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Compound operations are operations that consist of more than one discrete operation. Expressions that include postfix or prefix increment (++), postfix or prefix decrement (--), or compound assignment operators always result in compound operations. Compound assignment expressions use operators such as *=, /=, %=, +=, -=, <<=, >>=, ^= and , and |=. Compound operations on shared variables must be performed atomically to prevent data races.

Noncompliant Code Example (Logical Negation)

#include <stdbool.h>
 
static 

bool flag = 

false;
 
void toggle_flag(void) {
  flag = !flag;
}
 

bool get_flag(void) {
  return flag;
}

Execution of this code may result in a data race because because the value of of flag is is read, negated, and written back.

Consider, for example, two threads that call toggle_flag(). The expected effect of toggling flag twice is that it is restored to its original value. However, the following scenario leaves flag in the incorrect state:

As a result, the effect of the call by t ₂ is not reflected in flag; the program behaves as if toggle_flag() was called only once, not twice.

Noncompliant Code Example (Bitwise Negation)

The toggle_flag() method may also use the compound assignment operator ^= to negate the current value of flag.

static _Bool flag = 0;
 
void toggle_flag() {
  flag ^= 1;
}
 
_Bool get_flag() {
  return flag;
}

.

Compliant Solution (Mutex)

This compliant solution restricts access to flag under a mutex lock.

:

#include <threads.h>
#include <stdbool.h>
 
static 

bool flag = 

false;
mtx_t flag_mutex;

/* Initialize flag_mutex */
bool init_mutex(int type) {
  /* 

Check 

mutex type */
  if (

thrd_success != mtx_init(&flag_mutex, 

type))

 {
    return false;  /* 

Report error */
  }
  return true;
}
 
void toggle_flag(void) {

if (

thrd_success != mtx_lock(&flag_mutex)) 

{
    /* 

Handle error */
  }
  flag 

= 

!flag;
  if (

thrd_success != mtx_unlock(&flag_mutex)

) {
    /* 

Handle error */
  }
}
 

bool get_flag(void) {
  

bool temp_flag;
  if (

thrd_success != mtx_lock(&flag_mutex)

) {
    /* 

Handle error */
  }
  temp_flag = flag;
  if (

thrd_success != mtx_unlock(&flag_mutex)

) {
    /* 

Handle error */
  }
  return temp_flag;
}

This solution guards reads and writes to

the flag

field with a lock on the flag_mutex. This lock ensures that changes to flag are visible to all threads. Now, only two execution orders are possible

. In one execution order, t₁ obtains the mutex and completes the operation before t ₂ can acquire the mutex, as shown here:

The second other execution order involves the same operations, but is similar, except that t ₂ starts and finishes before t ₁.  

Compliant Solution (atomic

This noncompliant code example declares flag to be of type _Atomic _Bool.

_compare_exchange_weak() )

This compliant solution uses atomic variables and a compare-and-exchange operation to guarantee that the correct value is stored in flag. All updates are visible to other threads.

#include <stdatomic.h>
#include <stdbool.h>
 
static atomic_bool flag;

void init_flag(void) {
  

atomic_

init(&flag, false);
}

This code suffers from the same potential race condition as the first noncompliant code example.

Compliant Solution (atomic boolean)

This compliant solution declares flag to be of type _Atomic _Bool.

static _Atomic _Bool flag = 0;
  
void toggle_flag(void) {
  _Boolbool old_flag = atomic_load(&flag);
  _Boolbool new_flag;
  do {
    new_flag = !old_flag;
  } while (!atomic_compare_exchange_weak(&flag, &old_flag, new_flag));
}
  
_Boolbool get_flag(void) {
  return atomic_load(&flag);
}

The flag variable is updated using the atomic_compare_exchange_weak() function on the atomic boolean variable. All updates are visible to other threads.

An alternative solution is to use the atomic_flag

data type for managing

Boolean values atomically.

Noncompliant Code Example (Addition of Primitives)

In this noncompliant code example, multiple threads can invoke the set_values() method to set the a and b fields. Because this code fails to test for integer overflow, users of this code must also ensure that the arguments to the set_values() method can be added without overflow . (See rule see INT32-C. Ensure that operations on signed integers do not result in overflow for more information).)

static int a;
static int b;
 
int get_sum(void) {
  return a + b;
}
 
void set_values(int new_a, int new_b) {
  a = new_a;
  b = new_b;
}

The  get_sum() method contains a race condition. For example, when a and b currently have the values 0 and INT_MAX, respectively, and one thread calls get_sum() while another calls set_values(INT_MAX, 0), the get_sum() method might return either 0 or INT_MAX, or it might overflow. Overflow will occur when the first thread reads a and b after the second thread has set the value of a to INT_MAX

but before it has set the value of b to 0.

Noncompliant Code Example (Addition of Atomic Integers)

In this noncompliant code example,  a and  and b are are replaced with atomic integers.

#include <stdatomic.h>

static atomic_

int a;
static atomic_

int b;

void init_ab(void) {
  atomic_init(&a, 0);
  atomic_init(&b, 0);
}

int get_sum(void) {
  return atomic_load(&a) + atomic_load(&b);
}
 
void set_values(int new_a, int new_b) {
  atomic_store(&a, new_a);
  atomic_store(&b, new_b);
}

Compliant Solution (_Atomic struct)

This compliant solution uses an atomic struct, which guarantees that both numbers are read and written together.

#include <stdatomic.h>
  
static _Atomic struct ab_s {
  int a, b;
} ab;
 
void init_ab(void) {
  struct ab_s new_ab = {0, 0};
  atomic_init(&ab, new_ab);
}
 
int get_sum(void) {
  struct ab_s new_ab = atomic_load(&ab);
  return new_ab.a + new_ab.b;
}
 
void set_values(int new_a, int new_b) {
  struct ab_s new_ab = {new_a, new_b};
  atomic_store(&ab, new_ab);
}

On most modern platforms, this will compile to be lock-free.

Compliant Solution (

Mutex)

This compliant solution synchronizes the protects the set_values() and  and get_sum() methods methods with a mutex to ensure atomicity.:

#include <threads.h>
#include <stdbool.h>

static 

int a;
static

 int b;
mtx_t flag_mutex;

/* Initialize everything */
bool init_all(int type) {
  /* 

Check 

mutex type */
  a = 0;
  b = 0;
  if (

thrd_success != mtx_init(&flag_mutex, 

type)) 

{
    return false;  /* 

Report error */
  }
  return true;
}
 
int get_sum(void) {

  if (

thrd_success != mtx_lock(&flag_mutex))

{
    /* 

Handle error */
  }
  int sum = 

a

 + 

b

;
  if (

thrd_success != mtx_unlock(&flag_mutex))

{
    /* 

Handle error */
  }
  return sum;
}
 
void set_values(int new_a, int new_b) {

  if (

thrd_success != mtx_lock(&flag_mutex))

{
    /* 

Handle error */
  }
  

a = new_a

;
  

b = new_b

;
  if (

thrd_success != mtx_unlock(&flag_mutex)

) {
    /* 

Handle error */
  }
}

Thanks to the mutex, it is now possible to add overflow checking to the get_sum()  function without introducing the possibility of a race condition.

Risk Assessment

When operations on shared variables are not atomic, unexpected results can be produced. For example, information can be disclosed inadvertently because one user can receive information about other users.

Automated Detection

Related Guidelines

Bibliography

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