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3. Using and understanding the Valgrind core: Advanced Topics

3. Using and understanding the Valgrind core: Advanced Topics

This chapter describes advanced aspects of the Valgrind core services, which are mostly of interest to power users who wish to customise and modify Valgrind's default behaviours in certain useful ways. The subjects covered are:

  • The "Client Request" mechanism

  • Debugging your program using Valgrind's gdbserver and GDB

  • Function Wrapping

3.1. The Client Request mechanism

Valgrind has a trapdoor mechanism via which the client program can pass all manner of requests and queries to Valgrind and the current tool. Internally, this is used extensively to make various things work, although that's not visible from the outside.

For your convenience, a subset of these so-called client requests is provided to allow you to tell Valgrind facts about the behaviour of your program, and also to make queries. In particular, your program can tell Valgrind about things that it otherwise would not know, leading to better results.

Clients need to include a header file to make this work. Which header file depends on which client requests you use. Some client requests are handled by the core, and are defined in the header file valgrind/valgrind.h. Tool-specific header files are named after the tool, e.g. valgrind/memcheck.h. Each tool-specific header file includes valgrind/valgrind.h so you don't need to include it in your client if you include a tool-specific header. All header files can be found in the include/valgrind directory of wherever Valgrind was installed.

The macros in these header files have the magical property that they generate code in-line which Valgrind can spot. However, the code does nothing when not run on Valgrind, so you are not forced to run your program under Valgrind just because you use the macros in this file. Also, you are not required to link your program with any extra supporting libraries.

The code added to your binary has negligible performance impact: on x86, amd64, ppc32, ppc64 and ARM, the overhead is 6 simple integer instructions and is probably undetectable except in tight loops. However, if you really wish to compile out the client requests, you can compile with -DNVALGRIND (analogous to -DNDEBUG's effect on assert).

You are encouraged to copy the valgrind/*.h headers into your project's include directory, so your program doesn't have a compile-time dependency on Valgrind being installed. The Valgrind headers, unlike most of the rest of the code, are under a BSD-style license so you may include them without worrying about license incompatibility.

Here is a brief description of the macros available in valgrind.h, which work with more than one tool (see the tool-specific documentation for explanations of the tool-specific macros).


Returns 1 if running on Valgrind, 0 if running on the real CPU. If you are running Valgrind on itself, returns the number of layers of Valgrind emulation you're running on.


Discards translations of code in the specified address range. Useful if you are debugging a JIT compiler or some other dynamic code generation system. After this call, attempts to execute code in the invalidated address range will cause Valgrind to make new translations of that code, which is probably the semantics you want. Note that code invalidations are expensive because finding all the relevant translations quickly is very difficult, so try not to call it often. Note that you can be clever about this: you only need to call it when an area which previously contained code is overwritten with new code. You can choose to write code into fresh memory, and just call this occasionally to discard large chunks of old code all at once.

Alternatively, for transparent self-modifying-code support, use--smc-check=all, or run on ppc32/Linux, ppc64/Linux or ARM/Linux.


Returns the number of errors found so far by Valgrind. Can be useful in test harness code when combined with the --log-fd=-1 option; this runs Valgrind silently, but the client program can detect when errors occur. Only useful for tools that report errors, e.g. it's useful for Memcheck, but for Cachegrind it will always return zero because Cachegrind doesn't report errors.


If your program manages its own memory instead of using the standard malloc / new / new[], tools that track information about heap blocks will not do nearly as good a job. For example, Memcheck won't detect nearly as many errors, and the error messages won't be as informative. To improve this situation, use this macro just after your custom allocator allocates some new memory. See the comments in valgrind.h for information on how to use it.


This should be used in conjunction with VALGRIND_MALLOCLIKE_BLOCK. Again, see valgrind.h for information on how to use it.


Informs a Valgrind tool that the size of an allocated block has been modified but not its address. See valgrind.h for more information on how to use it.


These are similar to VALGRIND_MALLOCLIKE_BLOCK and VALGRIND_FREELIKE_BLOCK but are tailored towards code that uses memory pools. See Memory Pools for a detailed description.


Executes a function in the client program on the real CPU, not the virtual CPU that Valgrind normally runs code on. The function must take an integer (holding a thread ID) as the first argument and then 0, 1, 2 or 3 more arguments (depending on which client request is used). These are used in various ways internally to Valgrind. They might be useful to client programs.

Warning: Only use these if you really know what you are doing. They aren't entirely reliable, and can cause Valgrind to crash. See valgrind.h for more details.

VALGRIND_PRINTF(format, ...):

Print a printf-style message to the Valgrind log file. The message is prefixed with the PID between a pair of ** markers. (Like all client requests, nothing is output if the client program is not running under Valgrind.) Output is not produced until a newline is encountered, or subsequent Valgrind output is printed; this allows you to build up a single line of output over multiple calls. Returns the number of characters output, excluding the PID prefix.


Like VALGRIND_PRINTF (in particular, the return value is identical), but prints a stack backtrace immediately afterwards.


Execute the given monitor command (a string). Returns 0 if command is recognised. Returns 1 if command is not recognised. Note that some monitor commands provide access to a functionality also accessible via a specific client request. For example, memcheck leak search can be requested from the client program using VALGRIND_DO_LEAK_CHECK or via the monitor command "leak_search". Note that the syntax of the command string is only verified at run-time. So, if it exists, it is preferable to use a specific client request to have better compile time verifications of the arguments.


Changes the value of a dynamically changeable option (a string). See Dynamically Change Options.


Registers a new stack. Informs Valgrind that the memory range between start and end is a unique stack. Returns a stack identifier that can be used with other VALGRIND_STACK_* calls.

Valgrind will use this information to determine if a change to the stack pointer is an item pushed onto the stack or a change over to a new stack. Use this if you're using a user-level thread package and are noticing crashes in stack trace recording or spurious errors from Valgrind about uninitialized memory reads.

Warning: Unfortunately, this client request is unreliable and best avoided.


Deregisters a previously registered stack. Informs Valgrind that previously registered memory range with stack id id is no longer a stack.

Warning: Unfortunately, this client request is unreliable and best avoided.

VALGRIND_STACK_CHANGE(id, start, end):

Changes a previously registered stack. Informs Valgrind that the previously registered stack with stack id id has changed its start and end values. Use this if your user-level thread package implements stack growth.

Warning: Unfortunately, this client request is unreliable and best avoided.

3.2. Debugging your program using Valgrind gdbserver and GDB

A program running under Valgrind is not executed directly by the CPU. Instead it runs on a synthetic CPU provided by Valgrind. This is why a debugger cannot natively debug your program when it runs on Valgrind.

This section describes how GDB can interact with the Valgrind gdbserver to provide a fully debuggable program under Valgrind. Used in this way, GDB also provides an interactive usage of Valgrind core or tool functionalities, including incremental leak search under Memcheck and on-demand Massif snapshot production.

3.2.1. Quick Start: debugging in 3 steps

The simplest way to get started is to run Valgrind with the flag --vgdb-error=0. Then follow the on-screen directions, which give you the precise commands needed to start GDB and connect it to your program.

Otherwise, here's a slightly more verbose overview.

If you want to debug a program with GDB when using the Memcheck tool, start Valgrind like this:

valgrind --vgdb=yes --vgdb-error=0 prog

In another shell, start GDB:

gdb prog

Then give the following command to GDB:

(gdb) target remote | vgdb

You can now debug your program e.g. by inserting a breakpoint and then using the GDB continue command.

This quick start information is enough for basic usage of the Valgrind gdbserver. The sections below describe more advanced functionality provided by the combination of Valgrind and GDB. Note that the command line flag --vgdb=yes can be omitted, as this is the default value.

3.2.2. Valgrind gdbserver overall organisation

The GNU GDB debugger is typically used to debug a process running on the same machine. In this mode, GDB uses system calls to control and query the program being debugged. This works well, but only allows GDB to debug a program running on the same computer.

GDB can also debug processes running on a different computer. To achieve this, GDB defines a protocol (that is, a set of query and reply packets) that facilitates fetching the value of memory or registers, setting breakpoints, etc. A gdbserver is an implementation of this "GDB remote debugging" protocol. To debug a process running on a remote computer, a gdbserver (sometimes called a GDB stub) must run at the remote computer side.

The Valgrind core provides a built-in gdbserver implementation, which is activated using --vgdb=yes or --vgdb=full. This gdbserver allows the process running on Valgrind's synthetic CPU to be debugged remotely. GDB sends protocol query packets (such as "get register contents") to the Valgrind embedded gdbserver. The gdbserver executes the queries (for example, it will get the register values of the synthetic CPU) and gives the results back to GDB.

GDB can use various kinds of channels (TCP/IP, serial line, etc) to communicate with the gdbserver. In the case of Valgrind's gdbserver, communication is done via a pipe and a small helper program called vgdb, which acts as an intermediary. If no GDB is in use, vgdb can also be used to send monitor commands to the Valgrind gdbserver from a shell command line.

3.2.3. Connecting GDB to a Valgrind gdbserver

To debug a program "prog" running under Valgrind, you must ensure that the Valgrind gdbserver is activated by specifying either --vgdb=yes or --vgdb=full. A secondary command line option, --vgdb-error=number, can be used to tell the gdbserver only to become active once the specified number of errors have been shown. A value of zero will therefore cause the gdbserver to become active at startup, which allows you to insert breakpoints before starting the run. For example:

valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog

The Valgrind gdbserver is invoked at startup and indicates it is waiting for a connection from a GDB:

==2418== Memcheck, a memory error detector
==2418== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==2418== Using Valgrind-3.14.0.GIT and LibVEX; rerun with -h for copyright info
==2418== Command: ./prog
==2418== (action at startup) vgdb me ... 

GDB (in another shell) can then be connected to the Valgrind gdbserver. For this, GDB must be started on the program prog:

gdb ./prog

You then indicate to GDB that you want to debug a remote target:

(gdb) target remote | vgdb

GDB then starts a vgdb relay application to communicate with the Valgrind embedded gdbserver:

(gdb) target remote | vgdb
Remote debugging using | vgdb
relaying data between gdb and process 2418
Reading symbols from /lib/
Reading symbols from /usr/lib/debug/lib/
Loaded symbols for /lib/
[Switching to Thread 2418]
0x001f2850 in _start () from /lib/

Note that vgdb is provided as part of the Valgrind distribution. You do not need to install it separately.

If vgdb detects that there are multiple Valgrind gdbservers that can be connected to, it will list all such servers and their PIDs, and then exit. You can then reissue the GDB "target" command, but specifying the PID of the process you want to debug:

(gdb) target remote | vgdb
Remote debugging using | vgdb
no --pid= arg given and multiple valgrind pids found:
use --pid=2479 for valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog 
use --pid=2481 for valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog 
use --pid=2483 for valgrind --vgdb=yes --vgdb-error=0 ./another_prog 
Remote communication error: Resource temporarily unavailable.
(gdb)  target remote | vgdb --pid=2479
Remote debugging using | vgdb --pid=2479
relaying data between gdb and process 2479
Reading symbols from /lib/
Reading symbols from /usr/lib/debug/lib/
Loaded symbols for /lib/
[Switching to Thread 2479]
0x001f2850 in _start () from /lib/

If you want to use the --multi mode which makes vgdb start in extended-remote mode, set the following in GDB:

# gdb prog
(gdb) set remote exec-file prog
(gdb) set sysroot /
(gdb) target extended-remote | vgdb --multi --vargs -q
(gdb) start
Temporary breakpoint 1 at 0x24e0
Starting program: prog
relaying data between gdb and process 2999348

Temporary breakpoint 1, 0x000000000010a4a0 in main ()

Note that in --multi mode you don't have to start valgrind separately. vgdb will start valgrind for you. vgdb --multi mode is experimental and currently has some limitations like not being able to see program stdin and stdout. Also you have to explicitly set the remote exec-file and sysroot to tell GDB the "remote" and local files are the same.

Once GDB is connected to the Valgrind gdbserver, it can be used in the same way as if you were debugging the program natively:

  • Breakpoints can be inserted or deleted.

  • Variables and register values can be examined or modified.

  • Signal handling can be configured (printing, ignoring).

  • Execution can be controlled (continue, step, next, stepi, etc).

  • Program execution can be interrupted using Control-C.

And so on. Refer to the GDB user manual for a complete description of GDB's functionality.

3.2.4. Connecting to an Android gdbserver

When developping applications for Android, you will typically use a development system (on which the Android NDK is installed) to compile your application. An Android target system or emulator will be used to run the application. In this setup, Valgrind and vgdb will run on the Android system, while GDB will run on the development system. GDB will connect to the vgdb running on the Android system using the Android NDK 'adb forward' application.

Example: on the Android system, execute the following:

valgrind --vgdb-error=0 --vgdb=yes prog
# and then in another shell, run:
vgdb --port=1234

On the development system, execute the following commands:

adb forward tcp:1234 tcp:1234
gdb prog
(gdb) target remote :1234

GDB will use a local tcp/ip connection to connect to the Android adb forwarder. Adb will establish a relay connection between the host system and the Android target system. Be sure to use the GDB delivered in the Android NDK system (typically, arm-linux-androideabi-gdb), as the host GDB is probably not able to debug Android arm applications. Note that the local port nr (used by GDB) must not necessarily be equal to the port number used by vgdb: adb can forward tcp/ip between different port numbers.

In the current release, the GDB server is not enabled by default for Android, due to problems in establishing a suitable directory in which Valgrind can create the necessary FIFOs (named pipes) for communication purposes. You can stil try to use the GDB server, but you will need to explicitly enable it using the flag --vgdb=yes or --vgdb=full.

Additionally, you will need to select a temporary directory which is (a) writable by Valgrind, and (b) supports FIFOs. This is the main difficult point. Often, /sdcard satisfies requirement (a), but fails for (b) because it is a VFAT file system and VFAT does not support pipes. Possibilities you could try are /data/local, /data/local/Inst (if you installed Valgrind there), or /data/data/, if you are running a specific application and it has its own directory of that form. This last possibility may have the highest probability of success.

You can specify the temporary directory to use either via the --with-tmpdir= configure time flag, or by setting environment variable TMPDIR when running Valgrind (on the Android device, not on the Android NDK development host). Another alternative is to specify the directory for the FIFOs using the --vgdb-prefix= Valgrind command line option.

We hope to have a better story for temporary directory handling on Android in the future. The difficulty is that, unlike in standard Unixes, there is no single temporary file directory that reliably works across all devices and scenarios.

3.2.5. Monitor command handling by the Valgrind gdbserver

The Valgrind gdbserver provides additional Valgrind-specific functionality via "monitor commands". Such monitor commands can be sent from the GDB command line or from the shell command line or requested by the client program using the VALGRIND_MONITOR_COMMAND client request. See Valgrind monitor commands for the list of the Valgrind core monitor commands available regardless of the Valgrind tool selected.

The following tools provide tool-specific monitor commands:

An example of a tool specific monitor command is the Memcheck monitor command leak_check full reachable any. This requests a full reporting of the allocated memory blocks. To have this leak check executed, use the GDB command:

(gdb) monitor leak_check full reachable any

GDB sends as a single string all what follows 'monitor' to the Valgrind gdbserver. The Valgrind gdbserver parses the string and will execute the monitor command itself, if it recognises it to be a Valgrind core monitor command. If it is not recognised as such, it is assumed to be tool-specific and is handed to the tool for execution. For example:

(gdb) monitor leak_check full reachable any
==2418== 100 bytes in 1 blocks are still reachable in loss record 1 of 1
==2418==    at 0x4006E9E: malloc (vg_replace_malloc.c:236)
==2418==    by 0x804884F: main (prog.c:88)
==2418== LEAK SUMMARY:
==2418==    definitely lost: 0 bytes in 0 blocks
==2418==    indirectly lost: 0 bytes in 0 blocks
==2418==      possibly lost: 0 bytes in 0 blocks
==2418==    still reachable: 100 bytes in 1 blocks
==2418==         suppressed: 0 bytes in 0 blocks

Similarly to GDB, the Valgrind gdbserver will accept abbreviated monitor command names and arguments, as long as the given abbreviation is unambiguous. For example, the above leak_check command can also be typed as:

(gdb) mo l f r a

The letters mo are recognised by GDB as being an abbreviation for monitor. So GDB sends the string l f r a to the Valgrind gdbserver. The letters provided in this string are unambiguous for the Valgrind gdbserver. This therefore gives the same output as the unabbreviated command and arguments. If the provided abbreviation is ambiguous, the Valgrind gdbserver will report the list of commands (or argument values) that can match:

(gdb) mo v. n
v. can match v.set v.wait v.kill v.translate
(gdb) mo v.i n
n_errs_found 0 n_errs_shown 0 (vgdb-error 0)

Instead of sending a monitor command from GDB, you can also send these from a shell command line. For example, the following command lines, when given in a shell, will cause the same leak search to be executed by the process 3145:

vgdb --pid=3145 leak_check full reachable any
vgdb --pid=3145 l f r a

Note that the Valgrind gdbserver automatically continues the execution of the program after a standalone invocation of vgdb. Monitor commands sent from GDB do not cause the program to continue: the program execution is controlled explicitly using GDB commands such as "continue" or "next".

Many monitor commands (e.g. location, memcheck who_points_at, ...) require an address argument and an optional length: <addr> [<len>]. The arguments can also be provided by using a 'C array like syntax' by providing the address followed by the length between square brackets.

For example, the following two monitor commands provide the same information:

(gdb) mo xb 0x804a2f0 10
(gdb) mo xb 0x804a2f0[10]

3.2.6. GDB front end commands for Valgrind gdbserver monitor commands

As explained in Monitor command handling by the Valgrind gdbserver, valgrind monitor commands consist in strings that are not interpreted by GDB. GDB has no knowledge of these valgrind monitor commands. The GDB 'command line interface' infrastructure however provides interesting functionalities to help typing commands such as auto-completion, command specific help, searching for a command or command help matching a regexp, ...

To have a better integration of the valgrind monitor commands in the GDB command line interface, Valgrind provides python code defining a GDB front end command for each valgrind monitor command. Similarly, for each tool specific monitor command, the python code provides a matching GDB front end command.

Like other GDB commands, the GDB front end Valgrind monitor commands are hierarchically structured starting from 5 "top" GDB commands. Subcommands are defined below these "top" commands. To ease typing, shorter aliases are also provided.

  • valgrind (aliased by vg and v) is the top GDB command providing front end commands to the Valgrind general monitor commands.

  • memcheck (aliased by mc) is the top GDB command providing the front end commands corresponding to the memcheck specific monitor commands.

  • callgrind (aliased by cg) is the top GDB command providing the front end commands corresponding to the callgrind specific monitor commands.

  • massif (aliased by ms) is the top GDB command providing the front end commands corresponding to the massif specific monitor commands.

  • helgrind (aliased by hg) is the top GDB command providing the front end commands corresponding to the helgrind specific monitor commands.

The usage of a GDB front end command is compatible with a direct usage of the Valgrind monitor command. The below example shows a direct usage of the Memcheck monitor command xb to examine the definedness status of the some_mem array and equivalent usages based on the GDB front end commands.

(gdb) list
1	int main()
2	{
3	  char some_mem[5];
4	  return 0;
5	}
(gdb) p &some_mem
$2 = (char (*)[5]) 0x1ffefffe5b
(gdb) p sizeof(some_mem)
$3 = 5
(gdb) monitor xb 0x1ffefffe5b 5
		  ff	  ff	  ff	  ff	  ff
0x1FFEFFFE5B:	0x00	0x00	0x00	0x00	0x00
(gdb) memcheck xb 0x1ffefffe5b 5
		  ff	  ff	  ff	  ff	  ff
0x1FFEFFFE5B:	0x00	0x00	0x00	0x00	0x00
(gdb) mc xb &some_mem sizeof(some_mem)
		  ff	  ff	  ff	  ff	  ff
0x1FFEFFFE5B:	0x00	0x00	0x00	0x00	0x00

It is worth noting down that the third command uses the alias mc. This command also shows a significant advantage of using the GDB front end commands: as GDB "understands" the structure of these front end commands, where relevant, these front end commands will evaluate their arguments. In the case of the xb command, the GDB xb command evaluates its second argument (which must be an address expression) and its optional second argument (which must be an integer expression).

GDB will auto-load the python code defining the Valgrind front end commands as soon as GDB detects that the executable being debugged is running under valgrind. This detection is based on observing that the Valgrind process has loaded a specific Valgrind shared library. The loading of this library is done by the dynamic loader very early on in the execution of the process. If GDB is used to connect to a Valgrind process that has not yet started its execution (such as when Valgrind was started with the option --vgdb-stop-at=startup or --vgdb-error=0), then the GDB front end commands will not yet be auto-loaded. To have the GDB front end commands auto-loaded, you can put a breakpoint e.g. in main and use the GDB command continue. Alternatively, you can add in your .gdbinit a line that loads the python code at GDB startup such as:

source /path/to/valgrind/python/code/

The exact path to use in the source command depends on your Valgrind installation. The output of the shell command vgdb --help contains the absolute path name for the python file you can source in your .gdbinit to define the GDB valgrind front end monitor commands.

3.2.7. Valgrind gdbserver thread information

Valgrind's gdbserver enriches the output of the GDB info threads command with Valgrind-specific information. The operating system's thread number is followed by Valgrind's internal index for that thread ("tid") and by the Valgrind scheduler thread state:

(gdb) info threads
  4 Thread 6239 (tid 4 VgTs_Yielding)  0x001f2832 in _dl_sysinfo_int80 () from /lib/
* 3 Thread 6238 (tid 3 VgTs_Runnable)  make_error (s=0x8048b76 "called from London") at prog.c:20
  2 Thread 6237 (tid 2 VgTs_WaitSys)  0x001f2832 in _dl_sysinfo_int80 () from /lib/
  1 Thread 6234 (tid 1 VgTs_Yielding)  main (argc=1, argv=0xbedcc274) at prog.c:105

3.2.8. Examining and modifying Valgrind shadow registers

When the option --vgdb-shadow-registers=yes is given, the Valgrind gdbserver will let GDB examine and/or modify Valgrind's shadow registers. GDB version 7.1 or later is needed for this to work. For x86 and amd64, GDB version 7.2 or later is needed.

For each CPU register, the Valgrind core maintains two shadow register sets. These shadow registers can be accessed from GDB by giving a postfix s1 or s2 for respectively the first and second shadow register. For example, the x86 register eax and its two shadows can be examined using the following commands:

(gdb) p $eax
$1 = 0
(gdb) p $eaxs1
$2 = 0
(gdb) p $eaxs2
$3 = 0

Float shadow registers are shown by GDB as unsigned integer values instead of float values, as it is expected that these shadow values are mostly used for memcheck validity bits.

Intel/amd64 AVX registers ymm0 to ymm15 have also their shadow registers. However, GDB presents the shadow values using two "half" registers. For example, the half shadow registers for ymm9 are xmm9s1 (lower half for set 1), ymm9hs1 (upper half for set 1), xmm9s2 (lower half for set 2), ymm9hs2 (upper half for set 2). Note the inconsistent notation for the names of the half registers: the lower part starts with an x, the upper part starts with an y and has an h before the shadow postfix.

The special presentation of the AVX shadow registers is due to the fact that GDB independently retrieves the lower and upper half of the ymm registers. GDB does not however know that the shadow half registers have to be shown combined.

3.2.9. Limitations of the Valgrind gdbserver

Debugging with the Valgrind gdbserver is very similar to native debugging. Valgrind's gdbserver implementation is quite complete, and so provides most of the GDB debugging functionality. There are however some limitations and peculiarities:

  • Precision of "stop-at" commands.

    GDB commands such as "step", "next", "stepi", breakpoints and watchpoints, will stop the execution of the process. With the option --vgdb=yes, the process might not stop at the exact requested instruction. Instead, it might continue execution of the current basic block and stop at one of the following basic blocks. This is linked to the fact that Valgrind gdbserver has to instrument a block to allow stopping at the exact instruction requested. Currently, re-instrumentation of the block currently being executed is not supported. So, if the action requested by GDB (e.g. single stepping or inserting a breakpoint) implies re-instrumentation of the current block, the GDB action may not be executed precisely.

    This limitation applies when the basic block currently being executed has not yet been instrumented for debugging. This typically happens when the gdbserver is activated due to the tool reporting an error or to a watchpoint. If the gdbserver block has been activated following a breakpoint, or if a breakpoint has been inserted in the block before its execution, then the block has already been instrumented for debugging.

    If you use the option --vgdb=full, then GDB "stop-at" commands will be obeyed precisely. The downside is that this requires each instruction to be instrumented with an additional call to a gdbserver helper function, which gives considerable overhead (+500% for memcheck) compared to --vgdb=no. Option --vgdb=yes has neglectible overhead compared to --vgdb=no.

  • Processor registers and flags values.

    When Valgrind gdbserver stops on an error, on a breakpoint or when single stepping, registers and flags values might not be always up to date due to the optimisations done by the Valgrind core. The default value --vex-iropt-register-updates=unwindregs-at-mem-access ensures that the registers needed to make a stack trace (typically PC/SP/FP) are up to date at each memory access (i.e. memory exception points). Disabling some optimisations using the following values will increase the precision of registers and flags values (a typical performance impact for memcheck is given for each option).

    • --vex-iropt-register-updates=allregs-at-mem-access (+10%) ensures that all registers and flags are up to date at each memory access.

    • --vex-iropt-register-updates=allregs-at-each-insn (+25%) ensures that all registers and flags are up to date at each instruction.

    Note that --vgdb=full (+500%, see above Precision of "stop-at" commands) automatically activates --vex-iropt-register-updates=allregs-at-each-insn.

  • Hardware watchpoint support by the Valgrind gdbserver.

    The Valgrind gdbserver can simulate hardware watchpoints if the selected tool provides support for it. Currently, only Memcheck provides hardware watchpoint simulation. The hardware watchpoint simulation provided by Memcheck is much faster that GDB software watchpoints, which are implemented by GDB checking the value of the watched zone(s) after each instruction. Hardware watchpoint simulation also provides read watchpoints. The hardware watchpoint simulation by Memcheck has some limitations compared to real hardware watchpoints. However, the number and length of simulated watchpoints are not limited.

    Typically, the number of (real) hardware watchpoints is limited. For example, the x86 architecture supports a maximum of 4 hardware watchpoints, each watchpoint watching 1, 2, 4 or 8 bytes. The Valgrind gdbserver does not have any limitation on the number of simulated hardware watchpoints. It also has no limitation on the length of the memory zone being watched. Using GDB version 7.4 or later allow full use of the flexibility of the Valgrind gdbserver's simulated hardware watchpoints. Previous GDB versions do not understand that Valgrind gdbserver watchpoints have no length limit.

    Memcheck implements hardware watchpoint simulation by marking the watched address ranges as being unaddressable. When a hardware watchpoint is removed, the range is marked as addressable and defined. Hardware watchpoint simulation of addressable-but-undefined memory zones works properly, but has the undesirable side effect of marking the zone as defined when the watchpoint is removed.

    Write watchpoints might not be reported at the exact instruction that writes the monitored area, unless option --vgdb=full is given. Read watchpoints will always be reported at the exact instruction reading the watched memory.

    It is better to avoid using hardware watchpoint of not addressable (yet) memory: in such a case, GDB will fall back to extremely slow software watchpoints. Also, if you do not quit GDB between two debugging sessions, the hardware watchpoints of the previous sessions will be re-inserted as software watchpoints if the watched memory zone is not addressable at program startup.

  • Stepping inside shared libraries on ARM.

    For unknown reasons, stepping inside shared libraries on ARM may fail. A workaround is to use the ldd command to find the list of shared libraries and their loading address and inform GDB of the loading address using the GDB command "add-symbol-file". Example:

    (gdb) shell ldd ./prog => /lib/ (0x4002c000)
    	/lib/ (0x40000000)
    (gdb) add-symbol-file /lib/ 0x4002c000
    add symbol table from file "/lib/" at
    	.text_addr = 0x4002c000
    (y or n) y
    Reading symbols from /lib/ debugging symbols found)...done.

  • GDB version needed for ARM and PPC32/64.

    You must use a GDB version which is able to read XML target description sent by a gdbserver. This is the standard setup if GDB was configured and built with the "expat" library. If your GDB was not configured with XML support, it will report an error message when using the "target" command. Debugging will not work because GDB will then not be able to fetch the registers from the Valgrind gdbserver. For ARM programs using the Thumb instruction set, you must use a GDB version of 7.1 or later, as earlier versions have problems with next/step/breakpoints in Thumb code.

  • Stack unwinding on PPC32/PPC64.

    On PPC32/PPC64, stack unwinding for leaf functions (functions that do not call any other functions) works properly only when you give the option --vex-iropt-register-updates=allregs-at-mem-access or --vex-iropt-register-updates=allregs-at-each-insn. You must also pass this option in order to get a precise stack when a signal is trapped by GDB.

  • Breakpoints encountered multiple times.

    Some instructions (e.g. x86 "rep movsb") are translated by Valgrind using a loop. If a breakpoint is placed on such an instruction, the breakpoint will be encountered multiple times -- once for each step of the "implicit" loop implementing the instruction.

  • Execution of Inferior function calls by the Valgrind gdbserver.

    GDB allows the user to "call" functions inside the process being debugged. Such calls are named "inferior calls" in the GDB terminology. A typical use of an inferior call is to execute a function that prints a human-readable version of a complex data structure. To make an inferior call, use the GDB "print" command followed by the function to call and its arguments. As an example, the following GDB command causes an inferior call to the libc "printf" function to be executed by the process being debugged:

    (gdb) p printf("process being debugged has pid %d\n", getpid())
    $5 = 36

    The Valgrind gdbserver supports inferior function calls. Whilst an inferior call is running, the Valgrind tool will report errors as usual. If you do not want to have such errors stop the execution of the inferior call, you can use v.set vgdb-error to set a big value before the call, then manually reset it to its original value when the call is complete.

    To execute inferior calls, GDB changes registers such as the program counter, and then continues the execution of the program. In a multithreaded program, all threads are continued, not just the thread instructed to make the inferior call. If another thread reports an error or encounters a breakpoint, the evaluation of the inferior call is abandoned.

    Note that inferior function calls are a powerful GDB feature, but should be used with caution. For example, if the program being debugged is stopped inside the function "printf", forcing a recursive call to printf via an inferior call will very probably create problems. The Valgrind tool might also add another level of complexity to inferior calls, e.g. by reporting tool errors during the Inferior call or due to the instrumentation done.

  • Connecting to or interrupting a Valgrind process blocked in a system call.

    Connecting to or interrupting a Valgrind process blocked in a system call requires the "ptrace" system call to be usable. This may be disabled in your kernel for security reasons.

    When running your program, Valgrind's scheduler periodically checks whether there is any work to be handled by the gdbserver. Unfortunately this check is only done if at least one thread of the process is runnable. If all the threads of the process are blocked in a system call, then the checks do not happen, and the Valgrind scheduler will not invoke the gdbserver. In such a case, the vgdb relay application will "force" the gdbserver to be invoked, without the intervention of the Valgrind scheduler.

    Such forced invocation of the Valgrind gdbserver is implemented by vgdb using ptrace system calls. On a properly implemented kernel, the ptrace calls done by vgdb will not influence the behaviour of the program running under Valgrind. If however they do, giving the option --max-invoke-ms=0 to the vgdb relay application will disable the usage of ptrace calls. The consequence of disabling ptrace usage in vgdb is that a Valgrind process blocked in a system call cannot be woken up or interrupted from GDB until it executes enough basic blocks to let the Valgrind scheduler's normal checking take effect.

    When ptrace is disabled in vgdb, you can increase the responsiveness of the Valgrind gdbserver to commands or interrupts by giving a lower value to the option --vgdb-poll. If your application is blocked in system calls most of the time, using a very low value for --vgdb-poll will cause a the gdbserver to be invoked sooner. The gdbserver polling done by Valgrind's scheduler is very efficient, so the increased polling frequency should not cause significant performance degradation.

    When ptrace is disabled in vgdb, a query packet sent by GDB may take significant time to be handled by the Valgrind gdbserver. In such cases, GDB might encounter a protocol timeout. To avoid this, you can increase the value of the timeout by using the GDB command "set remotetimeout".

    Ubuntu versions 10.10 and later may restrict the scope of ptrace to the children of the process calling ptrace. As the Valgrind process is not a child of vgdb, such restricted scoping causes the ptrace calls to fail. To avoid that, Valgrind will automatically allow all processes belonging to the same userid to "ptrace" a Valgrind process, by using PR_SET_PTRACER.

    Unblocking processes blocked in system calls is not currently implemented on Mac OS X and Android. So you cannot connect to or interrupt a process blocked in a system call on Mac OS X or Android.

    Unblocking processes blocked in system calls is implemented via agent thread on Solaris. This is quite a different approach than using ptrace on Linux, but leads to equivalent result - Valgrind gdbserver is invoked. Note that agent thread is a Solaris OS feature and cannot be disabled.

  • Changing register values.

    The Valgrind gdbserver will only modify the values of the thread's registers when the thread is in status Runnable or Yielding. In other states (typically, WaitSys), attempts to change register values will fail. Amongst other things, this means that inferior calls are not executed for a thread which is in a system call, since the Valgrind gdbserver does not implement system call restart.

  • Unsupported GDB functionality.

    GDB provides a lot of debugging functionality and not all of it is supported. Specifically, the following are not supported: reversible debugging and tracepoints.

  • Unknown limitations or problems.

    The combination of GDB, Valgrind and the Valgrind gdbserver probably has unknown other limitations and problems. If you encounter strange or unexpected behaviour, feel free to report a bug. But first please verify that the limitation or problem is not inherent to GDB or the GDB remote protocol. You may be able to do so by checking the behaviour when using standard gdbserver part of the GDB package.

3.2.10. vgdb command line options

Usage: vgdb [OPTION]... [[-c] COMMAND]...

vgdb ("Valgrind to GDB") is a small program that is used as an intermediary between Valgrind and GDB or a shell. It has three usage modes:

  1. As a standalone utility, it is used from a shell command line to send monitor commands to a process running under Valgrind. For this usage, the vgdb OPTION(s) must be followed by the monitor command to send. To send more than one command, separate them with the -c option.

  2. In combination with GDB "target remote |" command, it is used as the relay application between GDB and the Valgrind gdbserver. For this usage, only OPTION(s) can be given, but no COMMAND can be given.

  3. In the --multi mode, vgdb uses the extended remote protocol to communicate with GDB. This allows you to view output from both valgrind and GDB in the GDB session. This is accomplished via the "target extended-remote | vgdb --multi". In this mode you no longer need to start valgrind yourself. vgdb will start up valgrind when gdb tells it to run a new program. For this usage, the vgdb OPTIONS(s) can also include --valgrind and --vargs to describe how valgrind should be started.

vgdb accepts the following options:


Specifies the PID of the process to which vgdb must connect to. This option is useful in case more than one Valgrind gdbserver can be connected to. If the --pid argument is not given and multiple Valgrind gdbserver processes are running, vgdb will report the list of such processes and then exit.


Must be given to both Valgrind and vgdb if you want to change the default prefix for the FIFOs (named pipes) used for communication between the Valgrind gdbserver and vgdb.


Instructs vgdb to search for available Valgrind gdbservers for the specified number of seconds. This makes it possible start a vgdb process before starting the Valgrind gdbserver with which you intend the vgdb to communicate. This option is useful when used in conjunction with a --vgdb-prefix that is unique to the process you want to wait for. Also, if you use the --wait argument in the GDB "target remote" command, you must set the GDB remotetimeout to a value bigger than the --wait argument value. See option --max-invoke-ms (just below) for an example of setting the remotetimeout value.


Gives the number of milliseconds after which vgdb will force the invocation of gdbserver embedded in Valgrind. The default value is 100 milliseconds. A value of 0 disables forced invocation. The forced invocation is used when vgdb is connected to a Valgrind gdbserver, and the Valgrind process has all its threads blocked in a system call.

If you specify a large value, you might need to increase the GDB "remotetimeout" value from its default value of 2 seconds. You should ensure that the timeout (in seconds) is bigger than the --max-invoke-ms value. For example, for --max-invoke-ms=5000, the following GDB command is suitable:

    (gdb) set remotetimeout 6


Instructs a standalone vgdb to exit if the Valgrind gdbserver it is connected to does not process a command in the specified number of seconds. The default value is to never time out.


Instructs vgdb to use tcp/ip and listen for GDB on the specified port nr rather than to use a pipe to communicate with GDB. Using tcp/ip allows to have GDB running on one computer and debugging a Valgrind process running on another target computer. Example:

# On the target computer, start your program under valgrind using
valgrind --vgdb-error=0 prog
# and then in another shell, run:
vgdb --port=1234

On the computer which hosts GDB, execute the command:

gdb prog
(gdb) target remote targetip:1234

where targetip is the ip address or hostname of the target computer.


Makes vgdb start in extended-remote mode and to wait for gdb to tell us what to run.


The path to valgrind to use, in extended-remote mode. If not specified, the system valgrind will be launched.


Options to run valgrind with, in extended-remote mode. For example -q. Everything following --vargs will be provided as arguments to valgrind as is.


To give more than one command to a standalone vgdb, separate the commands by an option -c. Example:

vgdb v.set log_output -c leak_check any

Instructs a standalone vgdb to report the list of the Valgrind gdbserver processes running and then exit.


Instructs vgdb to add timestamps to vgdb information messages.


Instructs a standalone vgdb to show the state of the shared memory used by the Valgrind gdbserver. vgdb will exit after having shown the Valgrind gdbserver shared memory state.


Instructs vgdb to produce debugging output. Give multiple -d args to increase the verbosity. When giving -d to a relay vgdb, you better redirect the standard error (stderr) of vgdb to a file to avoid interaction between GDB and vgdb debugging output.

3.2.11. Valgrind monitor commands

This section describes the Valgrind monitor commands, available regardless of the Valgrind tool selected. For the tool specific commands, refer to Memcheck Monitor Commands, Helgrind Monitor Commands, Callgrind Monitor Commands and Massif Monitor Commands.

The monitor commands can be sent either from a shell command line, by using a standalone vgdb, or from GDB, by using GDB's "monitor" command (see Monitor command handling by the Valgrind gdbserver) or by GDB's "valgrind" front end commands (see GDB front end commands for Valgrind gdbserver monitor commands). They can also be launched by the client program, using the VALGRIND_MONITOR_COMMAND client request.

Whatever the way the monitor command is launched, it will behave the same way. However, using the GDB's valgrind front end commands allows to benefit from the GDB infrastructure, such as expression evaluation. When relevant, the description of a monitor command below describes the additional flexibility provided by the GDB valgrind front end command. To launch a valgrind monitor command via its GDB front end command, instead of prefixing the command with "monitor", you must use the GDB valgrind command (or the shorter aliases vg or v). In GDB, you can use help valgrind to get help about the valgrind front end monitor commands and you can use apropos valgrind to get all the commands mentionning the word "valgrind" in their name or on-line help.

  • help [debug] instructs Valgrind's gdbserver to give the list of all monitor commands of the Valgrind core and of the tool. The optional "debug" argument tells to also give help for the monitor commands aimed at Valgrind internals debugging.

    Note that this monitor command produces the help information as provided by valgrind gdbserver. The GDB help given e.g. by help valgrind or help valgrind provides the help of the GDB front end command for the equivalent valgrind gdbserver monitor command. This "GDB help" describes the additional flexibility provided by the GDB front end command.

  • all_errors shows all errors found so far.

  • last_error shows the last error found.

  • location <addr> outputs information about the location <addr>. Possibly, the following are described: global variables, local (stack) variables, allocated or freed blocks, ... The information produced depends on the tool and on the options given to valgrind. Some tools (e.g. memcheck and helgrind) produce more detailed information for client heap blocks. For example, these tools show the stacktrace where the heap block was allocated. If a tool does not replace the malloc/free/... functions, then client heap blocks will not be described. Use the option --read-var-info=yes to obtain more detailed information about global or local (stack) variables.

    (gdb) monitor location 0x1130a0
     Location 0x1130a0 is 0 bytes inside global var "mx"
     declared at tc19_shadowmem.c:19
    (gdb) mo loc 0x1ffefffe10
     Location 0x1ffefffe10 is 0 bytes inside info.child,
     declared at tc19_shadowmem.c:139, in frame #1 of thread 1

    The GDB valgrind front end command valgrind location ADDR accepts any address expression for its ADDR argument. In the below examples, mx is a global struct and info is a pointer to a structure. Instead of having to print the addresses of the structure and printing the pointer variable, you can directly use the expressions in the GDB valgrind front end command argument.

    (gdb) valgrind location &mx
     Location 0x1130a0 is 0 bytes inside global var "mx"
     declared at tc19_shadowmem.c:19
    (gdb) v v.i lo info
     Location 0x1ffefffe10 is 0 bytes inside info.child,
     declared at tc19_shadowmem.c:139, in frame #1 of thread 1
  • n_errs_found [msg] shows the number of errors found so far, the nr of errors shown so far and the current value of the --vgdb-error argument. The optional msg (one or more words) is appended. Typically, this can be used to insert markers in a process output file between several tests executed in sequence by a process started only once. This allows to associate the errors reported by Valgrind with the specific test that produced these errors.

  • open_fds shows the list of open file descriptors and details related to the file descriptor. This only works if --track-fds=yes or --track-fds=all (to include stdin, stdout and stderr) was given at Valgrind startup.

  • v.clo <clo_option>... changes one or more dynamic command line options. If no clo_option is given, lists the dynamically changeable options. See Dynamically Change Options. The below shows example of changing the value of --vgdb-error using directly the valgrind monitor command, using the equivalent GDB valgrind front end command. It also shows how a more flexible setting can be done using the GDB eval command.

    (gdb) mo v.clo --vgdb-error=10
    ==2808839== Handling new value --vgdb-error=10 for option --vgdb-error
    (gdb) v v.clo --vgdb-error=11
    ==2808839== Handling new value --vgdb-error=11 for option --vgdb-error
    (gdb) set var $nnn = 15
    (gdb) eval "v v.clo --vgdb-error=%d", $nnn + 1
    ==2808839== Handling new value --vgdb-error=16 for option --vgdb-error
  • v.set {gdb_output | log_output | mixed_output} allows redirection of the Valgrind output (e.g. the errors detected by the tool). The default setting is mixed_output.

    With mixed_output, the Valgrind output goes to the Valgrind log (typically stderr) while the output of the interactive GDB monitor commands (e.g. last_error) is displayed by GDB.

    With gdb_output, both the Valgrind output and the interactive GDB monitor commands output are displayed by GDB.

    With log_output, both the Valgrind output and the interactive GDB monitor commands output go to the Valgrind log.

  • v.wait [ms (default 0)] instructs Valgrind gdbserver to sleep "ms" milli-seconds and then continue. When sent from a standalone vgdb, if this is the last command, the Valgrind process will continue the execution of the guest process. The typical usage of this is to use vgdb to send a "no-op" command to a Valgrind gdbserver so as to continue the execution of the guest process.

    The GDB valgrind front end command valgrind v.wait MS accepts any integer expression for its MS argument, while the monitor command accepts only integer numbers.

  • v.kill requests the gdbserver to kill the process. This can be used from a standalone vgdb to properly kill a Valgrind process which is currently expecting a vgdb connection.

  • v.set vgdb-error <errornr> dynamically changes the value of the --vgdb-error valgrind command line argument. A typical usage of this is to start with --vgdb-error=0 on the command line, then set a few breakpoints, set the vgdb-error value to a huge value and continue execution. Note that you can also change this value using e.g. v.clo --vgdb-error=12

    The GDB valgrind front end command valgrind v.set vgdb-error NUM accepts any integer expression for its ERRORNR argument, while the monitor command accepts only integer numbers.

  • v.set merge-recursive-frames <num> dynamically changes the value of the --merge-recursive-frames valgrind command line argument. Note that you can also change this value using e.g. v.clo --merge-recursive-frames=5

    The GDB valgrind front end command valgrind v.set merge-recursive-frames NUM accepts any integer expression for its NUM argument, while the monitor command accepts only integer numbers.

  • xtmemory [<filename> default xtmemory.kcg.%p.%n] requests the tool (Memcheck, Massif, Helgrind) to produce an xtree heap memory report. See Execution Trees for a detailed explanation about execution trees.

The following Valgrind monitor commands are useful for investigating the behaviour of Valgrind or its gdbserver in case of problems or bugs.

  • expensive_sanity_check_general executes various sanity checks. In particular, the sanity of the Valgrind heap is verified. This can be useful if you suspect that your program and/or Valgrind has a bug corrupting Valgrind data structure. It can also be used when a Valgrind tool reports a client error to the connected GDB, in order to verify the sanity of Valgrind before continuing the execution.

  • gdbserver_status shows the gdbserver status. In case of problems (e.g. of communications), this shows the values of some relevant Valgrind gdbserver internal variables. Note that the variables related to breakpoints and watchpoints (e.g. the number of breakpoint addresses and the number of watchpoints) will be zero, as GDB by default removes all watchpoints and breakpoints when execution stops, and re-inserts them when resuming the execution of the debugged process. You can change this GDB behaviour by using the GDB command set breakpoint always-inserted on.

  • memory [aspacemgr] shows the statistics of Valgrind's internal heap management. If option --profile-heap=yes was given, detailed statistics will be output. With the optional argument aspacemgr. the segment list maintained by valgrind address space manager will be output. Note that this list of segments is always output on the Valgrind log.

  • exectxt shows information about the "executable contexts" (i.e. the stack traces) recorded by Valgrind. For some programs, Valgrind can record a very high number of such stack traces, causing a high memory usage. This monitor command shows all the recorded stack traces, followed by some statistics. This can be used to analyse the reason for having a big number of stack traces. Typically, you will use this command if memory has shown significant memory usage by the "exectxt" arena.

  • scheduler shows various information about threads. First, it outputs the host stack trace, i.e. the Valgrind code being executed. Then, for each thread, it outputs the thread state. For non terminated threads, the state is followed by the guest (client) stack trace. Finally, for each active thread or for each terminated thread slot not yet re-used, it shows the max usage of the valgrind stack.

    Showing the client stack traces allows to compare the stack traces produced by the Valgrind unwinder with the stack traces produced by GDB+Valgrind gdbserver. Pay attention that GDB and Valgrind scheduler status have their own thread numbering scheme. To make the link between the GDB thread number and the corresponding Valgrind scheduler thread number, use the GDB command info threads. The output of this command shows the GDB thread number and the valgrind 'tid'. The 'tid' is the thread number output by scheduler. When using the callgrind tool, the callgrind monitor command status outputs internal callgrind information about the stack/call graph it maintains.

  • stats shows various valgrind core and tool statistics. With this, Valgrind and tool statistics can be examined while running, even without option --stats=yes.

  • unwind <addr> [<len>] shows the CFI unwind debug info for the address range [addr, addr+len-1]. The default value of <len> is 1, giving the unwind information for the instruction at <addr>.

    The GDB valgrind front end command valgrind unwind ADDR [LEN] accepts any address expression for its first ADDR argument, such as $pc. The second optional argument is any integer expression. Note that these 2 arguments must be separated by a space.

  • v.set debuglog <intvalue> sets the Valgrind debug log level to <intvalue>. This allows to dynamically change the log level of Valgrind e.g. when a problem is detected.

    The GDB valgrind front end command valgrind v.set debuglog LEVEL accepts any address expression for its LEVEL argument.

  • v.set hostvisibility [yes*|no] The value "yes" indicates to gdbserver that GDB can look at the Valgrind 'host' (internal) status/memory. "no" disables this access. When hostvisibility is activated, GDB can e.g. look at Valgrind global variables. As an example, to examine a Valgrind global variable of the memcheck tool on an x86, do the following setup:

    (gdb) monitor v.set hostvisibility yes
    Enabled access to Valgrind memory/status by GDB
    If not yet done, tell GDB which valgrind file(s) to use, typically:
    add-symbol-file /home/philippe/valgrind/git/improve/Inst/libexec/valgrind/memcheck-amd64-linux 0x58001000
    (gdb) add-symbol-file /home/philippe/valgrind/git/improve/Inst/libexec/valgrind/memcheck-amd64-linux 0x58001000
    add symbol table from file "/home/philippe/valgrind/git/improve/Inst/libexec/valgrind/memcheck-amd64-linux" at
    	.text_addr = 0x58001000
    (y or n) y
    Reading symbols from /home/philippe/valgrind/git/improve/Inst/libexec/valgrind/memcheck-amd64-linux...

    After that, variables defined in memcheck-x86-linux can be accessed, e.g.

    (gdb) p /x vgPlain_threads[1].os_state
    $3 = {lwpid = 0x4688, threadgroup = 0x4688, parent = 0x0, 
      valgrind_stack_base = 0x62e78000, valgrind_stack_init_SP = 0x62f79fe0, 
      exitcode = 0x0, fatalsig = 0x0}
    (gdb) p vex_control
    $5 = {iropt_verbosity = 0, iropt_level = 2, 
      iropt_register_updates = VexRegUpdUnwindregsAtMemAccess, 
      iropt_unroll_thresh = 120, guest_max_insns = 60, guest_chase_thresh = 10}
  • v.translate <address> [<traceflags>] shows the translation of the block containing address with the given trace flags. The traceflags value bit patterns have similar meaning to Valgrind's --trace-flags option. It can be given in hexadecimal (e.g. 0x20) or decimal (e.g. 32) or in binary 1s and 0s bit (e.g. 0b00100000). The default value of the traceflags is 0b00100000, corresponding to "show after instrumentation". The output of this command always goes to the Valgrind log.

    The additional bit flag 0b100000000 (bit 8) has no equivalent in the --trace-flags option. It enables tracing of the gdbserver specific instrumentation. Note that this bit 8 can only enable the addition of gdbserver instrumentation in the trace. Setting it to 0 will not disable the tracing of the gdbserver instrumentation if it is active for some other reason, for example because there is a breakpoint at this address or because gdbserver is in single stepping mode.

    The GDB valgrind front end command valgrind v.translate ADDR [TRACEFLAG] accepts any address expression for its first ADDR argument, such as $pc. The second optional argument is any integer expression. Note that these 2 arguments must be separated by a space.

3.3. Function wrapping

Valgrind allows calls to some specified functions to be intercepted and rerouted to a different, user-supplied function. This can do whatever it likes, typically examining the arguments, calling onwards to the original, and possibly examining the result. Any number of functions may be wrapped.

Function wrapping is useful for instrumenting an API in some way. For example, Helgrind wraps functions in the POSIX pthreads API so it can know about thread status changes, and the core is able to wrap functions in the MPI (message-passing) API so it can know of memory status changes associated with message arrival/departure. Such information is usually passed to Valgrind by using client requests in the wrapper functions, although the exact mechanism may vary.

3.3.1. A Simple Example

Supposing we want to wrap some function

int foo ( int x, int y ) { return x + y; }

A wrapper is a function of identical type, but with a special name which identifies it as the wrapper for foo. Wrappers need to include supporting macros from valgrind.h. Here is a simple wrapper which prints the arguments and return value:

#include <stdio.h>
#include "valgrind.h"
int I_WRAP_SONAME_FNNAME_ZU(NONE,foo)( int x, int y )
   int    result;
   OrigFn fn;
   printf("foo's wrapper: args %d %d\n", x, y);
   CALL_FN_W_WW(result, fn, x,y);
   printf("foo's wrapper: result %d\n", result);
   return result;

To become active, the wrapper merely needs to be present in a text section somewhere in the same process' address space as the function it wraps, and for its ELF symbol name to be visible to Valgrind. In practice, this means either compiling to a .o and linking it in, or compiling to a .so and LD_PRELOADing it in. The latter is more convenient in that it doesn't require relinking.

All wrappers have approximately the above form. There are three crucial macros:

I_WRAP_SONAME_FNNAME_ZU: this generates the real name of the wrapper. This is an encoded name which Valgrind notices when reading symbol table information. What it says is: I am the wrapper for any function named foo which is found in an ELF shared object with an empty ("NONE") soname field. The specification mechanism is powerful in that wildcards are allowed for both sonames and function names. The details are discussed below.

VALGRIND_GET_ORIG_FN: once in the wrapper, the first priority is to get hold of the address of the original (and any other supporting information needed). This is stored in a value of opaque type OrigFn. The information is acquired using VALGRIND_GET_ORIG_FN. It is crucial to make this macro call before calling any other wrapped function in the same thread.

CALL_FN_W_WW: eventually we will want to call the function being wrapped. Calling it directly does not work, since that just gets us back to the wrapper and leads to an infinite loop. Instead, the result lvalue, OrigFn and arguments are handed to one of a family of macros of the form CALL_FN_*. These cause Valgrind to call the original and avoid recursion back to the wrapper.

3.3.2. Wrapping Specifications

This scheme has the advantage of being self-contained. A library of wrappers can be compiled to object code in the normal way, and does not rely on an external script telling Valgrind which wrappers pertain to which originals.

Each wrapper has a name which, in the most general case says: I am the wrapper for any function whose name matches FNPATT and whose ELF "soname" matches SOPATT. Both FNPATT and SOPATT may contain wildcards (asterisks) and other characters (spaces, dots, @, etc) which are not generally regarded as valid C identifier names.

This flexibility is needed to write robust wrappers for POSIX pthread functions, where typically we are not completely sure of either the function name or the soname, or alternatively we want to wrap a whole set of functions at once.

For example, pthread_create in GNU libpthread is usually a versioned symbol - one whose name ends in, eg, @GLIBC_2.3. Hence we are not sure what its real name is. We also want to cover any soname of the form*. So the header of the wrapper will be

int I_WRAP_SONAME_FNNAME_ZZ(libpthreadZdsoZd0,pthreadZucreateZAZa)
  ( ... formals ... )
  { ... body ... }

In order to write unusual characters as valid C function names, a Z-encoding scheme is used. Names are written literally, except that a capital Z acts as an escape character, with the following encoding:

     Za   encodes    *
     Zp              +
     Zc              :
     Zd              .
     Zu              _
     Zh              -
     Zs              (space)
     ZA              @
     ZZ              Z
     ZL              (       # only in valgrind 3.3.0 and later
     ZR              )       # only in valgrind 3.3.0 and later

Hence libpthreadZdsoZd0 is an encoding of the soname and pthreadZucreateZAZa is an encoding of the function name pthread_create@*.

The macro I_WRAP_SONAME_FNNAME_ZZ constructs a wrapper name in which both the soname (first component) and function name (second component) are Z-encoded. Encoding the function name can be tiresome and is often unnecessary, so a second macro, I_WRAP_SONAME_FNNAME_ZU, can be used instead. The _ZU variant is also useful for writing wrappers for C++ functions, in which the function name is usually already mangled using some other convention in which Z plays an important role. Having to encode a second time quickly becomes confusing.

Since the function name field may contain wildcards, it can be anything, including just *. The same is true for the soname. However, some ELF objects - specifically, main executables - do not have sonames. Any object lacking a soname is treated as if its soname was NONE, which is why the original example above had a name I_WRAP_SONAME_FNNAME_ZU(NONE,foo).

Note that the soname of an ELF object is not the same as its file name, although it is often similar. You can find the soname of an object using the command readelf -a | grep soname.

3.3.3. Wrapping Semantics

The ability for a wrapper to replace an infinite family of functions is powerful but brings complications in situations where ELF objects appear and disappear (are dlopen'd and dlclose'd) on the fly. Valgrind tries to maintain sensible behaviour in such situations.

For example, suppose a process has dlopened (an ELF object with soname), which contains function1. It starts to use function1 immediately.

After a while it dlopens, which contains a wrapper for function1 in (soname) All subsequent calls to function1 are rerouted to the wrapper.

If is later dlclose'd, calls to function1 are naturally routed back to the original.

Alternatively, if is dlclose'd but remains, then the wrapper exported by becomes inactive, since there is no way to get to it - there is no original to call any more. However, Valgrind remembers that the wrapper is still present. If is eventually dlopen'd again, the wrapper will become active again.

In short, valgrind inspects all code loading/unloading events to ensure that the set of currently active wrappers remains consistent.

A second possible problem is that of conflicting wrappers. It is easily possible to load two or more wrappers, both of which claim to be wrappers for some third function. In such cases Valgrind will complain about conflicting wrappers when the second one appears, and will honour only the first one.

3.3.4. Debugging

Figuring out what's going on given the dynamic nature of wrapping can be difficult. The --trace-redir=yes option makes this possible by showing the complete state of the redirection subsystem after every mmap/munmap event affecting code (text).

There are two central concepts:

  • A "redirection specification" is a binding of a (soname pattern, fnname pattern) pair to a code address. These bindings are created by writing functions with names made with the I_WRAP_SONAME_FNNAME_{ZZ,_ZU} macros.

  • An "active redirection" is a code-address to code-address binding currently in effect.

The state of the wrapping-and-redirection subsystem comprises a set of specifications and a set of active bindings. The specifications are acquired/discarded by watching all mmap/munmap events on code (text) sections. The active binding set is (conceptually) recomputed from the specifications, and all known symbol names, following any change to the specification set.

--trace-redir=yes shows the contents of both sets following any such event.

-v prints a line of text each time an active specification is used for the first time.

Hence for maximum debugging effectiveness you will need to use both options.

One final comment. The function-wrapping facility is closely tied to Valgrind's ability to replace (redirect) specified functions, for example to redirect calls to malloc to its own implementation. Indeed, a replacement function can be regarded as a wrapper function which does not call the original. However, to make the implementation more robust, the two kinds of interception (wrapping vs replacement) are treated differently.

--trace-redir=yes shows specifications and bindings for both replacement and wrapper functions. To differentiate the two, replacement bindings are printed using R-> whereas wraps are printed using W->.

3.3.5. Limitations - control flow

For the most part, the function wrapping implementation is robust. The only important caveat is: in a wrapper, get hold of the OrigFn information using VALGRIND_GET_ORIG_FN before calling any other wrapped function. Once you have the OrigFn, arbitrary calls between, recursion between, and longjumps out of wrappers should work correctly. There is never any interaction between wrapped functions and merely replaced functions (eg malloc), so you can call malloc etc safely from within wrappers.

The above comments are true for {x86,amd64,ppc32,arm,mips32,s390}-linux. On ppc64-linux function wrapping is more fragile due to the (arguably poorly designed) ppc64-linux ABI. This mandates the use of a shadow stack which tracks entries/exits of both wrapper and replacement functions. This gives two limitations: firstly, longjumping out of wrappers will rapidly lead to disaster, since the shadow stack will not get correctly cleared. Secondly, since the shadow stack has finite size, recursion between wrapper/replacement functions is only possible to a limited depth, beyond which Valgrind has to abort the run. This depth is currently 16 calls.

For all platforms ({x86,amd64,ppc32,ppc64,arm,mips32,s390}-linux) all the above comments apply on a per-thread basis. In other words, wrapping is thread-safe: each thread must individually observe the above restrictions, but there is no need for any kind of inter-thread cooperation.

3.3.6. Limitations - original function signatures

As shown in the above example, to call the original you must use a macro of the form CALL_FN_*. For technical reasons it is impossible to create a single macro to deal with all argument types and numbers, so a family of macros covering the most common cases is supplied. In what follows, 'W' denotes a machine-word-typed value (a pointer or a C long), and 'v' denotes C's void type. The currently available macros are:

CALL_FN_v_v    -- call an original of type  void fn ( void )
CALL_FN_W_v    -- call an original of type  long fn ( void )

CALL_FN_v_W    -- call an original of type  void fn ( long )
CALL_FN_W_W    -- call an original of type  long fn ( long )

CALL_FN_v_WW   -- call an original of type  void fn ( long, long )
CALL_FN_W_WW   -- call an original of type  long fn ( long, long )

CALL_FN_v_WWW  -- call an original of type  void fn ( long, long, long )
CALL_FN_W_WWW  -- call an original of type  long fn ( long, long, long )

CALL_FN_W_WWWW -- call an original of type  long fn ( long, long, long, long )
CALL_FN_W_5W   -- call an original of type  long fn ( long, long, long, long, long )
CALL_FN_W_6W   -- call an original of type  long fn ( long, long, long, long, long, long )
and so on, up to 

The set of supported types can be expanded as needed. It is regrettable that this limitation exists. Function wrapping has proven difficult to implement, with a certain apparently unavoidable level of ickiness. After several implementation attempts, the present arrangement appears to be the least-worst tradeoff. At least it works reliably in the presence of dynamic linking and dynamic code loading/unloading.

You should not attempt to wrap a function of one type signature with a wrapper of a different type signature. Such trickery will surely lead to crashes or strange behaviour. This is not a limitation of the function wrapping implementation, merely a reflection of the fact that it gives you sweeping powers to shoot yourself in the foot if you are not careful. Imagine the instant havoc you could wreak by writing a wrapper which matched any function name in any soname - in effect, one which claimed to be a wrapper for all functions in the process.

3.3.7. Examples

In the source tree, memcheck/tests/wrap[1-8].c provide a series of examples, ranging from very simple to quite advanced.

mpi/libmpiwrap.c is an example of wrapping a big, complex API (the MPI-2 interface). This file defines almost 300 different wrappers.

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