Query for the available stack size of a program

AndrewCodeDev · July 20, 2023, 10:37pm

I’m working a lot with stack buffered allocators and they can get very large for my use case. I want to dynamically dispatch to heap memory at comptime if a requested buffer size is large enough to be compromising.

In C++, you have stuff like:

stackavail()

in C, you have this (can be used to find information about it):

#include <sys/resource.h>
int main (void)
{
  struct rlimit limit;

  getrlimit (RLIMIT_STACK, &limit);
  printf ("\nStack Limit = %ld and %ld max\n", limit.rlim_cur, limit.rlim_max);
}

And I also saw this as a proposal: Builtin function to tell you the maximum stack size of a given function · Issue #157 · ziglang/zig · GitHub

Are there any standard ways of querying for stack size at comptime?

Thanks!

squeek502 · July 21, 2023, 10:10am

std.os.getrlimit exists.

It doesn’t seem like it’d be possible to get this info at comptime, especially since setrlimit(.STACK, ...) is called in start.zig and can fail to change the stack size:

github.com

ziglang/zig/blob/c43ee5bb22298eefc3fae919807f5da8f7be70f1/lib/std/start.zig#L448-L452


      
          // The way Linux executables represent stack size is via the PT_GNU_STACK
          // program header. However the kernel does not recognize it; it always gives 8 MiB.
          // Here we look for the stack size in our program headers and use setrlimit
          // to ask for more stack space.
          expandStackSize(phdrs);

github.com

ziglang/zig/blob/c43ee5bb22298eefc3fae919807f5da8f7be70f1/lib/std/start.zig#L465-L477


      
          std.os.setrlimit(.STACK, .{
              .cur = wanted_stack_size,
              .max = wanted_stack_size,
          }) catch {
              // Because we could not increase the stack size to the upper bound,
              // depending on what happens at runtime, a stack overflow may occur.
              // However it would cause a segmentation fault, thanks to stack probing,
              // so we do not have a memory safety issue here.
              // This is intentional silent failure.
              // This logic should be revisited when the following issues are addressed:
              // https://github.com/ziglang/zig/issues/157
              // https://github.com/ziglang/zig/issues/1006
          };

AndrewCodeDev · July 21, 2023, 5:05pm

Interesting - thanks for the info. Either way, that eliminates some considerations I had, so thanks for the help!