Having easy access to parallel processing is a pleasant feature in programming languages. The thread syscalls of operating systems have notoriously been difficult to access, especially in C. The Open Multi-Processing (OpenMP) library started in 1997 to make things easier. It helps to mark sections of parallel code and loop that can be parallelized. It works well for C, C++, and FORTRAN code. It is easy to implement. Plus, your code can be compiled with or without the OpenMP library present. The downside is that your code requires OpenMP on the target. I recently had a case where C++ code needs to be installed on different platforms (i.e. systems with different major version level). OpenMP is tied to the C/C++ standard library and the compiler. The code is compiled by Clang, so in this particular case you need different OpenMP libraries. In order to reduce the dependency on OpenMP, the code was refactored to use C++ threads.
C++11 threads are easy to use. When switching from OpenMP, you only have to convert your #pragma statements to function calls. When using member functions, you have to code around a peculiarity of std::async and std::thread. Member functions of dynamically allocated objects cannot be called directly. If you try to do this, then you will get an error message. Consider the following object:
class hash_list {The class is used to access different Tokyo Cabinet databases. The function walkthrough() does heavy I/O work and updates the database file. Calling the function directly with std::async will not work. I fought with many compiler errors and was tempted to convert the member function to static, but this would have required a full rewrite of the class. Static member variables and function change access to encapsuled data. Instead, you will need a wrapper function to call the members.
private:
kyotocabinet::HashDB HDB;
kyotocabinet::HashDB::Cursor *pos;
…
public:
bool walkthrough( string directory ) {
…
}
#ifndef USE_OPENMP
bool wrap_walkthrough( hash_list *h, string d ) {
return( h->walkthrough(d) );
}
#endif
The function wrap_walkthrough() works fine, and it can be called with different dynamically allocated objects. The section calling the functions looks like this:
#ifndef USE_OPENMP
future<bool> f_rc_path = std::async( std::launch::async, wrap_walkthrough, path_orig, opt_path );
future<bool> f_rc_prfx = std::async( std::launch::async, wrap_walkthrough, path_prefix, prefix_path );
const bool rc_path = f_rc_path.get();
const bool rc_prfx = f_rc_prfx.get();
if ( ! rc_path ) {
cerr << "Walkthrough for " << opt_path << " failed!" << endl;
rc += 23;
}
if ( ! rc_prfx ) {
cerr << "Walkthrough for " << prefix_path << " failed!" << endl;
rc += 23;
}
#endif
Remember to write wrapper functions when you encounter the error message “reference to non-static member function must be called”.