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来自 standard library :
Both pointers must be derived from a pointer to the same object. (See below for an example.)
let ptr1 = Box::into_raw(Box::new(0u8));
let ptr2 = Box::into_raw(Box::new(1u8));
let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
// Make ptr2_other an "alias" of ptr2, but derived from ptr1.
let ptr2_other = (ptr1 as *mut u8).wrapping_offset(diff);
assert_eq!(ptr2 as usize, ptr2_other as usize);
// Since ptr2_other and ptr2 are derived from pointers to different
// objects, computing their offset is undefined behavior, even though
// they point to the same address!
unsafe {
let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
}
最佳答案
这与称为“出处”的概念有关,意思是“原产地”。 rust Unsafe Code Guidelines有一个关于 Pointer Provenance 的部分.它是一个非常抽象的规则,但它解释了它在编译期间使用的额外信息,有助于指导明确定义的指针转换。
// Let's assume the two allocations here have base addresses 0x100 and 0x200.
// We write pointer provenance as `@N` where `N` is some kind of ID uniquely
// identifying the allocation.
let raw1 = Box::into_raw(Box::new(13u8));
let raw2 = Box::into_raw(Box::new(42u8));
let raw2_wrong = raw1.wrapping_add(raw2.wrapping_sub(raw1 as usize) as usize);
// These pointers now have the following values:
// raw1 points to address 0x100 and has provenance @1.
// raw2 points to address 0x200 and has provenance @2.
// raw2_wrong points to address 0x200 and has provenance @1.
// In other words, raw2 and raw2_wrong have same *address*...
assert_eq!(raw2 as usize, raw2_wrong as usize);
// ...but it would be UB to dereference raw2_wrong, as it has the wrong *provenance*:
// it points to address 0x200, which is in allocation @2, but the pointer
// has provenance @1.
该指南链接到一篇好文章:
Pointers Are Complicated及其后续
Pointers Are Complicated II进入更多细节并创造了这句话:
Just because two pointers point to the same address, does not mean they are equal and can be used interchangeably.
fn main() {
let ptr1 = Box::into_raw(Box::new(0u8));
let ptr2 = Box::into_raw(Box::new(1u8));
let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
let ptr2_other = (ptr1 as *mut u8).wrapping_offset(diff);
assert_eq!(ptr2 as usize, ptr2_other as usize);
unsafe { println!("{} {} {}", *ptr1, *ptr2, *ptr2_other) };
}
error: Undefined Behavior: memory access failed: pointer must be in-bounds at offset 1200, but is outside bounds of alloc1444 which has size 1
--> src/main.rs:7:49
|
7 | unsafe { println!("{} {} {}", *ptr1, *ptr2, *ptr2_other) };
| ^^^^^^^^^^^ memory access failed: pointer must be in-bounds at offset 1200, but is outside bounds of alloc1444 which has size 1
|
= help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
= help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
关于pointers - 为什么 `offset_from` 使用的指针必须从指向同一对象的指针派生?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/68400492/
来自 standard library : Both pointers must be derived from a pointer to the same object. (See below fo
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