Afaik, that's a "portable" way to trigger a termination with dump collection :) (particularly on Windows if you just std::terminate you won't get a heap dump unless you do special WER registration and register handlers).
*(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
11–20 of 199 posts
Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
#12This statement would be technically legal on its own in x86 real mode if the compiler didn't do null pointer checks. However it would set the divide-by-zero IRQ handler to itself 0000:0000, and when the next division by zero happened, the machine run into UB (likely a reset or halt) because it would jump there, do 4x ADD byte ptr [BX + SI], AL (or ADD byte ptr [EAX], AL) followed by running the remaining interrupt vectors as instructions.
Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
#13Before I watch the full talk for the answer (what I've seen so far, he states there's not a right answer)... I know on some microcontrollers (e.g. Arm) addresses 0x0 and 0x4 are usually used to define the initial stack pointer and entry point and then are never needed again. Later, you probably want to detect the presence of null pointers by checking if &maybeAStruct is either 0 or valid. If you accidentally test the…
This is C code. Coder needs specifity hwo to align the char pointer to HW alignment expectations for memory address to prevent a segmentation fault.
0 is the only pointer value that's guaranteed to be aligned on all possible alignment boundaries, which is why it's so useful as a pointer in C.
Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
#14This statement would be technically legal on its own in x86 real mode if the compiler didn't do null pointer checks. However it would set the divide-by-zero IRQ handler to itself 0000:0000, and when the next division by zero happened, the machine run into UB (likely a reset or halt) because it would jump there, do 4x ADD byte ptr [BX + SI], AL (or ADD byte ptr [EAX], AL) followed by running the remaining interrupt ve…
Not quite. (char *) 0 is the null pointer. The null pointer is not necessarily a binary all-zero. On some compilers in x86, the null pointer intentionally points to something which will cause a crash when written to.
Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
#15Seg fault!
Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
#16This statement would be technically legal on its own in x86 real mode if the compiler didn't do null pointer checks. However it would set the divide-by-zero IRQ handler to itself 0000:0000, and when the next division by zero happened, the machine run into UB (likely a reset or halt) because it would jump there, do 4x ADD byte ptr [BX + SI], AL (or ADD byte ptr [EAX], AL) followed by running the remaining interrupt ve…
Not quite. (char *) 0 is the null pointer. The null pointer is not necessarily a binary all-zero. On some compilers in x86, the null pointer intentionally points to something which will cause a crash when written to.
Find me one contemporary example (ANSI C) with a disassembled screenshot.
This is writing sizeof(char) (== 1 almost everywhere) zero to address zero. It is not using a NULL macro or other predefined symbol.
In the real world, this would generally write a byte to address 0000:0000, leading to UB because it would fuck up the divide-by-zero IV.
PS: I used Borland C++ 3.1, Microsoft C++ 3.x and 4.5x, Watcom, and early GNU.
Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
#17I once used a (never executed) *(char*)0=0; to force GCC to generate async unwind tables for an otherwise leaf function that could throw from an inline asm statement. It was just an experiment never meant for production, but it was quite effective at doing exactly what I needed.
Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
#18Earlier quoted context omitted.
Not quite. (char *) 0 is the null pointer. The null pointer is not necessarily a binary all-zero. On some compilers in x86, the null pointer intentionally points to something which will cause a crash when written to.
Find me one contemporary example (ANSI C) with a disassembled screenshot. This is writing sizeof(char) (== 1 almost everywhere) zero to address zero. It is not using a NULL macro or other predefined symbol. In the real world, this would generally write a byte to address 0000:0000, leading to UB because it would fuck up the divide-by-zero IV. PS: I used Borland C++ 3.1, Microsoft C++ 3.x and 4.5x, Watcom, and early GN…
(void *) 0 is a null pointer constant and is not necessarily an all zeros representation. This has been defined virtually forever.
https://c-faq.com/null/null2.html
https://c-faq.com/null/machexamp.html
Actual ways to do what you want to do are described in
https://c-faq.com/null/accessloc0.html
but technically speaking the pointer with a constant zero assigned to it _is_ a null pointer (which can be implemented as whatever bit pattern), independent of the preprocessor macro.
Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
#19Is it still a programming language, or is it already a riddle (like sudoku)?
Re: *(char*)0 = 0; – What does the C++ programmer intend with this code? [video]
#20Earlier quoted context omitted.
Not quite. (char *) 0 is the null pointer. The null pointer is not necessarily a binary all-zero. On some compilers in x86, the null pointer intentionally points to something which will cause a crash when written to.
Find me one contemporary example (ANSI C) with a disassembled screenshot. This is writing sizeof(char) (== 1 almost everywhere) zero to address zero. It is not using a NULL macro or other predefined symbol. In the real world, this would generally write a byte to address 0000:0000, leading to UB because it would fuck up the divide-by-zero IV. PS: I used Borland C++ 3.1, Microsoft C++ 3.x and 4.5x, Watcom, and early GN…
> sizeof(char) (== 1 almost everywhere)
sizeof char is 1 by definition everywhere.
/pedantic