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

It is a misconception that the volatile qualifier provides 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.

Unfortunately, the volatile qualifier does not provide guarantees for any of these properties, neither by definition nor 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.

The C Standard, section 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.

In a nutshell, all this keyword does is inform the compiler that this variable may change in ways that cannot be determined; therefore, the compiler should not perform optimizations in memory areas marked as volatile. In other words, the compiler should not store the value in a register and use the register instead of doing expensive memory accesses. This concept is closely related to multithreading because if a shared variable is cached, a thread may change it, and the other threads may read stale data.

This property of the volatile keyword is sometimes misunderstood to provide atomicity of a variable that is shared between threads in a multithreaded program. A variable declared as volatile is not cached in a register, leading to this confusion that it can be used safely as a synchronization primitive. When a variable is declared volatile, the compiler does not reorder the sequence of reads and writes to that memory location. However, the compiler might reorder these reads and writes with those to other memory locations. This might result in nonatomic operations on the synchronization variable, causing errors.

Noncompliant Code Example

This noncompliant code example uses 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 volatile solves the problem of reading stale data but still does not provide atomicity 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 */

void debit(unsigned int amount) {
  mtx_lock(&flag);
  account_balance -= amount; /* Inside critical section */
  mtx_unlock(&flag);
}

Risk Assessment

Related Guidelines

CERT C++ Secure Coding Standard: CON01-CPP. Do not use volatile as a synchronization primitive

ISO/IEC 9899:2011 Section 5.1.2.3, "Program execution"

Sources

[Open Group 2004] Section 4.11, "Memory synchronization"