# Infinite loop

An infinite loop is a loop whose break condition is never fulfilled\. You should distinguish between infinite loops written purposefully and those created through a programming mistake\.

We will further discuss the second type of infinite loops caused by mistakes in C/C\+\+ programs\.

Here is the first sample of this type of errors\.

```cpp
void CXmlReader::UnsafePutCharsBack(
  const TCHAR* pChars, size_t nNumChars)
{
  if (nNumChars == 0)
    return;
  for (size_t nCharPos = nNumChars - 1;
       nCharPos >= 0;
       --nCharPos)
    UnsafePutCharBack(pChars[nCharPos]);
}
```

The 'nCharPos \>\= 0' condition is always true because the nCharPos variable has the [size\_t](https://pvs-studio.com/en/blog/terms/0044/) type\. This is an unsigned type, which means that the variable is always above or equal to zero\. That's why the loop becomes an infinite one\.



Have a look at one more example of a loop that may become infinite\.

```cpp
size_t n;
unsigned i;
for (i = 0; i != n; ++i)
{
  ...
}
```

The error in this case may reveal itself on the 64\-bit platform if the number of items being processed is bigger than UINT\_MAX \(4294967295\)\. If the 'n' variable is bigger than UINT\_MAX \(4294967295\), the 'i \!\= n' condition will always be true\. An overflow will occur at the next step, and the 'i' variable will be zeroed again\. It will never become equal to the 'n' variable\.

_Note\. The type of [64\-bit errors](https://pvs-studio.com/en/blog/terms/0002/) described here is pretty tricky\. The point is that the compiler sometimes builds a terminable loop while optimizing the code\. As a result, the error becomes an intermittent one and reveals itself depending on compiler settings and other factors\. To learn more about this interesting issue see the article "[A 64\-bit horse that can count](https://pvs-studio.com/en/blog/posts/cpp/a0043/)"\._

These errors are well diagnosed by static analyzers already at the stage of program writing\.

## References

1. Wikipedia\. [Infinite loop](https://en.wikipedia.org/wiki/Infinite_loop)\.
1. [Implicit type conversion to memsize type in an arithmetic expression](https://pvs-studio.com/en/docs/warnings/v104/)\.