首页 \ 问答 \ 分段故障(Segmentation Fault)

分段故障(Segmentation Fault)

 所以我需要制作一个符合特定算法的特定哈希码函数。 该算法在这个问题的背景下并不重要。 我遇到了一个段错误，我不确定如何修复它。 我在gdb中调试它，发现它来自访问无效的内存地址。  
 这是我的代码：  
int hash_code(const char* str){
   int len = strlen(str);
   char* dst;
   if(len == 0 )
   return 0;
 else{
   strncpy(dst, str, (len - 1));
   return (hash_code(dst) * 65599) + str[len-1];
  }
}
 
 我非常有信心它来自dst，但我不确定如何解决它，不能解决seg故障。 我将使用或初始化dst以避免这种情况？ 

So I need to make a specific hashcode function that meets a specific algorithm. The algorithm isn't really important in the context of this question. I'm getting a seg fault and am not sure how to fix it. I debugged it in gdb and found out it's from accessing an invalid memory address.  
here's my code: 
int hash_code(const char* str){
   int len = strlen(str);
   char* dst;
   if(len == 0 )
   return 0;
 else{
   strncpy(dst, str, (len - 1));
   return (hash_code(dst) * 65599) + str[len-1];
  }
}
 
I'm pretty confident that it's from the dst, but I'm not sure how to work around it, to not get the seg fault. What would I use or initialize dst with to avoid this?

原文：

更新时间：2022-01-22 18:01

最满意答案

 这就是全部. 。 它无法在函数之间传递多态参数，因此f . g 如果f是rank-2多态的话， f . g不起作用。 请注意以下工作：  
(~.) :: ((∀ m. Data m => m String) -> z) -> (x -> (∀ m. Data m => m String))
          -> x -> z
(~.) f g x = f (g x)

getKeyValWith :: SomeConn -> Key -> IO String
getKeyValWith c = useConnection c ~. getKeyVal
 
 理想情况下. 会有类似的类型  
(.) :: ∀ c . ((∀ y . c y => y) -> z) -> x -> ((∀ y . c y => y) -> x)
               -> x -> z
 
 因此涵盖所有特殊情况，如~. 以上。 但这是不可能的 - 它需要推断出在任何给定情况下最弱的约束条件 - 在传统情况下， cy = y~y₀ - 我很确定这一般是不可计算的。  
 （一个有趣的问题是，如果编译器在类型检查之前尽可能多地内联，我们可以得到多少，因为它现在已经用$ 。如果它进行了自动eta扩展，它肯定可以得到useConnection c . getKeyVal工作，但自动eta扩展通常不是一个好主意......）  
 通过将多态参数包装在GADT中来隐藏Rank-2多态性，就像使用SomeConn ，这是通常的解决方法。 

This is all about .. It's unable to pass a polymorphic argument between the functions, hence f . g doesn't work if f is rank-2 polymorphic. Notice the following works: 
(~.) :: ((∀ m. Data m => m String) -> z) -> (x -> (∀ m. Data m => m String))
          -> x -> z
(~.) f g x = f (g x)

getKeyValWith :: SomeConn -> Key -> IO String
getKeyValWith c = useConnection c ~. getKeyVal
 
Ideally, . would have a type like 
(.) :: ∀ c . ((∀ y . c y => y) -> z) -> x -> ((∀ y . c y => y) -> x)
               -> x -> z
 
and thus cover all special cases like ~. above. But this is not possible – it would require inferring the weakest possible constraint c to pick in any given situation – in the traditional case, c y = y~y₀ – and I'm pretty sure that is uncomputable in general. 
(An interesting question is how far we could get if the compiler just inlined . as much as possible before type-checking, as it right now already does with $. If it did automatic eta-expansion, it could certainly get useConnection c . getKeyVal to work, but automatic eta-expansion is in general not a good idea...) 
Hiding the Rank-2 polymorphism by wrapping the polymorphic argument in a GADT, as you did with SomeConn, is the usual workaround.

分段故障(Segmentation Fault)

最满意答案

相关问答

GHC为什么说这两种类型是不同的？(Why does GHC say that these two types are different? How can I get them to be the same?)[2023-04-17]

在存在GADT和类型家族时，GHC无法推断出类型(GHC could not infer types in presence of GADTs and Type Families)[2022-10-19]

为什么伊德里斯没有涉及“减去”类型检查的平等？(Why doesn't equality involving “minus” typecheck in Idris?)[2022-08-30]

GHC不能用GADT和存在类型推导出（a1~a）(GHC cannot deduce (a1 ~ a) with GADTs and existential types)[2024-02-14]

为什么在伊德里斯没有涉及“mod”的平等不是类型检查？(Why doesn't equality involving “mod” not typecheck in Idris?)[2023-10-25]

C ++与类型的多态性(C++ Polymorphism with types)[2021-10-10]

参数值（不是类型）的多态性？(Polymorphism over argument values (not types)?)[2023-05-07]

存在类型(existential types)[2024-03-09]

为什么GHC不能检查这个涉及多态性和存在类型的函数？(Why can't GHC typecheck this function involving polymorphism and existential types?)[2023-05-08]

GHC是否使用存在类型的动态调度？(Does GHC use dynamic dispatch with existential types?)[2023-05-17]

相关文章

最新问答