弗雷德可以不租吗?

在一个用C++编写并在Windows下用MinGW-w64编译的程序中，我在单独的线程中同时读取了几个文件。由于文件名可能包含非 ASCII 字符，因此我无法使用C++标准库std::ifstream因为它不支持wchar文件名。所以我需要将 C 库与 Win32 API 中的_wfopen一起使用。

但是，我得到了一个非常奇怪的错误，我已经在MCVE中重现了该错误。使用 fread() 读取 n 个字节后，_ftelli64的结果有时不会增加 n，而是少或多几个字节。

通过单线程读取，问题消失了，std::ifstream也

消失了。它的行为就好像 fread 中存在竞争条件，然后是不可重入的。

在下面的示例中，我用fopen替换了_wfopen，因为错误仍然存在。

#include <iostream>
#include <vector>
#include <string>
#include <sstream>
#include <fstream>
#include <thread>
constexpr const int numThreads = 8;
constexpr const int blockSize = 65536+8;
constexpr const int fileBlockCount = 48; //3MB files
void readFile(const std::string & path)
{
std::cout << "Reading file " << path << "n";
std::vector<char> buffer(blockSize);
FILE * f = fopen(path.c_str(), "rb");
for(int i=0;i<fileBlockCount;++i)
{
int64_t pos_before = _ftelli64(f);
int64_t n = fread(buffer.data(), 1, buffer.size(),f);
int64_t pos_after = _ftelli64(f);
int64_t posMismatch = (int64_t)pos_after-(pos_before+n);
if(ferror(f))
{
std::cout << "fread errorn";
}
if(posMismatch!=0)
{
std::cout << "Error " << path
<< " / ftell before " << pos_before
<< " / fread returned " << n
<< " / ftell after " << pos_after
<< " / mismatch " << posMismatch << "n";
}
}
fclose(f);
}
int main()
{
//Generate file names
std::vector<std::string> fileNames(numThreads);
for(int i=0;i<numThreads;++i)
{
std::ostringstream oss;
oss << i << ".dat";
fileNames[i] = oss.str();
}

//Create dummy data files
for(int i=0;i<numThreads;++i)
{
std::ofstream f(fileNames[i], std::ios_base::binary);
for(int j=0;j<blockSize*fileBlockCount;++j)
{
f.put((char)(j&255));
}
}

//Read data files in separate threads
std::vector<std::thread> threads;
for(int i=0;i<numThreads;++i)
{
threads.emplace_back(readFile, fileNames[i]);
}
//This waits for the threads to finish
for(int i=0;i<numThreads;++i)
{
threads[i].join();
}
threads.clear();
std::cout << "Done";
}

输出是随机的，如下所示：

Error 3.dat / ftell before 65544 / fread returned 65544 / ftell after 131089 / mismatch 1
Error 7.dat / ftell before 0 / fread returned 65544 / ftell after 65543 / mismatch -1
Error 7.dat / ftell before 65543 / fread returned 65544 / ftell after 131088 / mismatch 1
Error 3.dat / ftell before 2162953 / fread returned 65544 / ftell after 2228498 / mismatch 1
Error 7.dat / ftell before 2162952 / fread returned 65544 / ftell after 2228497 / mismatch 1
Error 3.dat / ftell before 3080570 / fread returned 65544 / ftell after 3146112 / mismatch -2
Error 7.dat / ftell before 3080569 / fread returned 65544 / ftell after 3146112 / mismatch -1
Error 2.dat / ftell before 65544 / fread returned 65544 / ftell after 131089 / mismatch 1
Error 6.dat / ftell before 0 / fread returned 65544 / ftell after 65543 / mismatch -1
Error 6.dat / ftell before 65543 / fread returned 65544 / ftell after 131088 / mismatch 1
Error 2.dat / ftell before 2162953 / fread returned 65544 / ftell after 2228498 / mismatch 1
Error 6.dat / ftell before 2162952 / fread returned 65544 / ftell after 2228497 / mismatch 1
Error 2.dat / ftell before 3080570 / fread returned 65544 / ftell after 3146112 / mismatch -2
Error 6.dat / ftell before 3080569 / fread returned 65544 / ftell after 3146112 / mismatch -1

编辑：这似乎与_ftelli64有关

如果我用ftell替换_ftelli64，问题不再存在 那么这是_ftelli64的破碎而不是重入实现吗？