Are all global variables static?

Yes, in the sense that global variables have a lifetime that spans the entire program execution, which is a characteristic associated with static storage duration.

Here's a more detailed breakdown:

Global Variables: These are variables declared outside of any function or class. They are accessible from anywhere in the program (unless access is restricted by scope).
Static Storage Duration: This means that the variable is allocated memory when the program starts and remains allocated until the program terminates. Global variables inherently possess static storage duration.

However, the term "static" also has another meaning related to scope and linkage, which can be confusing.

static keyword applied to Global Variables (file scope): When you explicitly declare a global variable as static (e.g., static int global_variable;), you are restricting its linkage to the current file. This means that the variable is only visible (accessible) within that specific source file. Other files in the program cannot access it, even though it's still a global variable in terms of its lifetime and storage. This internal linkage prevents naming collisions if another file happens to use the same variable name.
Global Variables (external linkage): When a global variable is declared without the static keyword (e.g., int global_variable;), it has external linkage. This means it's visible and accessible from other files in the program (provided they declare it with the extern keyword if it's not defined in that file).

Therefore:

Lifetime Perspective: From the perspective of lifetime, all global variables are indeed static because their memory is allocated for the duration of the program's execution.
Linkage Perspective: From the perspective of linkage, a global variable is only static if the static keyword is explicitly used in its declaration to limit its scope to the current file. Otherwise, it has external linkage.

In summary, while the lifetime aspect of all global variables aligns with the concept of "static" (existing throughout the program's execution), the static keyword used in their declaration primarily controls their visibility and linkage across different files within the program.