Why an executable running in shell output to stdout instead of stderr when crashes?

Question

I wrote a C++ demo

test.cpp

like:

int main()

{

    int num = 1 / 0;

}

then compiled it

$ g++ test.cpp -o test

then run it in shell:

$ ./test 2>error.txt

I expected the error messages to be redirected to error.txt, but they still print on the screen through stdout. Why did that happen?

The output shows as below:

Floating point exception (core dumped)

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Answer 1

Because the error message is not generated by the program. It is generated by the operating system.

Think: the program has died already. How can it generate extra output?

In fact, you'll observe output even if you redirect both stdout and stderr of the program to /dev/null.

If you create a sub-shell and redirect its stderr, you'll see the error message redirected:

( ./test ) 2>error.txt

Why an executable running in shell output to stdout instead of stderr when crashes?

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