Thread safety of C++ on MicroBlaze

Question

Has any one ever written a multi-threaded C++ application for MicroBlaze? The Xilinx documentation states that:

The standard C library provided with EDK is not built for a multi-threaded environment. STDIO functions like printf(), scanf() and memory management functions like malloc() and free() are common examples of functions that are not thread-safe. When using the C library in a multi-threaded environment, proper mutual exclusion techniques must be used to protect thread unsafe functions.

Additionally, MicroBlaze GCC reports that the thread model is "single."

If I'm using C++ standard library containers, this is surely unsafe, correct?

I'm having a heck of time even getting an answer to this simple question from Xilinx, let alone a way to fix it. It seems that this is a major deficiency of the Xilinx provided build system.

Answer 1

The answer from Xilinx (via email) is below. It makes no mention of multi-threading. It also references a their software tool 8.2i which was released in 2006 (6 years ago!!!). In short, it makes no sense.

Lessons learned:

• Xilinx support is terrible.
• Xilinx software tools are an afterthought for them.
• I can only assume that what is outlined in documentation is correct, specifically that you cannot use dynamic memory allocation in a multi-threaded environment. This means no c++ standard library containers.
• If you're the hardware guy about to select a MicroBlaze soft core processor, check with the firmware guy before you do that. What kind of applications is he expecting to write? Make sure he knows that he can't write a multi-threaded application in C++.

malloc()

Microblaze C library shipped with a small, minimal functionality malloc(). When used, memory could not be freed. Other functions such as calloc, realloc, etc., were not supported. There were also bugs when using both malloc() and routines such as printf, scanf, etc. To fix this, the minimal functionality malloc() has been removed. It has been replaced by original Newlib malloc(). As a result, you should see NO functionality issues. You might see a code size increase of around 4K. Because of the differences in which the new full functionality malloc() requests memory, user programs might need to review their heap size settings. If you see your malloc() calls returning NULL, where they used to work, try increasing your heap size. This change was essential to fix broken functionality.

For the rare cases where you still want the original light-weight, but broken malloc() functionality, the source code (malloc.S) can be included as one of the source files to be compiled to build your application. This will retain the old functionality, code size requirements, and dynamic memory requirements seen prior to EDK 8.2i.

xil_malloc()

MicroBlaze C library shipped with an alternative implementation of dynamic memory allocation called xil_malloc(). This routine has some limitations; it does not allocate memory from the heap, but rather from a fixed 64K buffer. This routine has now been deprecated. Though this routine is still available for linkage, its use is strongly discouraged. Please use malloc(); it is smaller than xil_malloc() and provides better functionality. When using malloc(), make sure to review your heap size settings to satisfy your dynamic memory requirements.

The standalone BSP contains a parameter "need_xil_malloc". This parameter was intended to allow you to write code that contains malloc(), yet wire it to the xil_malloc() implementation. Due to bugs in the implementation of the parameter and due to the deprecation of xil_malloc(), this parameter is deprecated as well.

Xilkernel contains a parameter "use_xil_malloc". This parameter was intended to allow the kernel message queue implementations to use xil_malloc() instead of malloc(). Due to the deprecation of xil_malloc(), this parameter is deprecated as well.

If you still want xil_malloc() source code for legacy reasons, the "xil_malloc.c" and "xil_sbrk.c" files can be downloaded and used.

C++ Applications

Prior to EDK 8.2i C++ applications might exhibit unusual behavior, memory corruption, etc. To fix these issues, include the attached source file (newlib_malloc.c) as part of your application compilation. This will fix the unexplained crashes. This work-around fixes bugs in the malloc() implementation in the MicroBlaze C library. This work-around has been incorporated into the C library starting in EDK 8.2i.

This information also appears to be available at: http://www.xilinx.com/support/answers/23345.html