Solving Segmentation Fault Errors with g++ in OI

Background

I’m used to solving algorithm problems in VSCode, but often when I need to allocate large memory locally (specifically in the main function), the IDE reports Segmentation fault at runtime.

For example, the following code:

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int blocks[1001][1001]{0};
 ...
}

At this point, the array actually only takes up 1001 * 1001 * 8B ≈ 7.64MB of memory, yet it throws a Segmentation fault. But on the OJ, it runs fine and gets AC.

The solution is actually simple: don’t define it inside main—just make it a global variable. But ~~in the spirit of getting to the bottom of things~~, I did some digging into this phenomenon.

Explanation

In C++ programs, global variables are stored on the heap, while local variables are stored on the stack. If you define a large array inside main, the limited stack space can easily cause a stack overflow, resulting in a Segmentation fault.

Then why does it work on OJ?

Actually, you can customize the upper limit of stack-allocated memory by passing arguments to the linker.

Using my VSCode configuration as an example: in the .vscode folder under your project directory, there should be a tasks.json file. Modify the configuration and add the following command to tasks.args:

1	-Wl,--stack,512000000

What this command means:

-Wl: tells the compiler that what follows is passed to the linker
--stack,512000000: sets the stack memory allocation limit to 512000000B

After the change, it should look like this:

{
    "tasks": [
        {
            "type": "cppbuild",
            "label": "C/C++: g++.exe 生成活动文件",
            "command": "C:\\MinGW\\bin\\g++.exe",
            "args": [
                "-fdiagnostics-color=always",
                "-g",
                "${file}",
                "-o",
                "${fileDirname}\\${fileBasenameNoExtension}.exe",
                "-Wl,--stack,512000000"
            ],
            "options": {
                "cwd": "${fileDirname}"
            },
            "problemMatcher": [
                "$gcc"
            ],
            "group": {
                "kind": "build",
                "isDefault": true
            },
            "detail": "调试器生成的任务。"
        }
    ],
    "version": "2.0.0"
}

From then on, even if you allocate large memory inside main, it won’t throw an error.