DCL34-C. Use volatile for data that cannot be cached

An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Asynchronous signal handling falls into this category. Without this type qualifier, unintended optimizations may occur.

The volatile keyword eliminates this confusion by imposing restrictions on access and caching. According to the C99 Rationale [[ISO/IEC 03]]:

No cacheing through this lvalue: each operation in the abstract semantics must be performed (that is, no cacheing assumptions may be made, since the location is not guaranteed to contain any previous value). In the absence of this qualifier, the contents of the designated location may be assumed to be unchanged except for possible aliasing.

Please keep in mind that while adding volatile will ensure that a compiler does not perform unintended reordering or optimization, it in no way guarantees synchronization between multiple threads or otherwise ward against simultaneous memory accesses.

Non-Compliant Coding Example

If the value of i is cached, the while loop may never terminate. When compiled on gcc with the -O optimization flag, the program fails to terminate even upon receiving a SIGINT.

#include <signal.h>

sig_atomic_t i;

void handler(int signum) {
  i = 0;
}

int main(void) {
  i = 1;
  signal(SIGINT, handler);
  while (i) {
   /* do something */
  }
  return 0;
}

Compliant Solution

By adding the volatile qualifier, i is guaranteed to be accessed from its original address for every iteration of the while loop.

#include <signal.h>

volatile sig_atomic_t i;

void handler(int signum) {
  i = 0;
}

int main(void) {
  i = 1;
  signal(SIGINT, handler);
  while (i) {
   /* do something */
  }
  return 0;
}

The sig_atomic_t type is the integer type of an object that can be accessed as an atomic entity, even in the presence of asynchronous interrupts. The type of sig_atomic_t is implementation-defined, although there are constraints. Only assign integer values from 0 through 127 to a variable of type sig_atomic_t to be fully portable.

Another acceptable (although less readable) solution involves specifying the volatile keyword temporarily by using a typecast.

#include <signal.h>

sig_atomic_t i;

void handler(int signum) {
  i = 0;
}

int main(void) {
  i = 1;
  signal(SIGINT, handler);
  while (*(volatile sig_atomic_t *)&i) {
   /* do something */
  }
  return 0;
}

Such a solution may be necessary to prevent the compiler from optimizing away the memory lookup while allowing for caching and optimizations elsewhere in the code.

Risk Assessment

Failing to use the volatile qualifier can result in race conditions in asynchronous portions of the code, causing unexpected values to be stored and leading to possible data integrity violations.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[ISO/IEC 9899-1999]] Section 6.7.3, "Type qualifiers," and Section 7.14, "Signal handling <signal.h>"
[[ISO/IEC 03]] Section 6.7.3, "Type qualifiers"
[[Sun 05]] Chapter 6, "Transitioning to ISO C"

DCL33-C. Ensure that restrict-qualified source and destination pointers in function arguments do not reference overlapping objects      02. Declarations and Initialization (DCL)       DCL35-C. Do not convert a function pointer to an incompatible type