3 ms·

Infinite loop without side effects == program stuck and not responding on user input and not outputting anything. That's not something a useful program will eve

Infinite loop without side effects == program stuck and not responding on user input and not outputting anything. That's not something a useful program will ever want to do.

xigoi 6mo ago
The problem is when you accidentally write an infinite loop. In a different language, you run the code, see that it gets stuck and fix it. In C, the compiler may delete the function, making it hard to realize what is happening.
- account42 6mo ago
  This is not a problem that C or C++ programmers actually encounter, ever.
  - ncruces 6mo ago
    I actually encountered it a couple weeks ago. Can you spot the infinite loop in this function? char* strcpy(char* restrict d, const char* restrict s) { stpcpy(d, s); return d; } I'll help. A call to `stpcpy` that ignores the return value can be swapped with a call to the (more likely to be optimized) `strcpy`. Since that's infinite recursion, and there is no forward progress, it's undefined behavior and anything goes. This isn't just theory, it actually broke things in practice for me.
    - rssoconnor 6mo ago
      Naming an externally linked function with the prefix "str" is by itself UB since that prefix is reserved for <string.h>.
      - ncruces 6mo ago
        Right. I am doing a standard library replacement, and forgot I needed to compile freestanding. Oops. My bad. So `str` and `mem` are reserved. But then so are `to` and `is` (by <ctype.h>). Just forget about having a function named `is_valid_user`. And so are `mtx_`, `cnd_`, `thrd_`, `atomic_`, `memory_`... Which is why... everything in C is UB.
- 1718627440 6mo ago
  Note, that this is not true for C.
Certhas 6mo ago
Not true, C++ made it so trivial infinite loops are not UB because it turns out they do have legitimate uses. https://lists.isocpp.org/std-proposals/2020/05/1322.php https://lists.isocpp.org/std-proposals/2020/05/1322.php https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2809r3.html https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p28...
- account42 6mo ago
  Yes, the C++ committee has been making some stupid decisions lately. This is not the only one. Low level platform-specific code that needs to hot spin until an interrupt happens can use assembly for that part which it will need to do for the interrupt handler anyway.
  - TuxSH 6mo ago
    You don't even need to use assembly for this, the wait for interrupt typically involves side effects.
zarzavat 6mo ago
https://9p.io/sources/plan9/sys/src/libc/9sys/abort.c https://9p.io/sources/plan9/sys/src/libc/9sys/abort.c
- account42 6mo ago
  This is already UB without an infinite loop.
deleted 6mo ago
[deleted]

Infinite loop without side effects == program stuck and not responding on user input and not outputting anything. That's not something a useful program will ever want to do.

xigoi 6mo ago

The problem is when you accidentally write an infinite loop. In a different language, you run the code, see that it gets stuck and fix it. In C, the compiler may delete the function, making it hard to realize what is happening.

account42 6mo ago
This is not a problem that C or C++ programmers actually encounter, ever.
- ncruces 6mo ago
  I actually encountered it a couple weeks ago. Can you spot the infinite loop in this function? char* strcpy(char* restrict d, const char* restrict s) { stpcpy(d, s); return d; } I'll help. A call to `stpcpy` that ignores the return value can be swapped with a call to the (more likely to be optimized) `strcpy`. Since that's infinite recursion, and there is no forward progress, it's undefined behavior and anything goes. This isn't just theory, it actually broke things in practice for me.
  - rssoconnor 6mo ago
    Naming an externally linked function with the prefix "str" is by itself UB since that prefix is reserved for <string.h>.
    - ncruces 6mo ago
      Right. I am doing a standard library replacement, and forgot I needed to compile freestanding. Oops. My bad. So `str` and `mem` are reserved. But then so are `to` and `is` (by <ctype.h>). Just forget about having a function named `is_valid_user`. And so are `mtx_`, `cnd_`, `thrd_`, `atomic_`, `memory_`... Which is why... everything in C is UB.
1718627440 6mo ago
Note, that this is not true for C.

Certhas 6mo ago

Not true, C++ made it so trivial infinite loops are not UB because it turns out they do have legitimate uses. https://lists.isocpp.org/std-proposals/2020/05/1322.php https://lists.isocpp.org/std-proposals/2020/05/1322.php https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2809r3.html https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p28...

account42 6mo ago
Yes, the C++ committee has been making some stupid decisions lately. This is not the only one. Low level platform-specific code that needs to hot spin until an interrupt happens can use assembly for that part which it will need to do for the interrupt handler anyway.
- TuxSH 6mo ago
  You don't even need to use assembly for this, the wait for interrupt typically involves side effects.

zarzavat 6mo ago

https://9p.io/sources/plan9/sys/src/libc/9sys/abort.c https://9p.io/sources/plan9/sys/src/libc/9sys/abort.c

account42 6mo ago
This is already UB without an infinite loop.

deleted 6mo ago

[deleted]