CON02-C. Do not use volatile as a synchronization primitive

The C Standard, subclause 5.1.2.3, paragraph 2 [ISO/IEC 9899:2011], says,

Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment. Evaluation of an expression in general includes both value computations and initiation of side effects. Value computation for an lvalue expression includes determining the identity of the designated object.

The volatile keyword informs the compiler that the qualified variable may change in ways that cannot be determined; consequently, compiler optimizations must be restricted for memory areas marked as volatile. For example, the compiler is forbidden to load the value into a register and subsequently reuse the loaded value rather than accessing memory directly. This concept relates to multithreading because incorrect caching of a shared variable may interfere with the propagation of modified values between threads, causing some threads to view stale data.

The volatile keyword is sometimes misunderstood to provide atomicity for variables that are shared between threads in a multithreaded program. Because the compiler is forbidden to either cache variables declared as volatile in registers or to reorder the sequence of reads and writes to any given volatile variable, many programmers mistakenly believe that volatile variables can correctly serve as synchronization primitives. Although the compiler is forbidden to reorder the sequence of reads and writes to a particular volatile variable, it may legally reorder these reads and writes with respect to reads and writes to other memory locations. This reordering alone is sufficient to make volatile variables unsuitable for use as synchronization primitives.

Further, the volatile qualifier lacks any guarantees regarding the following desired properties necessary for a multithreaded program:

• Atomicity: Indivisible memory operations.
• Visibility: The effects of a write action by a thread are visible to other threads.
• Ordering: Sequences of memory operations by a thread are guaranteed to be seen in the same order by other threads.

The volatile qualifier lacks guarantees for any of these properties, both by definition and by the way it is implemented in various platforms. For more information on how volatile is implemented, consult DCL17-C. Beware of miscompiled volatile-qualified variables.

Noncompliant Code Example

This noncompliant code example attempts to use flag as a synchronization primitive:

bool flag = false;

void test() {
  while (!flag) {
    sleep(1000);
  }
}

void wakeup(){
  flag = true;
}

void debit(unsigned int amount){
  test();
  account_balance -= amount;
}

In this example, the value of flag is used to determine whether the critical section can be executed. Because the flag variable is not declared volatile, it may be cached in registers. Before the value in the register is written to memory, another thread might be scheduled to run, resulting in that thread reading stale data.

Noncompliant Code Example

This noncompliant code example uses flag as a synchronization primitive but qualifies flag as a volatile type:

volatile bool flag = false;

void test() {
  while (!flag){
    sleep(1000);
  }
}

void wakeup(){
  flag = true;
}

void debit(unsigned int amount) {
  test();
  account_balance -= amount;
}

Declaring flag as volatile solves the problem of values being cached, which causes stale data to be read. However, volatile flag still fails to provide the atomicity and visibility guarantees needed for synchronization primitives to work correctly. The volatile keyword does not promise to provide the guarantees needed for synchronization primitives.

Compliant Solution

This code uses a mutex to protect critical sections:

#include <threads.h>

int account_balance;
mtx_t flag;

/* Initialize flag */

int debit(unsigned int amount) {
  if (mtx_lock(&flag) == thrd_error) {
    return -1;  /* Indicate error */
  }
 
  account_balance -= amount; /* Inside critical section */

  if (mtx_unlock(&flag) == thrd_error) {
    return -1;  /* Indicate error */
  }

  return 0;
}

Compliant Solution (Critical Section, Windows)

This compliant solution uses a Microsoft Windows critical section object to make operations involving account_balance atomic [MSDN].

#include <Windows.h>

static volatile LONG account_balance;
CRITICAL_SECTION flag;

/* Initialize flag */
InitializeCriticalSection(&flag);
 
int debit(unsigned int amount) {
  EnterCriticalSection(&flag); 
  account_balance -= amount; /* Inside critical section */
  LeaveCriticalSection(&flag);
 
  return 0;
}

Risk Assessment

Related Guidelines

Bibliography