C++ Handbook

`volatile` — C Keyword

`volatile` — C Keyword

The volatile keyword in C: prevents the compiler from optimizing away accesses.

How to use this reference page

Use reference pages to confirm names, categories, nearby facilities, and the constraints that matter before writing or reviewing code.

  • Scan the top of the page first to identify the primary types, functions, or algorithm families involved.
  • Use the nearby-page links when your question is really about a companion header, related algorithm family, or broader subsystem.
  • Validate tricky behavior with a small compileable example before relying on memory for details like invalidation, ordering, allocation, or lifetime rules.

volatile (C)

Marks a variable as potentially modified by external means (hardware registers, signal handlers). The compiler must not cache or eliminate reads/writes to volatile objects.

Syntax

volatile Type name;
volatile Type* ptr;

Example

#include <signal.h>
#include <stdio.h>

/* Signal handler pattern */
volatile sig_atomic_t g_stop = 0;

void handle_sigint(int sig) {
    (void)sig;
    g_stop = 1;
}

/* Memory-mapped register example */
volatile unsigned int* const STATUS_REG =
    (volatile unsigned int*)0x40000000u;

void wait_ready(void) {
    while (!(*STATUS_REG & 1u)) {
        /* busy-wait – volatile prevents the compiler hoisting the read */
    }
}

int main(void) {
    signal(SIGINT, handle_sigint);
    while (!g_stop) {
        /* main loop */
    }
    printf("stopped\n");
    return 0;
}

Notes

Example in practice

int main() {
    // Pick one facility from this reference page.
    // Write the smallest program that exercises its main precondition,
    // complexity rule, or lifetime constraint before scaling up.
    return 0;
}