User Defined Malloc Replacement

General Programming Concepts: Writing and Debugging Programs

User Defined Malloc Replacement

The AIX memory subsystem (malloc(), calloc(), realloc(), free(), mallopt() and mallinfo()) has been modified to allow users to replace it with one of their own design.

NOTE: Replacement Memory Subsystems written in C++ are not supported due to the use of the libc.a memory subsystem in the C++ library libC.a.

The existing memory subsystem works for both threaded and non-threaded applications. The user defined memory subsystem should be thread-safe so that it works in both threaded and non-threaded processes, however, there are no checks to verify that it is. So, if a non-thread safe memory module is loaded in a threaded application, memory and data may be corrupted.

The user defined memory subsystem 32 and 64 bit objects must be placed in an archive with the 32bit shared object named mem32.o and the 64bit shared object named mem64.o.

The user shared objects must export the following symbols :

__malloc__
__free__
__realloc__
__calloc__
__mallinfo__
__mallopt__
__malloc_init__
__malloc_prefork_lock__
__malloc_postfork_unlock__

The functions are defined as follows:

void *__malloc__(size_t) :: This function is the user equivalent of malloc() as described in the AIX documentation.
void __free__(void *) :: This function is the user equivalent of free() as described in the AIX documentation.
void *__realloc__(void *, size_t) :: This function is the user equivalent of realloc() as described in the AIX documentation.
void *__calloc__(size_t, size_t) :: This function is the user equivalent of calloc() as described in the AIX documentation.
int __mallopt__(int, int) :
: This function is the user equivalent of mallopt() as described in the AIX documentation.
struct mallinfo __mallinfo__() :: This function is the user equivalent of mallinfo() as described in the AIX documentation.

The following interfaces are used by the thread subsystem to manage the user defined memory subsystem in a multi-threaded environment. The are only called if the application and/or the user defined module are bound with libpthreads.a. Even if the the user defined subsystem is not thread-safe and not bound with libpthreads.a these symbols must be defined and exported or the object will not be loaded.

void __malloc_init__(void): Called by the pthread initialization routine. This function is used to initialize the threaded user memory subsystem. In most cases, this includes creating and initializing some form of locking data. Even if the user defined memory subsystem module is bound with libpthreads.a the user defined memory subsystem MUST work before __malloc_init__() is called.
void __malloc_prefork_lock__(void): Called by pthreads when fork() is called. This function is used to insure that the memory subsystem is in a known state before the fork() and stays that way until the fork() has returned. In most cases this includes acquiring the memory subsystem locks.
void __malloc_postfork_unlock__(void): Called by pthreads when fork() is called. This function is used to make the memory subsystem available in the parent and child after a fork(). This should undo the work done by __malloc_prefork_lock__(). In most cases this includes releasing the memory subsystem locks.

All of the functions must be exported from a shared module. There must be separate modules for 32 and 64 bit implementations place in an archive. For example:

mem.exp:

	__malloc__
	__free__
	__realloc__
	__calloc__
	__mallopt__
	__mallinfo__
	__malloc_init__
	__malloc_prefork_lock__
	__malloc_postfork_unlock__

mem_functions32.o:
Contains all of the required 32 bit functions
mem_functions64.o:
Contains all of the required 64 bit functions

Creating 32bit shared object:

	ld -b32 -m -o mem32.o mem_functions32.o \
	-bE:mem.exp \
	-bM:SRE -lpthreads -lc

Creating 64bit shared object:

	ld -b64 -m -o mem64.o mem_functions64.o \
	-bE:mem.exp \
	-bM:SRE -lpthreads -lc

Creating the archive (the shared objects name must be mem32.o for the 32bit object and mem64.o for the 64bit object):
```
	ar -X32_64 -r <archive_name> mem32.o mem64.o
```
NOTE: In the above examples for creating the shared objects -lpthreads is only needed if the object uses pthread functions.

Enablement

The user defined memory subsystem is enable by using the MALLOCTYPE environment variable. The archive containing the user defined memory subsystem is specified by setting MALLOCTYPE to user:<archive_name> where <archive_name> is in the application's libpath or the path is specified in the LIBPATH environment variable.

NOTES:
When a setuid application is exec'd, the LIBPATH environment variable is ignored so the archive must be in the applications libpath.

<archive_name> cannot contain path information.

32/64bit Considerations

If the archive does not contain both the 32 and 64 bit shared objects and the user defined memory subsystem was enabled using the MALLOCTYPE environment variable, there will be problems exec'ing 64bit processes from 32bit applications and 32bit processes from 64bit applications. When an new process is created using exec(), it inherits the environment of the calling application. This means that MALLOCTYPE will be inherited and the new process will attempt to load the user defined memory subsystem. If the archive member does not exist for this type of executable, the load will fail and the new process will exit.

Thread Considerations

All of the provided memory functions must work in a multi-threaded environment. Even if the module is linked with libpthreads.a, at least __malloc__() MUST work before __malloc_init__() is called and pthreads is initialized. This is required because the pthread initialization requires malloc() before __malloc_init__() is called.

All provided memory functions must work in both threaded and non-threaded environments. The __malloc__() function should be able to run to completion without having any dependencies on __malloc_init__() (i.e. __malloc__() should initially assume that __malloc_init__() has NOT yes run.) Once __malloc_init__() has completed, then __malloc__() can rely on any work done by __malloc_init__(). This is required because the pthread initialization uses malloc() before __malloc_init__() is called.

There are two variables provided to keep from calling thread related routines when they are not needed. The first, __multi_threaded, is zero until a thread is created when it becomes non-zero and for that process will not be reset to zero. The other variable, __n_pthreads, is -1 until pthreads has been initialized when it is set to 1. From that point on it is a count of the number of active threads.

Example:
If __malloc__() uses pthread_mutex_lock() the code may look something like this:
if (__multi_threaded)
pthread_mutex_lock(mutexptr);

/* ..... work ....... */

if (__multi_threaded)
pthread_mutex_unlock(mutexptr);

Not only does this keep __malloc__() from executing pthread functions before pthreads is fully initialized, it also speeds up single threaded applications because locking is not done until a second thread is started.

Limitations

Memory subsystems written in C++ are not supported due to initialization and the dependencies of libC.a and the libc.a memory subsystem.

Error messages are not translated due to setlocale() using malloc() to initialize the locales. If malloc() fails then setlocale() cannot finish and the application is still in the POSIX locale so only the default English messages will be displayed.

Existing statically built executables will not be able to use the user defined memory subsystem without recompiling.

Error Reporting

The first time malloc() is called, the 32 or 64 bit object in the archive specified by the MALLOCTYPE environment variable is loaded. If the load fails, a message will be displayed and the application will exit. If the load is successful, an attempt is made to verify that all of the required symbols are present. If any symbols are missing the application will be terminated and the list of missing symbols will displayed.

Related Information

Creating a Shared Library

Chapter 11. Threads Programming Guidelines

The malloc, free, realloc, calloc, mallopt, mallinfo, alloca, or valloc subroutine.