C++ 为什么这会给出一个 Use of uninitialised value of size 8
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Why would this give a Use of uninitialised value of size 8
提问by Yotam
In my code I have a class named membranewith 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_Uwhich is generated in valgrind.
在我的代码中，我有一个名为 的类membrane，一个名为exciteMod()的函数，一个名为 的函数decide()和一个名为 的变量delta_U。的第一行exciteMod()是this->delta_U = 0。在decide()我有一个-delta_U( exp(-this->delta_U))的指数。这会导致错误 使用大小为 8 的未初始化值。什么可能导致这种情况？我没有任何关于delta_Uvalgrind 生成的错误。
Edit:Here are the relevant segment of the code:
编辑：这是代码的相关部分：
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*/
}
}
This is the warning:
这是警告：
==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)
==467==
在 0x300B00D75D 处使用大小为 8 的未初始化值==467==：__ieee754_exp（在 /lib64/libm-2.5.so 中）
==467== by 0x300B022FA3：exp（在 /lib64/libm-2.5.so 中）所以）
==467== by 0x40BB9A：membrane::decide() (membrane.cpp:813)
==467== by 0x40EBB1：membrane::exciteMod() (membrane.cpp:639)
==467== 0x413994：membrane::MCstep(int) (membrane.cpp:486)
==467== by 0x402767: main (main.cpp:14)
Edit:
I should have mentioned that the only place where I call decide()is inside of exciteMod().
编辑：
我应该提到我打电话的唯一地方decide()是exciteMod().
回答by Employed Russian
The most likely cause of uninitialized value is that at least one of b->nextUor b->Uthat you are adding to delta_Uis itself uninitialized. That is:
未初始化值的最可能原因是至少其中之一b->nextU或b->U您添加的delta_U值本身未初始化。那是：
foo = 0;
foo += some_uninitialized_value;
if (foo) // Valgrind warns here
You would like Valgrind to report when foo becomes uninitialized. Unfortunately, doing so produces too many "false positive" warnings to be practical.
您希望 Valgrind 在 foo 未初始化时进行报告。不幸的是，这样做会产生太多的“误报”警告，不切实际。
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_Ubecomes undefined.
您可以插入到循环调用中VALGRIND_CHECK_MEM_IS_DEFINED（请参阅Valgrind 用户手册），Valgrind 会在delta_U未定义时发出确切的信号。
回答by Hemjal
lets say you have t number of values needed to be stored. then use this:
假设您需要存储 t 个值。然后使用这个：
int *p = (int *) malloc(t*(sizeof(int)+1));
memset(p,0,t*(sizeof(int)+1));
