动态变量的内存分配过程是怎样的？

Question

When a function is called, a space in memory is reserved for local variables (formal parameters and those declared within the function's scope). I understand that in ANSI C, because it is required that the variables are declared at the beginning of a block.

However, in the case of the following C code compiled with GCC, will the z variable will have its space allocated at the beginning of the block or only when y is equal to 42 ?

void foo(int x) {
    int y;
    scanf("%d%*c", &y);
    if (y != 42)
        return;
    int z;
    return;
}

Is the behavior the same for other higher level languages such as Python and Ruby, with similar code?

Answer 1

This is typically implemented by reserving space on the stack for all variables that are declared in the method. It would certainly be possible to do it dynamically, but that would require each "potential" variable to internally be represented as a pointer (since its address cannot be known in advance), and the overhead would almost certainly not be worth it. If you really want "dynamic" variables, you can implement it yourself with pointers and dynamic memory allocation.

Java and C# do the same thing: they reserve space for the total collection of local variables.

I don't really know about Python or Ruby, but in these languages, there is no such thing as a primitive data type: all values are references and stored on the heap. As such, it is entirely possible that the storage space for the value referred to by a variable won't appear until the variable "declaration" is executed (although "declaration" isn't really a thing in dynamic languages; it's more of an assignment to a variable that happens do not exist yet). Note, though, that the variable itself also requires storage space (it's a pointer, after all) - however, the variables of dynamic languages are often implemented as hashmaps, so the variables themselves may also dynamically appear and disappear.