stringstream and multithreading

Question

I'm new to multithreading and C++ and I'm having a problem when trying to use threads in my application which save files. The code is as follows:

#include <iostream> #include <thread> #include <fstream> #include <vector> #include <sstream> using namespace std; void writeCSV(vector<vector<double> > & vec, const char* filename) { ofstream output(filename); for (vector<vector<double> >::const_iterator i = vec.begin(); i != vec.end(); ++i) { for (vector<double>::const_iterator j = i->begin(); j != --i->end(); ++j) { output << *j << ", "; } output << *(--i->end()) << "\n"; } } void testFn(int id) { std::ostringstream filename; vector<vector<double> > v(460, vector<double>(460,0)); filename << "test" << id << ".csv"; const char* fileStr = filename.str().c_str(); cout << id << " : " << fileStr << endl; writeCSV(v, fileStr); } int main() { int numOfThreads = 180; std::thread t[numOfThreads]; for (int i= 0; i< numOfThreads; i++) { t[i] = std::thread (testFn, i); } for (int i = 0; i< numOfThreads; i++) { t[i].join(); } return 0; }

When I run this program, it prints out in the terminal (subsection of results):

66 : 0�c 97 : test97.csv 90 : �'�ǳ 85 : �'�ǳ 43 : 9695 : �'�ǳ 67 : �'�ǳ 93 : : �_ �� 115 : test115.csv 144 : test144.csv 99 : test99.c0 68 : 91 : )� 98 : test98.c0

as well as saving the files with weird/wrong filenames. This seems to be an issue from multithreading and ostringstream I guess, but any ideas why/how to fix?

No, it is not a data race. Speculation rarely leads to the right answer. — Sam Varshavchik
– Sam Varshavchik, Commented Jul 5, 2016 at 13:41

Sam Varshavchik · Accepted Answer · 2016-07-05 14:53:41Z

This has nothing to do with multithreading.

const char* fileStr = filename.str().c_str();

std::ostringstream's str() method returns a std::string representing the contents of the string stream.

A std::string's c_str() method returns an internal pointer to the string data.

What you're missing is that the pointer returned by c_str() is only valid until either the std::string is modified, or it is destroyed, whichever comes first.

Here std::string gets destroyed immediately, because it is a temporary value. As such, a pointer to its internal data is immediately invalidated.

You must store the returned string in an object, that exists as long as the string data is needed. Simply:

std::string str = filename.str(); const char* fileStr = str.c_str();

str continues to exist for the remainder of its automatic scope, which is long enough, here.

John Burger · Accepted Answer · 2016-07-05 13:52:26Z

In your file initiialisation code, you have the following excerpt:

std::ostringstream filename; ... filename << "test" << id << ".csv"; const char* fileStr = filename.str().c_str();

That final line is a killer. What you're doing is taking your hard-earned filename, getting the string representation from it, extracting the c_str() pointer from it - and then destroying the temporary string object. From that point on, any attempt to access fileStr is fraught.

What you need to do is assign the temporary string object to a local variable:

filename << "test" << id << ".csv"; std::string fileString = filename.str(); const char* fileStr = fileString.c_str();

That guarantees the lifetime of the temporary string object - at least until the end of the function!

Without doing that, fileStr points into the middle of the unallocated heap - and the heap changes underneath it.

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