volatile in C

volatile tells the compiler the value can change behind its back.

#include <signal.h>

static volatile sig_atomic_t interrupted = 0;
static volatile unsigned int *const hardware_register =
    (volatile unsigned int *) 0x1000;

void spin_until_flag(void) {
    while (!interrupted) {
    }
}

unsigned int read_twice(void) {
    unsigned int first = *hardware_register;
    unsigned int second = *hardware_register;
    return second - first;
}

How it works

1. Without it a loop reading a flag may be optimized away.
2. It disables caching in a register, not much else.
3. It is not a substitute for an atomic or a lock.

Keywords and builtins used here

• const
• int
• read_twice
• return
• sig_atomic_t
• spin_until_flag
• static
• unsigned
• void
• volatile
• while

The run, in numbers

Lines

16

Characters to type

354

Tokens

67

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 38 seconds.

Type this snippet

Step 1 of 2 in Qualifiers, step 6 of 13 in Unions, bitfields & undefined behavior.

← Previous Next →