C：为什么从void指针转换为函数指针未定义？

Question

  

可能重复：
  Why are function pointers and data pointers incompatible in C/C++?

在dlsym的手册页中，提供了以下代码段。

       double (*cosine)(double);

       handle = dlopen("libm.so", RTLD_LAZY);
       /* Writing: cosine = (double (*)(double)) dlsym(handle, "cos");
          would seem more natural, but the C99 standard leaves    
          casting from "void *" to a function pointer undefined.  
          The assignment used below is the POSIX.1-2003 (Technical
          Corrigendum 1) workaround; see the Rationale for the    
          POSIX specification of dlsym(). */

       *(void **) (&cosine) = dlsym(handle, "cos");

我查找了相关的spec page，但仍然无法理解不允许从void指针转换为函数指针的原因。 Aren的void指针应该足够大以容纳所有类型的指针。如果是这样，为什么要保留这个铸件未定义？