How to know/limit static stack size in C program with GCC/Clang compiler?

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I'm writing an embedded program that uses a static limited stack area of a known size (in other words, I have X bytes for the stack, and there's no overlaying OS that can allocate more stack on demand for me). I want to avoid errors during runtime, and catch them in build time instead - to have some indication if I mistakenly declared too much variables in some function block that won't fit in the stack during the runtime.

Given that I don't use recursive calls in my program, can I somehow know during compilation time how much space on stack all my local variables will take on the deepest function call path? Or at least know how much space my variables will take in a single block (function) if the compiler is not smart enough to analyze it on all the nested calls?

embedded clang stack gcc c
1 Answers

Given that I don't use recursive calls in my program, can I somehow know during compilation time how much space on stack all my local variables will take on the deepest function call path?

Only if you don't use interrupts. Which is extremely likely in any embedded system. So you'll have to find out stack use with dynamic analysis.

The old school way is to set your whole stack area to a value like 0xAA upon reset from a debugger, then let the program run for a while, make sure to provoke all use-cases. Then halt and inspect how far down you still have 0xAA in memory. It isn't a 100% scientific, fool-proof method but works just fine in practice, in the vast majority of cases.

Other methods involve setting write breakpoints at certain stack locations where you don't expect the program to end up, sort of like a "hardware stack canary". Run the program and ensure that the breakpoint never triggers. If it does, then investigate from there, move the breakpoint further down the memory map to see exactly where.

Another good practice is to always memory map your stack so that it can only overflow into forbidden memory or at least into read-only flash etc - ideally you'd get a hardware exception for stack overflow. You definitely want to avoid the stack overflowing into other RAM sections like .data/.bss, as that will cause severe and extremely subtle error scenarios.

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