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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, for example, may cause objects to be modified in a manner unknown to the compiler. 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 \[[ISO/IEC 03|AA. C References#ISO/IEC 03]\]:

. These optimizations may cause race conditions because a programmer may write code that prevents a race condition, yet the compiler is not aware of the programmer's data model and may modify the code during compilation to permit race conditions.

The volatile keyword eliminates this confusion by imposing restrictions on access and caching. According to the C99 Rationale [C99 Rationale 2003],

No caching No cacheing through this lvalue: each operation in the abstract semantics must be performed (that is, no cacheing caching assumptions may be made, since because 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 Type qualifying objects as volatile does not guarantee synchronization between multiple threads or otherwise ward , protect against simultaneous memory accesses, or, unless used to declare objects of type sig_atomic_t, guarantee atomicity of accesses to the object.

Non-Compliant Code Example

For restrictions specific to signal handlers, see SIG31-C. Do not access shared objects in signal handlers. However, type qualifying objects as volatile does ensure that a conforming compiler will not elide or reorder access to the object.

Noncompliant Code Example

In this noncompliant code example, the programmer is targeting a custom piece of hardware that controls an LED by writing values into a register bank. The register bank is memory mapped into the process such that writing to a specific memory location will actually place a value into a hardware register to be read by the LED controller. The programmer intends to turn the LED on by placing value 1 into the first register, and then turn the LED off later by placing the value 0 into the first registerThe following non-compliant code relies on the reception of a SIGINT signal to toggle a flag to terminate a loop.

#include <stddef.h>


#include <signal<stdint.h>

sig_atomicextern void get_register_bank(int32_t i;

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

int main**bank,
                              size_t *num_registers);
extern void external_wait(void);

void func(void) {
  int32_t bank[3];
   isize_t num_regs = 13;

  signal(SIGINT, handlerget_register_bank((int32_t **)&bank, &num_regs);
  whileif (inum_regs < 3) {
    /* doHandle somethingerror */
   }

  bank[0]  return= 1;
  external_wait();
  bank[0] = 0;
}

However, 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.

Non-Compliant Code Example

The following non-compliant code prevents the compiler from optimizing away the loop condition, by type casting the variable to volatile within the while loop.

#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.

However, this solution violates SIG31-C. Do not access or modify shared objects in signal handlers.

Compliant Solution

By adding the volatile qualifier to the variable declaration, i is guaranteed to be accessed from its original address for every iteration of the while loop, as well as from within the signal handler.

#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, though it has some guarantees. Integer values from 0 through 127 can be safely stored to a variable of type sig_atomic_t safely.

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.

The compiler is free to optimize what it perceives as being a dead store to bank[0] by removing the first assignment to the variable. This would cause the LED to never be turned on in an optimized build. 

Compliant Solution

In this compliant solution, the register bank's memory is qualified with the volatile keyword, ensuring the compiler does not optimize access to the memory.

#include <stddef.h>
#include <stdint.h>

extern void get_register_bank(volatile int32_t **bank,
                              size_t *num_registers);
extern void external_wait(void);

void func(void) {
  volatile int32_t bank[3];
  size_t num_regs = 3;

  get_register_bank((volatile int32_t **)&bank, &num_regs);
  if (num_regs < 3) {
    /* Handle error */
   }

  bank[0] = 1;
  external_wait();
  bank[0] = 0;
}

Risk Assessment

Failure to declare variables containing data that cannot be cached as volatile can result in unexpected runtime behavior resulting from compiler optimizations.

Automated Detection

Related Vulnerabilities

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

References

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Related Guidelines

Bibliography

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Image Added Image Added Image Added qualifiers" \[[Sun 05|AA. C References#Sun 05]\] [Chapter 6, "Transitioning to ISO C"|http://docs.sun.com/source/819-3688/tguide.html#pgfId-997898]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 invoke a function using a type that does not match the function definition