here). This means that probe code can access type definitions of the application code, and thus access structures, classes, and even their methods, in a natural way.

  • Doesn't require special privileges to run.

    Probe modules are loaded to the application itself, not to the operating system kernel.

  • Doesn't require debug info.

    Works with stripped binaries.

  • Neglectable inactive probe site overhead.

    Inactive probe site is a single if (unlikely(site == active)) statement, i.e., only few instructions.

  • Minimal active probe site overhead.

    Active probe execution is simply a call to a function in another shared library. No trap instruction (breakpoint) or system call is used.

  • Type safe.

    On application side compilation will fail if site prototype doesn't match site argument list.

    On probe module side compilation will fail if probe prototype doesn't match actual probe function.

    At runtime probe module won't be enabled if one of its probe prototypes doesn't match the site prototype it was going to attach.

  • Thread safe and synchronous cancellation safe.

    You may use sites in multi-threaded code, even with enabled synchronous cancellation.

  • Lock free and non-blocking on probe path.

    Probes are executed without any locks. It's a user's responsibility to serialize concurrent calls to probe callbacks if desired.

  • Lock contention free and non-blocking on control path.

    Commands initiated with xprobes control utility (for instance, load new probe module) are executed by one of the application threads. Other threads are not blocked during this execution. There is one exception: when you execute a command with xprobes utility, and some other application thread concurrently calls dlopen() on a library blessed with XProbes, there's a tiny time window during which one of this threads has to wait for the other.

  • Several non-conflicting probe modules may be used simultaneously.

    You may enable several probe modules at once, unless they try to attach to the same site. This provides for better modularity: you can keep a repository of different probe modules for different aspects of the application, and/or distribute probe modules with your library.

  • Portable (to some extent).

    XProbes requires GNU compiler (GCC) version 4.1 or higher, and GNU assembler and linker (or compatible tools) to build, not only XProbes itself, but also the applications that use it and any probe modules. Some of the attributes that it uses require ELF object file format. On the other hand, it doesn't require GNU C library, so it may be used on any ELF system with installed GCC suite.


Here's a short example (you can also play with the code in test/ directory, build it with make check).

NOTE: xprobes control utility uses Unix socket and a real-time signal to communicate with the application that links with libxprobes. Default signal is SIGRTMAX. For the application (i.e., libxprobes) you override the signal number with XPROBES_OPTIONS='s=NUM' environment setting, for xprobes utility use --signal=NUM option.

To bless you application with XProbes, include <xprobes/site.h> and define probe sites with XPROBES_SITE(provider, name, proto, args) macro.

#include <xprobes/site.h>

my_func(int i, int j, char *s)
  XPROBES_SITE(my_app, my_func,
               (const char *, int, bool),
               (s, 3, i > j));

Link you application with -lxprobes. That's all.

Now suppose you want to attach a probe to that site. First you have to write one:

#include <xprobes/probe.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>

static int greater_count = 0;

my_probe(const char *str, int three, bool greater)
  if (greater)

command(const char *cmd, int (*out)(const char *msg))
  if (strcmp(cmd, "dump") == 0)
      char buf[64];
      sprintf(buf, "greater_count: %d", greater_count);
      out("unknown command ");

XPROBES_MODULE(command, 0,
  XPROBES_PROBE("my_app_*", my_probe, (const char *, int, bool)));

Macro prototypes are

XPROBES_MODULE(command_callback, unload_delay, probes...);
XPROBES_PROBE(pattern, callback, proto);

From this source you build probe module as a shared library, with something like

gcc -shared -fPIC -DPIC -g -O2 my_probe.c -o

Note that you don't have to link with -lxprobes (libxprobes has internal ABI version check, so you won't be able to enable a probe that was compiled with incompatible version of header file).

Now, if you wish to enable the probe right from the application start, then you do

$ LD_PRELOAD=/path/to/ my_app

Alternatively, suppose we are talking about a server application that is already running, and you are not allowed to restart it. Then you use xprobes utility:

bash$ xprobes $(pidof my_app)
Connecting... done

First we want to learn what sites are available:

xprobes> sites
  my_app_my_func(const char*,int,bool)

Good to know that the site we defined with XPROBES_SITE() is there. Note that unnecessary spaces were removed from the prototype, to allow us compare prototypes disregarding differences in whitespace (also note that prototypes are compared as strings, so (int), (signed int) and (int32_t) are different prototypes).

Now let's load the probe module:

xprobes> load /path/to/
command in progress, waiting...

When you see "command in progress, waiting..." message this means that the command didn't execute immediately, but instead was scheduled to the time the first probe site is hit (regardless of whether there's a probe attached to it or not). Since my_app_my_func is our only site, won't be loaded until application calls my_func(). You may cancel command in progress by pressing C-c.

xprobes> modules
* /path/to/

Loaded modules are automatically enabled. An asterisk before the module name indicates that the module is enabled. Let's see what probes it has:

xprobes> probes
  my_app_* (const char*,int,bool) -> my_probe

Here it is. Now let's see what sites is has attached to:

xprobes> sites
  my_app_my_func(const char*,int,bool) => my_app_* -> my_probe [/path/to/]

This tells us that my_app_my_func site was matched by my_app_* pattern, and my_probe function from /path/to/ was attached to it (prototype check passed).

xprobes> command /path/to/ dump
command in progress, waiting...
greater_count: 16

We called the command method of our probe module, and it output a message telling that the probe was hit with i > j 16 times so far.

xprobes> disable /path/to/
xprobes> unload /path/to/
module may be unloaded in 58 secs

As said above, there are no locks on probe path. We just disabled the module, but other threads may not yet see the effect, and still call probes from it. So for safety the module is locked for 60 seconds since the moment it was disabled (you can set other timeout with XPROBES_OPTIONS='t=NUM', also you may set it per module with the second argument of XPROBES_MODULE(), but per module value is used only if it is greater than that of libxprobes, i.e., you may only increase the delay).

xprobes> modules
  /path/to/ (may be unloaded in 52 secs)

It is left as an exercise to the reader to wait for that time...


Copyright (C) 2010 Tomash Brechko. All rights reserved.

libxprobes and corresponding header files are released under LGPLv3+. xprobes control utility is released under GPLv3+ (it uses libreadline). Essentially this means that all the functionality is available for non-(L)GPLd code too. See for further details.

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