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 |=
. Compound operations on shared variables must be performed atomically to prevent data races.
Noncompliant Code Example (Logical Negation)
This noncompliant code example declares a shared_Bool
flag
variable and provides a toggle_flag()
method that negates the current value of flag
:#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 the value of flag
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:
Time |
| Thread | Action |
---|---|---|---|
1 |
| t1 | Reads the current value of |
2 |
| t2 | Reads the current value of |
3 |
| t1 | Toggles the temporary variable in the cache to |
4 |
| t2 | Toggles the temporary variable in the different cache to |
5 |
| t1 | Writes the cache variable's value to |
6 |
| t2 | Writes the different cache variable's value to |
As a result, the effect of the call by t2 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
:
#include <stdbool.h> static bool flag = false; void toggle_flag(void) { flag ^= 1; } bool get_flag(void) { return flag; }
^=
is a nonatomic compound operation.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 ^= 1; 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, one of which is shown in the following scenario.
Time |
| Thread | Action |
---|---|---|---|
1 |
| t1 | Reads the current value of |
2 |
| t1 | Toggles the cache variable to |
3 |
| t1 | Writes the cache variable's value to |
4 |
| t2 | Reads the current value of |
5 |
| t2 | Toggles the different cache variable to |
6 |
| t2 | Writes the different cache variable's value to |
The second execution order involves the same operations but t2 starts and finishes before t1.
Noncompliant Code Example (atomic_bool
)
This noncompliant code example declares flag
to be of type atomic_bool
:
#include <stdatomic.h> #include <stdbool.h> static atomic_bool flag; void init_flag(void) { atomic_init(&flag, false); } void toggle_flag(void) { bool temp_flag = atomic_load(&flag); temp_flag = !temp_flag; atomic_store( &flag, temp_flag); } bool get_flag(void) { return atomic_load(&flag); }
This code suffers from the same potential race condition as the first noncompliant code example, because the read and subsequent write in toggle_flag()
do not constitute a single atomic operation.
Compliant Solution (atomic_compare_exchange_weak()
)
This compliant solution uses a compare and exchange 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); } void toggle_flag(void) { bool old_flag = atomic_load(&flag); bool new_flag; do { new_flag = !old_flag; } while (!atomic_compare_exchange_weak(&flag, &old_flag, new_flag)); } bool get_flag(void) { return atomic_load(&flag); }
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 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 b
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); }
int
fields with atomic integers fails to eliminate the race condition in the sum because the compound operation a.get() + b.get()
is still nonatomic.Compliant Solution (Addition)
This compliant solution protects the set_values()
and get_sum()
methods with a mutex to ensure atomicity:
#include <stdatomic.h> #include <threads.h> #include <stdbool.h> static atomic_int a; static atomic_int b; mtx_t flag_mutex; /* Initialize everything */ bool init_all(int type) { /* Check mutex type */ atomic_init(&a, 0); atomic_init(&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 = atomic_load(&a) + atomic_load(&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 */ } atomic_store(&a, new_a); atomic_store(&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.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
CON42-C | Medium | Probable | Medium | P8 | L2 |
Related Guidelines
CERT Oracle Secure Coding Standard for Java | VNA02-J. Ensure that compound operations on shared variables are atomic |
CWE-366, Race condition within a thread |
Bibliography
Subclause 7.17, "Atomics" |