What does this error mean: `somefile.c:200: error: the frame size of 1032 bytes is larger than 1024 bytes`?

Question

During a make, I\'m seeing an error along the lines of:

cc1: warnings being treated as errors
somefile.c:200: error: the frame size of 1032 bytes is larger than         


        

Answer 1

-Wframe-larger-than

The warning is generated by -Wframe-larger-than. man gcc of GCC 7 says:

Warn if the size of a function frame is larger than len bytes. The computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than len even if you do not get a warning. In addition, any space allocated via "alloca", variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning.

Minimal example

main.c

int main(void) {
    char s[1024];
    return 0;
}

and:

$ gcc -std=c99 -O0 -Wframe-larger-than=1 main.c
main.c: In function ‘main’:
main.c:4:1: warning: the frame size of 1040 bytes is larger than 1 bytes [-Wframe-larger-than=]
 }
 ^
$ gcc -std=c99 -O0 -Wframe-larger-than=2048 main.c
# No warning.

Why this exists

Operating systems must limit the stack size, otherwise it would grow until it reaches the heap / mmaps and everything would break unpredictably.

Linux sends a signal if the program tries to grow beyond that maximum stack size.

-Wframe-larger-than= is a way to help to prevent the stack from overflowing, by keeping function local variables (which are placed on the stack) small.

There is no compile time guarantee however, since the problem is likely to happen when calling recursive functions, and it all comes down to how many times it recurses.

The solution is to allocate memory with malloc instead of using large arrays as local variables. This ends up using mmap memory.

The key difference between the stack and malloc memory is that the stack must be contiguous, which is simple leads to great memory packing efficiency, while malloc requires complex heuristics. See also:

• What is the function of the push / pop instructions used on registers in x86 assembly?
• https://unix.stackexchange.com/questions/145557/how-does-stack-allocation-work-in-linux/239323#239323