In regular Linux the MMU allows the stack to grow dynamically, the MMU just allocates more physical pages. However in uClinux, the correct amount of stack for each thread must be allocated before the thread is created.
Too little stack and your program will corrupt the system in nasty, unpredictable ways. Thread stack gets malloced from the system heap, so an overflow means writes to an arbitrary address just outside the block of memory allocated to the thread. This memory could possibly be in use by other parts of the system, perhaps for a different application. These sorts of bugs can be very difficult to track down.
If you allocate too much stack, then you are wasting memory, a valuable resource on embedded systems. For example I discovered I was allocating far too much stack and wasting Mbytes of memory, especially when multiple threads were running.
The standard approach is to try random values of stack until you find one that works. However I thought it might be a better idea to actually measure the amount of stack used by each thread. Then I could tweak the stack allocation to optimise memory usage and even check for stack overflows at run time.
I have written a small library (called threadstack):
unsigned int threadstack_free(pthread_t *thread);
unsigned int threadstack_used(pthread_t *thread);
Here is a sample run on my Blackfin BF537 STAMP, when a 100k stack was allocated to a thread:
stack used: 1300
stack free: 100620
How it Works
The functions work by examining memory allocated to the stack. The theory is that if the memory is non-zero, then it must have been used by the thread at some time (the entire block of memory used for the stack is initially set to 0 before the thread starts). So the routines search the stack memory for the first non-zero value, and that is declared the “high water mark” – the point where the stack reached it’s maximum.
0xff <- stack top
0xfe <- high water mark
0x00 <- stack bottom
The high water mark will change over time, so after your thread has been running for a while is the best time to measure stack usage.
One weakness with this approach is that if stack allocation is way too low your program may bomb before these routines get a chance to run. However in that case you will at least know there is a problem, and can increase stack to some high number (e.g. Mbytes) to get the program running, before using these functions to determine actual stack requirements.
In my uClinux Asterisk port I have added code to check for stack overflow just before a thread ends:
pthread_t thread = pthread_self();
assert(threadstack_free(&thread) > 10*1024);
This code checks that while the thread was running, the minimum free stack was 10k. The assert will kill the program with an error message and tell me straight away I need more stack. Much nicer than getting an obscure bug in the system due to a stack overflow on a thread. Now the program finds stack overflow bugs for me!
This example above runs from within the actual thread itself, hence the call to pthread_self() to discover the threads handle. You can also call the functions from another thread (e.g. the main thread), for example to periodically meter stack usage.
More information on multi-threaded applications for uClinux
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