This document describes the options available in goto-instrument for modelling the behaviour of volatile variables. The following options are provided:

--nondet-volatile
--nondet-volatile-variable <variable>
--nondet-volatile-model <variable>:<model>

By default, cbmc treats volatile variables the same as non-volatile variables. That is, it assumes that a volatile variable does not change between subsequent reads, unless it was written to by the program. With the above options, goto-instrument can be instructed to instrument the given goto program such as to (1) make reads from all volatile expressions non-deterministic (--nondet-volatile), (2) make reads from specific variables non-deterministic (--nondet-volatile-variable), or (3) model reads from specific variables by given models (--nondet-volatile-model).

Below we give two usage examples for the options. Consider the following test, for function get_celsius() and with harness test_get_celsius():

#include <assert.h>
#include <limits.h>
#include <stdint.h>
 
// hardware sensor for temperature in kelvin
extern volatile uint16_t temperature;
 
int get_celsius()
{
  if(temperature > (1000 + 273))
  {
    return INT_MIN; // value indicating error
  }
  
  return temperature - 273;
}
 
void test_get_celsius()
{
  int t = get_celsius();
  
  assert(t == INT_MIN || t <= 1000);
  assert(t == INT_MIN || t >= -273);
}

Here the variable temperature corresponds to a hardware sensor. It returns the current temperature on each read. The get_celsius() function converts the value in Kelvin to degrees Celsius, given the value is in the expected range. However, it has a bug where it reads temperature a second time after the check, which may yield a value for which the check would not succeed. Verifying this program as is with cbmc would yield a verification success. We can use goto-instrument to make reads from temperature non-deterministic:

goto-cc -o get_celsius_test.gb get_celsius_test.c
goto-instrument --nondet-volatile-variable temperature get_celsius_test.gb get_celsius_test-mod.gb
cbmc --function test_get_celsius get_celsius_test-mod.gb

Here the final invocation of cbmc correctly reports a verification failure.

However, simply treating volatile variables as non-deterministic may for some use cases be too inaccurate. Consider the following test, for function get_message() and with harness test_get_message():

#include <assert.h>
#include <stdint.h>
 
extern volatile uint32_t clock;
 
typedef struct message {
  uint32_t timestamp;
  void *data;
} message_t;
 
void *read_data();
 
message_t get_message()
{
  message_t msg;
  
  msg.timestamp = clock;
  msg.data = read_data();
  
  return msg;
}
 
void test_get_message()
{
  message_t msg1 = get_message();
  message_t msg2 = get_message();
  
  assert(msg1.timestamp <= msg2.timestamp);
}

The harness verifies that get_message() assigns non-decreasing timestamps to the returned messages. However, simply treating clock as non-deterministic would not allow to prove this property. Thus, we can supply a model for reads from clock:

// model for reads of the variable clock
uint32_t clock_read_model()
{
  static uint32_t clock_value = 0;
  
  uint32_t increment;
  __CPROVER_assume(increment <= 100);
  
  clock_value += increment;
  
  return clock_value;
}

The model is stateful in that it keeps the current clock value between invocations in the variable clock_value. On each invocation, it increments the clock by a non-determinstic value in the range 0 to 100. We can tell goto-instrument to use the model clock_read_model() for reads from the variable clock as follows:

goto-cc -o get_message_test.gb get_message_test.c
goto-instrument --nondet-volatile-model clock:clock_read_model get_message_test.gb get_message_test-mod.gb
cbmc --function get_message_test get_message_test-mod.gb

Now the final invocation of cbmc reports verification success.