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

There is a misconception that the volatile qualifier will provide 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 by 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, neither by definition nor by the way it is implemented in various platforms. For more information on how volatile is implemented, consult recommendation DCL17-C. Beware of miscompiled volatile-qualified variables.

The C99 standard, section 5.1.2.3, paragraph 2, 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 may produce side effects. At certain specified points in the execution sequence called sequence points, all side effects of previous evaluations shall be complete, and no side effects of subsequent evaluations shall have taken place.

In a nutshell, all this keyword does is to inform the compiler that this variable may change in ways that cannot be determined; therefore, the compiler should not perform optimizations in these 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 confused as providing 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 declared volatile, the compiler does not re-order the sequence of reads and writes to that memory location. However, the compiler might re-order these reads and writes with those to other memory locations. This might result in non-atomic operations on the synchronization variable resulting in 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 if the critical section can be executed or not. 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 9899:1999 Section 5.1.2.3 "Program Execution"

Bibliography

Open Group 2004] Section 4.11 "Memory Synchronization"