volatile and non-volatile bitfields - c

I'm writing a code for Cortex-M0 CPU and gcc. I've the following structure:
struct {
volatile unsigned flag1: 1;
unsigned flag2: 1;
unsigned foo; // something else accessed in main loop
} flags;
flag1 is read and written from both GPIO interrupt handler and main loop. flag2 is only read and written in main loop.
The ISR looks like this:
void handleIRQ(void) {
if (!flags.flag1) {
flags.flag1 = 1;
// enable some hw timer
}
}
The main loop looks like this:
for (;;) {
// disable IRQ
if (flags.flag1) {
// handle IRQ
flags.flag1 = 0;
// access (rw) flag2 many times
}
// wait for interrupt, enable IRQ
}
When accessing flag2 in main loop, will the compilier optimize access to it so it won't be fetched or stored to memory every time it is read or written to in code?
It's not clear to me because to set flag1 in ISR, it will need to load whole char, set a bit and store it back.

It is my reading of the C11 standard that it is not proper to use a bitfield for this - even if both of them were declared as volatile. The following excerpt is from 3.14 Memory location:
Memory location
Either an object of scalar type, or a maximal sequence of adjacent bit-fields all having nonzero width
NOTE 1 Two threads of execution can update and access separate memory locations without interfering with each other.
NOTE 2 It is not safe to concurrently update two non-atomic bit-fields in the same structure if all
members declared between them are also (non-zero-length) bit-fields, no matter what the sizes of those
intervening bit-fields happen to be.
There is no exception given for volatile. Thus it wouldn't be safe to use the above bitfield if both threads of execution (i.e. the main and the ISR) if ISR will update one flag and the main will update another. The solution given is to add a member of size 0 in between to force them be placed in different memory locations. But then again, it would mean that both flags would consume at least one byte of memory, so it is again just simpler to use a non-bit-field unsigned char or bool for them:
struct {
volatile bool flag1;
bool flag2;
unsigned foo; // something else accessed in main loop
} flags;
Now they will be placed in different memory locations and they can be updated without them interfering with each other.
However the volatile for flag1 is still strictly necessary because otherwise updates to flag1 would be side-effect free in the main thread, and the compiler could deduce that it can keep that field in a register only - or that nothing need to be updated at all.
However, one needs to note that under C11, even the guarantees of volatile might not be enough: 5.1.2.3p5:
When the processing of the abstract machine is interrupted by receipt of a signal, the values of objects that are neither lock-free atomic objects nor of type volatile sig_atomic_t are unspecified, as is the state of the floating-point environment. The value of any object modified by the handler that is neither a lock-free atomic object nor of type volatile sig_atomic_t becomes indeterminate when the handler exits, as does the state of the floating-point environment if it is modified by the handler and not restored to its original state.
Thus, if full compatibility is required, flag1 ought to be for example of type volatile _Atomic bool; it might even be possible to use an _Atomic bitfield. Both of these require a C11 compiler, however.
Then again, you can check the manuals of your compiler if they guarantee that an access to such volatile objects is also guaranteed to be atomic.

The volatile flag for just one bit isn't all that meaningful - it is possibly even harmful. What the compiler might do in practice is to allocate two chunks of memory, possibly each 32 bits wide. Because the volatile flag blocks it from combining the two bits inside the same allocated area, since there is no bit-level access instruction available.
When accessing flag2 in main loop, will the compilier optimize access to it so it won't be fetched or stored to memory every time it is read or written to in code?
That's hard to tell, depends on how many data registers there are available. Disassemble the code and see.
Overall, bit-fields are not recommended since they are so poorly defined by the standard. And in this case, the individual volatile bit might lead to extra memory getting allocated.
Instead, you should do this:
volatile bool flag1;
bool flag2;
Assuming those flags aren't part of a hardware register, in which case the code was incorrect from the start and they should both be volatile.

Related

Read optimizations on shared memory

Suppose you have a function that make several read access to a shared variable whose access is atomic. All in running in the same process. Imagine them as threads of a process or as a sw running on bare metal platform with no MMU.
As a requirement you must ensure that the value of that read is consistent for all the length of the function so the code must not re-read the memory location and have to put in a local variable or on a register. How can we ensure that this behaviour is respected?
As an example...
shared is the only shared variable
extern uint32_t a, b, shared;
void useless_function()
{
__ASM volatile ("":::"memory");
uint32_t value = shared;
a = value *2;
b = value << 3;
}
Can value be optimized out by direct readings of shared variable in some contexts? If yes, how can I be sure this cannot happen?
As a requirement you must ensure that the value of that read is consistent for all the length of the function so the code must not re-read the memory location and have to put in a local variable or on a register. How can we ensure that this behaviour is respected?
You can do that with READ_ONCE macro from Linux kernel:
/*
* Prevent the compiler from merging or refetching reads or writes. The
* compiler is also forbidden from reordering successive instances of
* READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
* particular ordering. One way to make the compiler aware of ordering is to
* put the two invocations of READ_ONCE or WRITE_ONCE in different C
* statements.
*
* These two macros will also work on aggregate data types like structs or
* unions. If the size of the accessed data type exceeds the word size of
* the machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will
* fall back to memcpy(). There's at least two memcpy()s: one for the
* __builtin_memcpy() and then one for the macro doing the copy of variable
* - '__u' allocated on the stack.
*
* Their two major use cases are: (1) Mediating communication between
* process-level code and irq/NMI handlers, all running on the same CPU,
* and (2) Ensuring that the compiler does not fold, spindle, or otherwise
* mutilate accesses that either do not require ordering or that interact
* with an explicit memory barrier or atomic instruction that provides the
* required ordering.
*/
E.g.:
uint32_t value = READ_ONCE(shared);
READ_ONCE macro essentially casts the object you read to be volatile because the compiler cannot emit extra reads or writes for volatile objects.
The above is equivalent to:
uint32_t value = *(uint32_t volatile*)&shared;
Alternatively:
uint32_t value;
memcpy(&value, &shared, sizeof value);
memcpy breaks the dependency between shared and value, so that the compiler cannot re-load shared instead of loading value.
In the example given you are not using the variable value in the function at all. So it will definitely be optimised.
Also, as mentioned in comments, in a multitasking system, the value of shared can be changed within the function.
What I need is that shared is read only once and it local value keeped for all function length and not re-evaluated
I would suggest something like this below.
extern uint32_t a, b, shared;
void useless_function()
{
__ASM volatile ("":::"memory");
uint32_t value = shared;
a = value*2;
b = value << 3;
}
Here shared is read only once in the function. It will be read again on next call of the function.

What is the GCC builtin for an atomic set?

I see the list of builtins at https://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html. But for an atomic set, do you need to use the pair __sync_lock_test_and_set and __sync_lock_release?
I have seen this example of this on https://attractivechaos.wordpress.com/2011/10/06/multi-threaded-programming-efficiency-of-locking/.
volatile int lock = 0;
void *worker(void*)
{
while (__sync_lock_test_and_set(&lock, 1));
// critical section
__sync_lock_release(&lock);
}
But if I use this example, and do my atomic set inside the critical section, then atomic sets to different variables will be unnecessarily serialized.
Appreciate any input on how to do an atomic set where I have multiple atomic variables.
As per definition need to use both
__sync_synchronize (...)
This builtin issues a full memory barrier. type
__sync_lock_test_and_set (type *ptr, type value, ...)
This builtin, as described by Intel, is not a traditional test-and-set
operation, but rather an atomic exchange operation. It writes value
into *ptr, and returns the previous contents of *ptr. Many targets
have only minimal support for such locks, and do not support a full
exchange operation. In this case, a target may support reduced
functionality here by which the only valid value to store is the
immediate constant 1. The exact value actually stored in *ptr is
implementation defined.
This builtin is not a full barrier, but rather an acquire barrier.
This means that references after the builtin cannot move to (or be
speculated to) before the builtin, but previous memory stores may not
be globally visible yet, and previous memory loads may not yet be
satisfied.
void __sync_lock_release (type *ptr, ...)
This builtin releases the
lock acquired by __sync_lock_test_and_set. Normally this means writing
the constant 0 to *ptr. This builtin is not a full barrier, but rather
a release barrier. This means that all previous memory stores are
globally visible, and all previous memory loads have been satisfied,
but following memory reads are not prevented from being speculated to
before the barrier.
I came up with this solution. Please reply if you know a better one:
typedef struct {
volatile int lock; // must be initialized to 0 before 1st call to atomic64_set
volatile long long counter;
} atomic64_t;
static inline void atomic64_set(atomic64_t *v, long long i)
{
// see https://attractivechaos.wordpress.com/2011/10/06/multi-threaded-programming-efficiency-of-locking/
// for an explanation of __sync_lock_test_and_set
while (__sync_lock_test_and_set(&v->lock, 1)) { // we don't have the lock, so busy wait until
while (v->lock); // it is released (i.e. lock is set to 0)
} // by the holder via __sync_lock_release()
// critical section
v->counter = i;
__sync_lock_release(&v->lock);
}

Clear variable on the stack

Code Snippet:
int secret_foo(void)
{
int key = get_secret();
/* use the key to do highly privileged stuff */
....
/* Need to clear the value of key on the stack before exit */
key = 0;
/* Any half decent compiler would probably optimize out the statement above */
/* How can I convince it not to do that? */
return result;
}
I need to clear the value of a variable key from the stack before returning (as shown in the code).
In case you are curious, this was an actual customer requirement (embedded domain).
You can use volatile (emphasis mine):
Every access (both read and write) made through an lvalue expression of volatile-qualified type is considered an observable side effect for the purpose of optimization and is evaluated strictly according to the rules of the abstract machine (that is, all writes are completed at some time before the next sequence point). This means that within a single thread of execution, a volatile access cannot be optimized out or reordered relative to another visible side effect that is separated by a sequence point from the volatile access.
volatile int key = get_secret();
volatile might be overkill sometimes as it would also affect all the other uses of a variable.
Use memset_s (since C11): http://en.cppreference.com/w/c/string/byte/memset
memset may be optimized away (under the as-if rules) if the object modified by this function is not accessed again for the rest of its lifetime. For that reason, this function cannot be used to scrub memory (e.g. to fill an array that stored a password with zeroes). This optimization is prohibited for memset_s: it is guaranteed to perform the memory write.
int secret_foo(void)
{
int key = get_secret();
/* use the key to do highly privileged stuff */
....
memset_s(&key, sizeof(int), 0, sizeof(int));
return result;
}
You can find other solutions for various platforms/C standards here: https://www.securecoding.cert.org/confluence/display/c/MSC06-C.+Beware+of+compiler+optimizations
Addendum: have a look at this article Zeroing buffer is insufficient which points out other problems (besides zeroing the actual buffer):
With a bit of care and a cooperative compiler, we can zero a buffer — but that's not what we need. What we need to do is zero every location where sensitive data might be stored. Remember, the whole reason we had sensitive information in memory in the first place was so that we could use it; and that usage almost certainly resulted in sensitive data being copied onto the stack and into registers.
Your key value might have been copied into another location (like a register or temporary stack/memory location) by the compiler and you don't have any control to clear that location.
If you go with dynamic allocation you can control wiping that memory and not be bound by what the system does with the stack.
int secret_foo(void)
{
int *key = malloc(sizeof(int));
*key = get_secret();
memset(key, 0, sizeof(int));
// other magical things...
return result;
}
One solution is to disable compiler optimizations for the section of the code that you dont want optimizations:
int secret_foo(void) {
int key = get_secret();
#pragma GCC push_options
#pragma GCC optimize ("O0")
key = 0;
#pragma GCC pop_options
return result;
}

Volatile keyword in C [duplicate]

This question already has answers here:
Why is volatile needed in C?
(18 answers)
Closed 9 years ago.
I am writing program for ARM with Linux environment. its not a low level program, say app level
Can you clarify me what is the difference between,
int iData;
vs
volatile int iData;
Does it have hardware specific impact ?
Basically, volatile tells the compiler "the value here might be changed by something external to this program".
It's useful when you're (for instance) dealing with hardware registers, that often change "on their own", or when passing data to/from interrupts.
The point is that it tells the compiler that each access of the variable in the C code must generate a "real" access to the relevant address, it can't be buffered or held in a register since then you wouldn't "see" changes done by external parties.
For regular application-level code, volatile should never be needed unless (of course) you're interacting with something a lot lower-level.
The volatile keyword specifies that variable can be modified at any moment not by a program.
If we are talking about embedded, then it can be e.g. hardware state register. The value that it contains may be modified by the hardware at any unpredictable moment.
That is why, from the compiler point of view that means that compiler is forbidden to apply optimizations on this variable, as any kind of assumption is wrong and can cause unpredictable result on the program execution.
By making a variable volatile, every time you access the variable, you force the CPU to fetch it from memory rather than from a cache. This is helpful in multithreaded programs where many threads could reuse the value of a variable in a cache. To prevent such reuse ( in multithreaded program) volatile keyword is used. This ensures that any read or write to an volatile variable is stable (not cached)
Generally speaking, the volatile keyword is intended to prevent the compiler from applying any optimizations on the code that assume values of variables cannot change "on their own."
(from Wikipedia)
Now, what does this mean?
If you have a variable that could have its contents changed at any time, usually due to another thread acting on it while you are possibly referencing this variable in a main thread, then you may wish to mark it as volatile. This is because typically a variable's contents can be "depended on" with certainty for the scope and nature of the context in which the variable is used. But if code outside your scope or thread is also affecting the variable, your program needs to know to expect this and query the variable's true contents whenever necessary, more than the normal.
This is a simplification of what is going on, of course, but I doubt you will need to use volatile in most programming tasks.
In the following example, global_data is not explicitly modified. so when the optimization is done, compiler thinks that, it is not going to modified anyway. so it assigns global_data with 0. And uses 0, whereever global_data is used.
But actually global_data updated through some other process/method(say through ptrace ). by using volatile you can force it to read always from memory. so you can get updated result.
#include <stdio.h>
volatile int global_data = 0;
int main()
{
FILE *fp = NULL;
int data = 0;
printf("\n Address of global_data:%x \n", &global_data);
while(1)
{
if(global_data == 0)
{
continue;
}
else if(global_data == 2)
{
;
break;
}
}
return 0;
}
volatile keyword can be used,
when the object is a memory mapped io port.
An 8 bit memory mapped io port at physical address 0x15 can be declared as
char const ptr = (char *) 0x15;
Suppose that we want to change the value at that port at periodic intervals.
*ptr = 0 ;
while(*ptr){
*ptr = 4;//Setting a value
*ptr = 0; // Clearing after setting
}
It may get optimized as
*ptr = 0 ;
while(0){
}
Volatile supress the compiler optimization and compiler assumes that tha value can
be changed at any time even if no explicit code modify it.
Volatile char *const ptr = (volatile char * )0x15;
Used when the object is a modified by ISR.
Sometimes ISR may change tha values used in the mainline codes
static int num;
void interrupt(void){
++num;
}
int main(){
int val;
val = num;
while(val != num)
val = num;
return val;
}
Here the compiler do some optimizations to the while statement.ie the compiler
produce the code it such a way that the value of num will always read form the cpu
registers instead of reading from the memory.The while statement will always be
false.But in actual scenario the valu of num may get changed in the ISR and it will
reflect in the memory.So if the variable is declared as volatile the compiler will know
that the value should always read from the memory
volatile means that variables value could be change any time by any external source. in GCC if we dont use volatile than it optimize the code which is sometimes gives unwanted behavior.
For example if we try to get real time from an external real time clock and if we don't use volatile there then what compiler do is it will always display the value which is stored in cpu register so it will not work the way we want. if we use volatile keyword there then every time it will read from the real time clock so it will serve our purpose....
But as u said you are not dealing with any low level hardware programming then i don't think you need to use volatile anywhere
thanks

How to safely convert/copy volatile variable?

volatile char* sevensegment_char_value;
void ss_load_char(volatile char *digits) {
...
int l=strlen(digits);
...
}
ss_load_char(sevensegment_char_value);
In the above example I've got warning from avr-gcc compiler
Warning 6 passing argument 1 of 'strlen' discards 'volatile' qualifier from pointer target type [enabled by default]
So I have to somehow copy the value from volatile to non-volatile var? What is the safe workaround?
There is no such thing like a "built in" Workaround in C. Volatile tells the compiler, that the contents of a variable (or in your case the memory the variable is pointing at) can change without the compiler noticing it and forces the compiler to read the data direct from the data bus rather than using a possibly existing copy in the registers.
Therefore the volatile keyword is used to avoid odd behaviour induced through compiler optimizations. (I can explain this further if you like)
In your case, you have a character buffer declared as volatile. If your program changes the contents of this buffer in a different context like an ISR for example, you have to implement sort of a synchronisation mechanism (like disabling the particular interrupt or so) to avoid inconsistency of data. After aquiring the "lock" (disabling the interrupt) you can copy the data byte by byte to a local (non-volatile) buffer and work on this buffer for the rest of the routine.
If the buffer will not change "outside" of the context of your read accesses I suggest to omit the volatile keyword as there is no use for it.
To judge the correct solution, a little bit more information about your exact use case would be needed.
Standard library routines aren't designed to work on volatile objects. The simplest solution is to read the volatile memory into normal memory before operating on it:
void ss_load_char(volatile char *digits) {
char buf[BUFSIZE];
int i = 0;
for (i = 0; i < BUFSIZE; ++i) {
buf[i] = digits[i];
}
int l=strlen(buf);
...
}
Here BUFSIZE is the size of the area of volatile memory.
Depending on how the volatile memory is configured, there may be routines you are supposed to call to copy out the contents, rather than just using a loop. Note that memcpy won't work as it is not designed to work with volatile memory.
The compiler warning only means that strlen() will not treat your pointer as volatile, i.e. it will maybe cache the pointer in a register when computing the length of your string. I guess, that's ok with you.
In general, volatile means that the compiler will not cache the variable. Look at this example:
extern int flag;
while (flag) { /* loop*/ }
This would loop forever if flag != 0, since the compiler assumes that flag is not changed "from the outside", like a different thread. If you want to wait on the input of some other thread, you must write this:
extern volatile int flag;
while (flag) { /* loop*/ }
Now, the compiler will really look at flag each time the loop loops. This may be must more what we intended in this example.
In answer to your question: if you know what you're doing, just cast the volatile away with int l=strlen((char*)digits).

Resources