MqTap is a small "shim" library that can be interposed between application code using POSIX message queues and the librt.so.1 shared library which provides the mq_* function implementations.
The library is used by setting the LD_PRELOAD environment variable to the full path to the libmq_tap:
export LD_PRELOAD=/path/to/libmq_tap.so
You can then start your application(s) which use POSIX queues and calls to mq_open(), mq_send(), mq_receive(), and mq_receive() will be intercepted to allow for your own processing in addition to the expected processing.
Each mq function in the mq_tap library maintains a static function pointer to the real mq function in the rt library; these pointers are set on the first call by first loading the actual librt.so.1 library via dlopen() and then calling dlsym() to get the address of the desired function in that library. The function pointers are then used to call the actual implementation of the mq function before or after "alternative" processing is performed.
The project makefile builds 32-bit and 64-bit versions of the mq_tap library, as well as 32-bit and 64-bit versions of test code which creates a queue, calls mq_send() and calls mq_receive().
The overall intent of a library like this is to be able to tap into POSIX message queues without modifying the application(s) using them.
In the base implementation of the mq_tap library all data passed to mq_send() and received by mq_receive() will be logged to stdout. Alternatives could include:
- logging the data to files in the mq_send() and mq_receive() functions
- creating a "shadow" message queue on each call to mq_open( O_CREAT) and outputing the same data from each mq_send() to the shadow queue.
- collecting queue usage statistics