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;
   VALGRIND_GET_ORIG_FN(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 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 tends to kill the program in short order by stack overflow. 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.

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 libpthread.so*. 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 libpthread.so.0 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 libfoo.so using the command readelf -a libfoo.so | grep soname.

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) object1.so, which contains function1. It starts to use function1 immediately.

After a while it dlopens wrappers.so, which contains a wrapper for function1 in (soname) object1.so. All subsequent calls to function1 are rerouted to the wrapper.

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

Alternatively, if object1.so is dlclose'd but wrappers.so remains, then the wrapper exported by wrapper.so 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 object1.so 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.

Figuring out what's going on given the dynamic nature of wrapping can be difficult. The --trace-redir=yes flag 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:

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 flags.

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->.

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}-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}-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.

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       -- void fn ( long )
CALL_FN_W_W       -- long fn ( long )

CALL_FN_v_WW      -- void fn ( long, long )
CALL_FN_W_WW      -- long fn ( long, long )

CALL_FN_v_WWW     -- void fn ( long, long, long )
CALL_FN_W_WWW     -- long fn ( long, long, long )

CALL_FN_W_WWWW    -- long fn ( long, long, long, long )
CALL_FN_W_5W      -- long fn ( long, long, long, long, long )
CALL_FN_W_6W      -- long fn ( long, long, long, long, long, long )
and so on, up to 
CALL_FN_W_12W

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 ickyness. 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 of course 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.

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

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