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Activity for alx
| Type | On... | Excerpt | Status | Date |
|---|---|---|---|---|
| Comment | Post #288020 |
> since p and q are now indeterminate, nothing can be assumed about them - their values are unspecified.
Nothing can be assumed about the value of p.
> since p used to hold a valid value and since free() can't change p, then p cannot hold a trap representation in this specific case.
Yet we c... (more) |
— | over 1 year ago |
| Comment | Post #286189 |
strncat(3) is a good function. It's just that is has been misused. I thought it was a useless broken-by-design function, until I found by chance a good use of it in groff(1)'s source code (https://git.savannah.gnu.org/cgit/groff.git/tree/src/roff/troff/input.cpp?id=f720813c5f512627a246115a255989ad6... (more) |
— | over 1 year ago |
| Comment | Post #286189 |
@#8176 linux.die.net has quite outdated versions of the man pages; at least I can confirm that of the Linux man-pages, which I maintain. The only official online version of the Linux man-pages is this PDF: https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/book/man-pages-6.04.01.pdf. There's... (more) |
— | over 1 year ago |
| Comment | Post #287083 |
> Someone somewhere might still have the great idea to define a macro called "index" in a public header file...)
And someone might be 4.3BSD and POSIX.1-2001 :D.
It's a function, though, but that already makes it reserved, so I wouldn't call anything `index`, except maybe a structure member.
O... (more) |
— | over 1 year ago |
| Edit | Post #288017 |
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— | over 1 year ago |
| Edit | Post #288017 |
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— | over 1 year ago |
| Edit | Post #288017 |
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— | over 1 year ago |
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— | over 1 year ago |
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— | over 1 year ago |
| Edit | Post #288017 | Initial revision | — | over 1 year ago |
| Question | — |
Can freed pointers undergo lvalue conversion? ``` char p, q; p = malloc(1); free(p); q = p; // lvalue conversion ``` Is the last lvalue conversion (`= p;`) Undefined Behavior or not? We didn't take the address of the local `p`. C11::6.3.2.1/1 contains the following sentence regarding lvalue conversions: > If the lvalue des... (more) |
— | over 1 year ago |
| Comment | Post #287917 |
`int* ptr = original; free(original); if(ptr == original)`: That would actually be different. That one is in fact Undefined Behavior, since you're reading the pointer value and not its representation. To make it unspecified behavior, you's need to call memcmp(3). (more) |
— | almost 2 years ago |
| Comment | Post #287917 |
I guess using a `free(3)`d pointer in the same way, instead of an uninitialized `int32_t` local, would still be *unspecified* behavior, since we can't trap because `memcmp(3)` uses char to read the representation. Am I correct? I used `int32_t` just to simplify the question, but it's true that `int... (more) |
— | almost 2 years ago |
| Comment | Post #287908 |
I chose 0 on purpose to avoid endianness issues, and also to make sure I know the translation from representation to value. I could have also chosen maybe `0x1111` or something similar which isn't affected by endianness, but `0` was just simpler.
I'm almost sure representation 0 has to correspond... (more) |
— | almost 2 years ago |
| Comment | Post #287908 |
memcmp(3) really compares bytes rather than bits. The proof is that whatever the bit endianness is, I won't be able to detect it using memcmp(3). I'd need bitfields for that. (more) |
— | almost 2 years ago |
| Comment | Post #287908 |
I don't think your answer covers the part about the variable containing an invalid value (possibly a trap representation). This answer would be fine for the same question if the question didn't mention reading from an uninitialized variable (which was the main point I was interested in). (more) |
— | almost 2 years ago |
| Edit | Post #287905 |
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— | almost 2 years ago |
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— | almost 2 years ago |
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— | almost 2 years ago |
| Edit | Post #287905 | Initial revision | — | almost 2 years ago |
| Question | — |
memcmp(3) memory containing invalid values What does it mean that we can use memcmp(3) on invalid values? ISO C allows comparing an invalid value through memcmp(3), because it doesn't read the value, but rather its representation, and "reading the representation is never Undefined Behavior" (or so I've been told). The following code inv... (more) |
— | almost 2 years ago |
| Comment | Post #287760 |
If instead of a `struct` containing an array, it would be a `struct` containing many fields (e.g., `struct s2 {int a; int b; int c; int d;};`), would it change your answer in any sense? Let's say I do `struct s2 *s2 = malloc(offsetof(struct s2, c));`, and later I only use fields `a` and `b`. Would ... (more) |
— | almost 2 years ago |
| Edit | Post #287754 |
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— | almost 2 years ago |
| Edit | Post #287754 | Initial revision | — | almost 2 years ago |
| Question | — |
Is partial allocation of an object Undefined Behavior? Is it valid to partly allocate an object, as long as you only use the allocated part of it? ```c #include #include struct s { int i[100]; }; int main(void) { struct s s; s = malloc(50 sizeof(int)); s->i[30] = 7; printf("%d\n", s->i[30]); free(s); } ``` ```sh al... (more) |
— | almost 2 years ago |
| Edit | Post #287748 |
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— | almost 2 years ago |
| Edit | Post #287748 |
Post edited: Use wider types, which will more-likely show atomicity problems |
— | almost 2 years ago |
| Edit | Post #287748 |
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| Edit | Post #287748 | Initial revision | — | almost 2 years ago |
| Question | — |
Strict aliasing rules and function boundaries Let's analyze this code, assuming an architecture where the alignment of `int64t` is the same as that of `double`: ```c void bar(double f, int64t j) { (int64t )f = j; } void foo(void) { int64t i = 1; bar((double ) &i, &i); } ``` - The object is of type `int64t`. - ... (more) |
— | almost 2 years ago |
| Edit | Post #287522 |
Post edited: ffix |
— | about 2 years ago |
| Edit | Post #287522 |
Post edited: reorder params |
— | about 2 years ago |
| Edit | Post #285946 |
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— | about 2 years ago |
| Edit | Post #287522 |
Post edited: revert last change. ***cat() functions have the common meaning of appending after finding a NUL; this one doesn't. |
— | about 2 years ago |
| Edit | Post #287522 |
Post edited: Rename to stpecat() |
— | about 2 years ago |
| Edit | Post #287522 |
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— | about 2 years ago |