Are local static variables automatically thread local, or are they shared between threads?

void f() {
    static int x; // <-- need explicit _Thread_local ?
}

CodePudding user response：

There was static for block-scope variables before the C language specification acknowledged threads or had any support for them, much less _Thread_local specifically. In that context, when not combined with _Thread_local, it specifies static storage duration, meaning that the variable comes into existence (as if) at the beginning of program execution and exists and maintains its last-stored value for the entire run of the program. An object with static storage duration is shared by all threads.

On the other hand, _Thread_local always specifies thread storage duration, which means that the object so declared exists and maintains its last-stored value for the entire lifetime of a thread, and that the declared identifier designates a different object in each thread. When an object is declared _Thread_local at block scope, it must also bear either the extern or static qualifier, which conveys its linkage -- external or none.

extern declarations of any kind at block scope are unusual, but they do occasionally serve a useful purpose. Most of the time, though, static _Thread_local is what you will want for thread-local, block-scope variables.

CodePudding user response：

static and _Thread_local specify two different things.

A variable declared as static has static storage duration and has full program lifetime. A variable declared _Thread_local has thread storage duration and an instance of it exists for each thread.

CodePudding user response：

They are shared value between threads. As you already mentioned by yourself, if you want to make have independently value for each of thread then declare it something line that _Thread_local static int x;