Parallel Read Write to C++ stringstream

Question

I am using std::stringstream for reading/writing binary data.

std::stringstream strm(std::stringstream::binary|std::stringstream::in|std::stringstream::out);

    strm.write(...) //happens in one thread
    strm.read(...)  //happens in another thread

Does C++ standards guarantees that parallel read write to stringstream work? Or not.

Answer 1

My fstream.h file at /usr/local/pgi/linux86-64/13.10/include/CC/fstream.h contains no mention of mutex locks. Further, in programs I have written output using << operator to stringstream files can become interleaved if written at the 'same' time.

Since you're reading from and writing to the same file, I imagine the line order is important? As such, I think you want a global mutex lock between threads.

Something like:

#include ....

pthread_mutex_t FileMutex = PTHREAD_MUTEX_INITIALIZER;

std::stringstream strm(std::stringstream::binary|...)

int main()
{

blah blah

pthread_create(&threads, NULL, function, voidPtrToArguments);

blah blah

}

void *function(void *voidPtrToArguments)
{
blah blah some more

pthread_mutex_lock(&FileMutex);
strm.write(...);
pthread_mutex_unlock(&FileMutex);

}

and then the same for a function to read.

Answer 2

This is covered in [iostreams.threadsafety] in the Standard. The C++17 text reads:

Concurrent access to a stream object, stream buffer object, or C Library stream by multiple threads may result in a data race unless otherwise specified. [Note: Data races result in undefined behavior . —end note ]

There is no "otherwise specified" for std::stringstream so your case would be undefined behaviour.