为什么这会给使用未初始化的大小为8的值 [英] Why would this give a Use of uninitialised value of size 8

问题描述

在我的代码中，我有一个类名为 membrane 的函数名为 exciteMod（） code> decisions（）和一个名为 delta_U 的变量。 exciteMod（）的第一行是 this-> delta_U = 0 。在 decisions（）我有一个指数 -delta_U （ exp（-this-> ; delta_U））。这会导致错误使用大小为8的未初始化值。可能导致这种情况的原因是什么？我没有关于在valgrind中生成的 delta_U 的任何错误。

In my code I have a class named membrane with a function named exciteMod(), a function named decide() and a variable named delta_U. The first line of exciteMod() is this->delta_U = 0. In decide() I have an exponent of -delta_U (exp(-this->delta_U)). which cause an error Use of uninitialised value of size 8. What might cause this? I don't have any error about delta_U which is generated in valgrind.

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下面是代码的相关段：

Here are the relevant segment of the code:

void membrane::exciteMod(){
  this->delta_U = 0;
  /* Do some stuff which does not directly affect this->delta_U*/
  std::tr1::shared_ptr<bead> bit = this->beads.begin();
  while (bit != this->nead.end()){
    std::tr1::shared_ptr<bead> b = *bit++;
    //calculate the doubles U and nextU on b, nothing here gives a warning in valgrind,     anyhow U and nextU on b are always defined
   this->delta_U += (b->nextU - b->U);
  }
  decide();
}

void membrane::decide(){
  double r = P.r.ran3() // the random function from numerical recepies
  double f = - this->delta_U;
  if (r > exp(f)){ //this gives the warning even though delta_U is valid
    /*stuff*/
  }
}

这是警告：

== 467 ==大小为8的未初始化值的使用

== 467 ==在0x300B00D75D：__ieee754_exp（在/lib64/libm-2.5.so中）

== 467 == by 0x300B022FA3：exp（in /lib64/libm-2.5.so）

== 467 == by 0x40BB9A：membrane :: decide（）（membrane.cpp：813）

== 467 == by 0x40EBB1：membrane :: exciteMod（）（membrane.cpp：639）

== 467 == by 0x413994：membrane :: MCstep（int）（membrane.cpp： 486）

== 467 == by 0x402767：main（main.cpp：14）

==467== Use of uninitialised value of size 8
==467== at 0x300B00D75D: __ieee754_exp (in /lib64/libm-2.5.so)
==467== by 0x300B022FA3: exp (in /lib64/libm-2.5.so)
==467== by 0x40BB9A: membrane::decide() (membrane.cpp:813)
==467== by 0x40EBB1: membrane::exciteMod() (membrane.cpp:639)
==467== by 0x413994: membrane::MCstep(int) (membrane.cpp:486)
==467== by 0x402767: main (main.cpp:14)

Edit：

我应该提到我调用 decision（）的唯一地方在 exciteMod（）。

推荐答案

未初始化值的最可能原因是， code> b_> nextU 或 b-> U code>本身未初始化。也就是：

The most likely cause of uninitialized value is that at least one of b->nextU or b->U that you are adding to delta_U is itself uninitialized. That is:

foo = 0;
foo += some_uninitialized_value;
if (foo)  // Valgrind warns here

你希望Valgrind在foo变为未初始化。

You would like Valgrind to report when foo becomes uninitialized. Unfortunately, doing so produces too many "false positive" warnings to be practical.

您可以插入到 VALGRIND_CHECK_MEM_IS_DEFINED 的循环调用中$ c>（请参阅 Valgrind用户手册），Valgrind将发出确切的信号 delta_U 变为未定义的时刻。

You can insert into your loop calls to VALGRIND_CHECK_MEM_IS_DEFINED (see Valgrind user manual), and Valgrind will signal the exact moment when delta_U becomes undefined.

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