EXC_BAD_ACCESS - C++ substr 函数

Question

我有一个使用 c++ substr 的函数。在 Linux 和 macOS 上，此函数有时会在程序结束时（从单独的线程调用时）随机崩溃。

函数如下：

bool mkpath( string path )
{
bool bSuccess = false;
int nRC = ::mkdir( path.c_str(), 0775 );
if( nRC == -1 )
{
    switch( errno )
    {
        case ENOENT:
            //parent didn't exist, try to create it
            if( mkpath( path.substr(0, path.find_last_of('/')) ) )
            {
                //Now, try to create again.
                int status = ::mkdir( path.c_str(), 0775 );
                bSuccess = (0 == status || errno == EEXIST);
            }
            else
            {
                bSuccess = false;
            }
            break;
        case EEXIST:
            //Done!
            bSuccess = true;
            break;
        default:
            bSuccess = false;
            break;
    }
}
else
    bSuccess = true;
return bSuccess;
}

lldb回溯如下：

* thread #4, stop reason = EXC_BAD_ACCESS (code=2, address=0x70000097fff8)
* frame #0: 0x00007fff73d2a81a libsystem_malloc.dylib`tiny_malloc_from_free_list + 8
frame #1: 0x00007fff73d2a297 libsystem_malloc.dylib`tiny_malloc_should_clear + 288
frame #2: 0x00007fff73d290c6 libsystem_malloc.dylib`szone_malloc_should_clear + 66
frame #3: 0x00007fff73d27d7a libsystem_malloc.dylib`malloc_zone_malloc + 104
frame #4: 0x00007fff73d27cf5 libsystem_malloc.dylib`malloc + 21
frame #5: 0x00007fff70ea0dea libc++abi.dylib`operator new(unsigned long) + 26
frame #6: 0x00007fff70e73d70 libc++.1.dylib`std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> 
>::basic_string(std::__1::basic_string<char, std::__1::char_traits<char>, 
std::__1::allocator<char> > const&, unsigned long, unsigned long, std::__1::allocator<char> 
const&) + 132
frame #7: 0x0000000103821467 libSampleLibrary.dylib`std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >::substr(unsigned long, unsigned 
long) const + 87
frame #8: 0x0000000103820c77 libSampleLibrary.dylib`mkpath(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >) + 135
frame #9: 0x0000000103820c80 libSampleLibrary.dylib`mkpath(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >) + 144
frame #10: 0x0000000103820c80 libSampleLibrary.dylib`mkpath(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >) + 144
frame #11: 0x0000000103820c80 libSampleLibrary.dylib`mkpath(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >) + 144
frame #12: 0x0000000103820c80 libSampleLibrary.dylib`mkpath(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >) + 144
frame #13: 0x0000000103820c80 libSampleLibrary.dylib`mkpath(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >) + 144
frame #14: 0x0000000103820c80 libSampleLibrary.dylib`mkpath(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >) + 144
frame #15: 0x0000000103820c80 libSampleLibrary.dylib`mkpath(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >) + 144
...

发生这种情况的任何可能原因？正如我们添加的那样，调用它的线程可以从外部取消

pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, NULL);

在函数中调用该函数的第一行。

每当发生这种崩溃时，我都会在回溯中看到以下行超过 5400 次，这非常令人惊讶：

frame #15: 0x0000000103820c80 libSampleLibrary.dylib`mkpath(std::__1::basic_string<char, 
std::__1::char_traits<char>, std::__1::allocator<char> >) + 144

我进一步调试，结果发现一个静态变量（homePath）被破坏（返回垃圾）。 mkpath() 函数从以下函数中获取它的值：

string GetSettingFilePath()
{
    static string homePath = "";
    if(!homePath.empty()){
        
        // sometimes homePath variable returns junk at the program exit
        return homePath;
    }

    struct passwd* pwd = getpwuid(getuid());

    if (pwd)
    {
        homePath = pwd->pw_dir;
    }
    else
    {
        // try the $HOME environment variable
        homePath = getenv("HOME");
    }
    if (homePath.empty())
    {
        homePath = "./";
    }
    return homePath
}

由于 homePath 变量虽然设置正确，但有时在程序退出时返回垃圾，导致无限递归。但是为什么在程序退出时调用函数内部的这个静态变量会返回垃圾。

Answer 1

如果您递归调用mkpath，并且path 中的字符串没有有斜线/。那么你将有无限递归。

递归将是无限的，因为您调用 mkpath 时使用的参数完全相同。

在进行递归调用之前，您应该检查path 是否真的有斜线/。