Setting stack size with setrlimit fails with gcc

Question

I'm using gcc 10.1 on Ubuntu 18.04. I'm getting segfaults when defining a large stack allocated variable, even though my stack seems to be large enough to accommodate it. Here is a code snippet:

#include <iostream>
#include <array>
#include <sys/resource.h>

using namespace std;

int main() {
  if (struct rlimit rl{1<<28, 1l<<32}; setrlimit(RLIMIT_STACK, &rl))
    cout << "Can not set stack size! errno = " << errno << endl;
  else
    cout << "Stack size: " << rl.rlim_cur/(1<<20) << "MiB to " << rl.rlim_max/(1<<20) << "MiB\n";

  array<int8_t, 100'000'000> a;

  cout << (int)a[42] << endl;
}

which segfaults when compiled with gcc, but runs fine when compiled with clang 11.0.1 and outputs:

Stack size: 256MiB to 4096MiB
0

EDIT

Clang was eliding allocation of a. Here is a better example:

#include <iostream>
#include <array>
#include <sys/resource.h>

using namespace std;

void f() {
  array<int8_t, 100'000'000> a;

  cout << (long)&a[0] << endl;    
}

int main()
{
  if (struct rlimit rl{1<<28, 1l<<32}; setrlimit(RLIMIT_STACK, &rl))
    cout << "Can not set stack size! errno = " << errno << endl;
  else
    cout << "Stack size: " << rl.rlim_cur/(1<<20) << "MiB to " << rl.rlim_max/(1<<20) << "MiB" << endl;

  array<int8_t, 100'000'000> a; // line 21

  cout << (long)&a[0] << endl;  // line 23
  f();
}

which you can find at: https://wandbox.org/permlink/XMaGFMa7heWfI9G8. It runs fine when lines 21 and 23 are commented out, but segfaults otherwise.

Basile Starynkevitch · Accepted Answer · 2020-12-05 08:55:31Z

1

Use proc(5) and pmap(1) and strace(1) to understand the limitations of your computer.

array<int8_t, 100'000'000> a;

requires about 100Mbytes of space on call stack, which is generally limited to a few megabytes (perhaps even by your Linux kernel, but I am not sure)

Try also cat /proc/$$/limits in your terminal. On mine I am getting

 Limit                     Soft Limit           Hard Limit           Units     
 Max cpu time              unlimited            unlimited            seconds   
 Max file size             unlimited            unlimited            bytes     
 Max data size             unlimited            unlimited            bytes     
 Max stack size            8388608              unlimited            bytes

The difference of behavior between compilers might be attributed to various optimizations (e.g. permitted by some C++ standard like n4849). A clever enough compiler is allowed to use just a few words for a inside your function f (e.g. because it would figure out, maybe with some abstract interpretation techniques, that locations a[1024] ... a[99999999] are useless).

If you compile with a recent GCC (e.g. GCC 10), you could invoke it as g++ -O -Wall -Wextra -fanalyzer -Wstack-usage=2048 to get useful warnings. See also this draft report funded by CHARIOT & DECODER projects.

In practice, use dynamic allocation for huge data (e.g. placement new with mmap(2) and smart pointers)

For a real application, consider writing your GCC plugin to get ad-hoc warnings.

Or at least compile your source code foo.cc with g++ -O2 -fverbose-asm -S foo.cc and look inside the generated foo.s and repeat with clang++ : the generated assembler files are different.

edited Dec 5, 2020 at 8:55

answered Dec 5, 2020 at 7:44

Basile Starynkevitch

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8 Comments

Paul Jurczak Over a year ago

My computer has 32GB RAM, so there is no practical limitation for 256MB stack. Are you saying that maximum stack size is limited by Linux and can't be changed by calling setrlimit(RLIMIT_STACK, ...)?

Basile Starynkevitch Over a year ago

IIRC, the maximal stack size might be limited by the Linux kernel. Consider recompiling it.

Paul Jurczak Over a year ago

Thanks. That would explain sparcity of information about setting stack size on Linux. I was bulding my application with 256MB stack on Windows with no problems. I'm surprised that setrlimit is misleading in this case.

Paul Jurczak Over a year ago

I tested the super smart optimizer theory and it doesn't hold. See: wandbox.org/permlink/D1nK7zM5zjPHTZF6, where large a in function f is just fine, but segfaults in main.

Basile Starynkevitch Over a year ago

What makes you believe that your process has a large call stack?

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