ofstream variable cannot appear in OpenMP firstprivate?

Question

code

ofstream myfile("file_path");
#pragma omp parallel for default(none) schedule(dynamic) firstprivate(myfile) private(i)
for(i=0; i<10000; i++) {
    myfile<<omp_get_thread_num()+100<<endl;
}

but the compiler show me the error:

error: use of deleted function 'std::basic_ofstream<_CharT, _Traits>::basic_ofstream(const std::basic_ofstream<_CharT, _Traits>&) [with _CharT = char; _Traits = std::char_traits]'

/usr/include/c++/5/fstream:723:7: note: declared here basic_ofstream(const basic_ofstream&) = delete;

error: 'myfile' not specified in enclosing parallel

Answer 1

firstprivate works by making a thread-private copy of the value. This does not work with streams, because you cannot copy them. You cannot safely write to a file just by having multiple streams of it open. There are basically two options:

• Have a shared stream , protect all threaded access to it with #pragma omp critical .

   ofstream myfile("file_path"); #pragma omp parallel for for (int i=0; i < 10000; i++) { #pragma omp critical myfile << (omp_get_thread_num()+100) << endl; } 

• Open one stream for each thread on a different file .

   #pragma omp parallel { ofstream myfile(std::string("file_path.") + std::to_string(omp_get_thread_num())); #pragma omp for for (int i=0; i < 10000; i++) { myfile << (omp_get_thread_num()+100) << endl; } }