Question

I’m having trouble passing a C++0x lambda function as the second argument to makecontext (from ucontext.h). The signature of makecontext is:

void makecontext(ucontext_t*, void (*)(), int, ...);

Previously, I was able to apply a C-style (void (*)(void)) cast, to the global scope functions I used. A reinterpret_cast would do the trick in C++. However, with a C++0x lambda function, I get the following error:

error: invalid cast from type ‘main(int, char**)::<lambda(int)>’ to type ‘void (*)()’

I’m using G++ 4.6. The following code is sufficient to produce the compile error:

#include <ucontext.h>

void f1(int i) {}

int main(int argc, char *argv[]) {
  ucontext_t c;
  makecontext(&c, (void (*)(void))f1, 1, 123); // ok
  makecontext(&c, reinterpret_cast<void (*)(void)>(f1), 1, 123); // ok

  auto f2 = [](int i){};
  makecontext(&c, (void (*)(void))f2, 1, 123); // error
  makecontext(&c, reinterpret_cast<void (*) (void)>(f2), 1, 123); // error
  return 0;
}

Answer 1

[](int i){} is a captureless lambda that has a single int parameter and returns nothing; it is thus implicitly convertible to void(*)(int): a pointer to a function that takes a single int and returns nothing.

Assuming the makecontext function is capable of handling functions with different types (it appears from its documentation that it is, though the documentation is a bit vague), then you’ll need to use two casts to use it with a lambda: one to cast the lambda to a function pointer, and one to cast the lambda to the void(*)() type:

auto f2 = [](int i) { };
makecontext(&c, (void (*)(void))(void (*)(int))f2, 1, 123);

This is all rather unsafe (casting between function pointer types isn’t exactly the safest thing to be doing), so it would be best to wrap this sort of functionality into a utility library. You can wrap the makecontext function with a function template so that you can be sure that all of your uses of the function are type safe, and all of the unsafe code is encapsulated in one place.