Giving a function argument dynamically in C++

Question

I have the following code where I create threads based on the for loop below, the 3rd argument in pthread takes a void* ()(void) and I am giving it a string arg. The problem is that every thread has it's own method i.e thread 0 has its method thread0(). I want to be able to set the method based on the t of the for loop without getting the following error:

main2.cpp:40:51: error: cannot convert ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ to ‘void* (*)(void*)’ for argument ‘3’ to ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void*), void*)’
 err = pthread_create(&threads[t], &attr,method, &t);

for(t = 0; t <5; t++){;
    printf("Creating thread %d\n",t);
    std::string method = "thread"+t;
    err = pthread_create(&threads[t], &attr,method, &t);
    if(err != 0) exit(-1);
}

Answer 1

The error message is pretty clear about the function pointer, that you can't translate c++ symbols to strings or vice versa.

Here's the signature of pthread_create() as given in the documentation:

SYNOPSIS

intpthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg);

You can do the following:

typedef void* (*start_routine_t)(void*);

void* foo_0(void*) { return nullptr; }
void* foo_1(void*) { return nullptr; }    
void* foo_2(void*) { return nullptr; }
void* foo_3(void*) { return nullptr; }
void* foo_4(void*) { return nullptr; }

constexpr std::map<std::string,start_routine_t> thread_funcs {
    { "thread_0" , foo_0 } ,
    { "thread_1" , foo_1 } ,
    { "thread_2" , foo_2 } ,
    { "thread_3" , foo_3 } ,
    { "thread_4" , foo_4 } ,
};

pthread_t threads[5];


// ....

for(t = 0; t <5; t++){;
    printf("Creating thread %d\n",t);
    std::ostringstream method;
    method << "thread_" <<t;
    err = pthread_create(&threads[t], &attr,method, &thread_funcs[method.str()],nullptr);
    if(err != 0) exit(-1);
}

Or the more straightforward way without using strings at all:

start_routine_t thread_funcs[5] = { foo_0, foo_1, foo_2, foo_3, foo_4 }; 
pthread_t threads[5];

// ...

for(t = 0; t <5; t++){
    printf("Creating thread %d\n",t);
    err = pthread_create(&threads[t], &attr,method, thread_funcs[t], nullptr);
    if(err != 0) exit(-1);
}

---

As you also asked for c++-11 facilities:

1. Use std::thread instead of the pthread-API directly at all. If your target environment supports pthreads properly, you usually can use the std::thread ABI.

2. Use lambda functions to refer to specific routines on the fly:

   std::vector<std::thread> threads(5);
   for(t = 0; t <5; t++){
      printf("Creating thread %d\n",t);
      auto thread_func = [t]() {
          switch(t) {
          case 0: foo_0(); break;
          case 1: foo_1(); break;
          case 2: foo_2(); break;
          case 3: foo_3(); break;
          case 4: foo_4(); break;
          }               
      };
      threads[t] = std::thread(thread_func);
   }
   

   The above code example probably isn't the best (most efficient), but demonstrates how to map function calls on the fly.