Thread-local storage in C

_Thread_local gives every thread its own copy of a variable.

#include <stdio.h>

static _Thread_local int calls = 0;
static _Thread_local char scratch[64];

int bump_local(void) {
    calls++;
    snprintf(scratch, sizeof scratch, "call %d", calls);
    return calls;
}

const char *last_message(void) {
    return scratch;
}

How it works

1. Each thread reads and writes an independent instance.
2. No lock is needed, because nothing is shared.
3. It suits per-thread scratch buffers and counters.

Keywords and builtins used here

• _Thread_local
• bump_local
• char
• const
• int
• last_message
• return
• sizeof
• static
• void

The run, in numbers

Lines

14

Characters to type

248

Tokens

58

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 33 seconds.

Type this snippet

Step 2 of 2 in Lock-free & local, step 9 of 10 in Threads & concurrency.

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