2,002 questions
Best practices
1
vote
5
replies
139
views
Ways to reliably read an object in C
I have a regular byte buffer declared in file scope:
static unsigned char buffer[32];
At some point, the function to start a DMA read operation into this buffer is called:
void func(void) {
...
- 509
0
votes
1
answer
88
views
How does the JVM instruction getfield work?
ThreadA getfield Stop.flag is false. At a certain point, another thread modifies Stop.flag to true, but ThreadA getfield Stop.flag always returns false.
I don’t understand: Under the CPU MESI protocol,...
- 17
2
votes
1
answer
113
views
Accessing Union Members with Different Qualifiers in C
In the C standard (at least, I'm looking at draft C23 WG14/N3088), section 6.7.3 Type Qualifiers states:
7 If an attempt is made to modify an object defined with a const-qualified type through the ...
- 509
6
votes
1
answer
168
views
is it valid for std::copy_n to do a memmove when a struct has volatile in it?
I have this kind of structure in my code:
struct data
{
volatile int a;
volatile int b;
};
// Some hardware that require specific access
static data *hardware_data = 0x100000;
void function(...
- 73
5
votes
2
answers
195
views
Is there a difference between the C# Volatile.ReadBarrier vs Volatile.Read?
In the released .NET 10 there is a new method added to the Volatile class with the name ReadBarrier().
And I don't know whether from the compilers perspective there is a difference if I do a read with ...
0
votes
3
answers
72
views
Trying to read character and turn it into a string
static char* convert_to_string(const volatile uint8_t *ch)
{
static char read[40]; // static so pointer remains valid after return
uint8_t i = 0;
while (*ch != ESC || *ch != END_OF_TEXT ||...
- 27
Best practices
0
votes
7
replies
173
views
How to efficiently initialize a volatile struct
I would like use bit-fields to access low level memory. I'm aware of the non-portability of bitfields but they do appear to be consistently implemented on my platform (cortex-m4) on both clang and gcc....
- 51
2
votes
0
answers
225
views
Difficulty reading volatile memory
I have code that has worked well when compiled with gcc but fails to work correctly when compiled with clang. I tracked down the issue to a read of volatile memory (on a microcontroller). I found by ...
- 51
4
votes
1
answer
129
views
Interpretation of "refer to" in 6.7.3 7 in the C standard
The standard is quite pedantic in defining any terms used in its text. For example, the term "access" is defined in the context dictionary at the beginning of the standard as modifying or ...
- 509
1
vote
0
answers
99
views
Does critical section protected by semaphore, mutex, etc, implicitly volatile? [duplicate]
Say if I have an array of integers, int array[NUM_ELEMENTS];, access to it is encapsulated as setter and getter function well protected by synchronization such as semaphore, mutex, etc, do I need to ...
- 2,301
2
votes
0
answers
104
views
Does closure variable capture introduce thread safety risks due to field implementation?
Follow-up to: Detailed Explanation of Variable Capture in Closures
Given that captured variables are implemented as fields in compiler-generated classes (as explained in the referenced answer), does ...
- 39
2
votes
1
answer
135
views
Must T be unqualified in std::allocator<T>?
gcc, clang, and msvc all reject the following code:
#include <memory>
#include <vector>
int main() {
auto _ = std::vector<int const>{}; // error
auto _ = std::vector<...
- 44.6k
4
votes
2
answers
133
views
Multiple unsequenced modifications following argument evaluation - defined or undefined behavior?
The standard states that the behavior of unsequenced operations that have side effect on the same memory location is undefined [intro.execution§10].
Does the following code have undefined behavior, ...
- 14.4k
2
votes
4
answers
151
views
Declaring volatile pointer to a pointer which points to non-volatile data
When accessing a hardware register in C/C++, I usually create #defines like this:
#define HW_REGISTER ((volatile uint16_t *)0xFFFF8A00)
I.e. 0xFFFF8A00 is a pointer to a volatile uint16_t value and ...
- 2,973
1
vote
2
answers
105
views
CppInsight redundant static_cast of temporary expression to volatile
I was using CppInsight to check an example involing decltype and volatile with the following function.
decltype(auto) triple(volatile int &f) {
f *= 3;
return f;
}
The insight turned the ...
- 101
5
votes
3
answers
242
views
Use of volatile with Interlocked.CompareExchange
I came across this blog post by Stephen Toub, demonstrating how to implement an AsyncManualResetEvent:
https://devblogs.microsoft.com/dotnet/building-async-coordination-primitives-part-1-...
- 135
2
votes
3
answers
210
views
The difference between set() and setVolatile of VarHandle
public class MyClass {
private volatile Object refValue;
private static final VarHandle REF_VALUE_HANDLE;
static {
try {
REF_VALUE_HANDLE = MethodHandles.lookup()
...
- 29
1
vote
4
answers
163
views
How to make a variable volatile in main loop but not in IR handler
I have a variable which is read from my main loop, and is both read and written from an interrupt handler. The interrupt can change the value at any time, so clearly it needs to be volatile in the ...
- 121
4
votes
0
answers
144
views
Why is a volatile symbol placed differently?
When an array is declared with volatile it ends up somewhere different than without:
static const char ARRAY[100];
$ nm a.elf | grep ARRAY
00010d00 t ARRAY
But:
static volatile const char ARRAY[100];...
- 459
1
vote
1
answer
102
views
Does assigning a volatile variable to itself suppress compiler optimization in a loop?
I'm working on a delay loop in C and come across an odd case with volatile. Consider this code:
void delay(unsigned int count){
volatile unsigned int timer = count;
while (timer > 0){
...
- 506
5
votes
2
answers
195
views
Is dereferencing a volatile null pointer value undefined behavior?
It seems unclear, but there are claims that dereferencing a nullpointer is undefined behavior:
A comment by M.M.
This note was in fact removed as the result of DR 1102 with the stated reasoning being ...
- 6,552
1
vote
1
answer
92
views
Does Volatile.Read() still apply if it is nested in a getter?
Consider this code in C#:
private int _backingField;
public int SomeNumber => Volatile.Read(ref _backingField);
public void Test_1()
{
var someNumber = Volatile.Read(ref _backingField);
/...
- 3,968
4
votes
0
answers
130
views
C#'s new "field" keyword: Are "volatile" properties now possible by calling "Volatile.Read(ref field)"?
Traditionally, using the volatile keyword on properties in C# was not possible. In other words, this was not possible:
// CS0106: The modifier 'volatile' is not valid for this item
public volatile int ...
- 3,968
2
votes
1
answer
83
views
Can a @Volatile lateinit var be atomically initialized using a DCL pattern in Kotlin?
Consider the following DCL example written in Kotlin:
@Volatile
private var property: String? = null
private val lock = ReentrantLock()
fun singletonValue(): String {
if (property == null) {
...
- 5,460
2
votes
2
answers
117
views
Java thread confinement with Executors.newSingleThreadExecutor()
The question is about memory visibility. I have some doubts about whether a Kotlin program like the next would be thread-safe:
class MyApi {
private val singleThreadExecutor = Executors....
0
votes
0
answers
67
views
Are local variables always volatile? [duplicate]
Are local variables always volatile in C#?
In other words, is the volatility of num the same in these two classes:
public sealed class ThreadSafeObjectA
{
private volatile int num;
public int ...
- 3,968
2
votes
2
answers
70
views
volatile access through cast with volatile-qualified type
I've seen constructs like the following to write to memory-mapped I/O.
*((volatile unsigned int *)0xDEADBEEF) = 0x00;
But is this guaranteed to be a volatile access?1
I started to think about this ...
- 68
1
vote
1
answer
77
views
Volatile and optimization with different compilation units
Please have a look at this more or less standard example that i see people use when talking about the usage of volatile in the context of embedded c++ firmware development on a baremetal target:
// ...
- 43
1
vote
1
answer
117
views
Perform a function only once in a method call across multiple threads in C#
I have a class from an assembly loaded at runtime that is instantiated using reflection (and therefore using a parameter-less constructor) across multiple threads (using Channels). Each thread ...
- 1,987
4
votes
1
answer
183
views
Is volatile needed with how smart compilers are these days?
Compilers are getting smarter and smarter these days. So, is volatile needed?
Compilers are smart enough to ask if we still need volatile ?
I tried many scenarios and it seemed like the compiler would ...
- 196
2
votes
3
answers
189
views
Does a C pointer to volatile treat the addressed memory as volatile when it is subscripted?
Consider these two definitions:
volatile int a[10];
volatile int *p = a;
EDIT: clarify question.
If we refer to a[3] that memory location will be treated as volatile storage, that is, the compiler ...
- 2,945
2
votes
1
answer
178
views
Java volatile and multithreading: what are the possible outputs for the program?
I've stumbled upon an old university exercise in which the aim was to reason about what can happen in absence of synchronization.
Consider the following code:
public class Main {
public ...
- 1,150
1
vote
1
answer
136
views
If I populate a dictionary and assign it to a field - can I guarantee the field will not contain a half populated dictionary?
In the below code, I populate the contents of the dictionary then assign it to Data. As such, my mental expectation is that Data will contain either null or a Dictionary with two elements in it.
But ...
- 24.1k
1
vote
2
answers
120
views
Continuous memory reads C compilation optimization questions
I was thinking about the two register timer interview question that goes as follows:
There is a hardware memory mapped timer with the value of the timer stored in two registers: one holds the most ...
- 79
-1
votes
1
answer
91
views
How to prove the volatile visibility guarantee?
I am reading an article about the Java Volatile keyword, got some questions. click here
public class SharedObject {
public int counter = 0;
}
Imagine too, that only Thread 1 increments the ...
- 29
0
votes
1
answer
158
views
Some questions about JUC
public class TestJUC {
private int x;
public void actor1(){
x = 1;
}
public void actor2() {
System.out.println(x);
}
}
If thread A executes actor1 method and ...
- 29
1
vote
1
answer
134
views
Accessing one byte of a multi-byte volatile register
Suppose we have a pointer p, of type volatile int*. Assume that this is a currently valid pointer, i.e., reading from *p performs an implementation-defined operation, and is not UB.
What can happen if ...
- 123k
1
vote
1
answer
81
views
About StoreLoad Memory Barrier
I don't understand StoreLoad.
Does store1 StoreLoad load2 mean that the CPU's store instruction cannot be reordered after StoreLoad and the load instruction cannot be reordered before StoreLoad?
If ...
- 29
1
vote
1
answer
111
views
How to stop nvcc from reordering clock instructions?
In the following snippet nvcc (CUDA 12.5) 'helpfully' reorders the clock statement. This causes the timings to be off by a factor 26x.
#include <cuda.h>
#include <stdio.h>
__device__ int ...
- 77.4k
0
votes
1
answer
127
views
LLVM kCFI sanitizer with function of volatile arguments
Here is the minimal reproducible example:
// my_func.h
typedef volatile struct {
int a;
} my_vdata_t;
typedef struct {
int a;
} my_data_t;
extern void (*vfunc)(my_vdata_t* data);
extern void (*...
0
votes
1
answer
173
views
In CUDA, why is performance similar with write-through (with 'volatile') and write-back under light load?
I think under light load, because write back requires writing data to the cache first and waiting for it to be flushed to the global memory, this mode should cause performance waste because there will ...
- 33
0
votes
2
answers
114
views
Passing `volatile` buffers to library functions which do not take `volatile`
I have a multi-processor system with a "shared" memory region used for communication. In some cases one of the processors needs to process some considerably large data in the shared memory ...
- 18.7k
3
votes
2
answers
151
views
Rationale behind declaring FILE * volatile to stdin in C
I wonder what reason might be to declare FILE *volatile fp as volatile pointer:
int main(int argc, char **argv) {
int gc;
fe_Object *obj;
FILE *volatile fp = stdin;
fe_Context *ctx = fe_open(...
- 594
11
votes
0
answers
209
views
Semantics of volatile _Atomic
Generally _Atomic does not imply semantics of volatile, i.e. operations on the atomic object are not observable side effects that the compiler needs to preserve.
As a consequence the compiler can ...
- 78.1k
18
votes
1
answer
437
views
Semantics of volatile std::atomic<T>
Generally std::atomic<T> does not imply semantics of volatile, i.e. operations on the atomic object are not observable side effects that the compiler needs to preserve.
As a consequence the ...
- 78.1k
5
votes
1
answer
455
views
Deeply understanding Volatile and Memory Barriers
I have been reading a ton on this topic over the past view days but would really like clarification on what I learned so far in relations to C# and C. Firstly, Atomicity seems to be fairly intrinsic ...
- 148
4
votes
2
answers
296
views
Volatile access of struct member
If accessing a struct member marked as volatile from an interrupt, does the whole chain of access need to be marked as volatile ?
For example
struct bar
{
volatile uint16_t a;
volatile uint16_t b;
...
- 1,464
0
votes
1
answer
109
views
Do I need to mark my getter/setter and/or class volatile if one of its members is volatile?
I run a complex code on my Teensy board which does beat detection with a microphone and displays some cool LED effects on a bicycle by reacting to them, all in C++ code. But I currently experience a ...
2
votes
1
answer
145
views
How volatile keyword works incase of PG_TRY and PG_CATCH?
Here is the below code which i have tried.
Datum sample_func(PG_FUNCTION_ARGS)
{
int32 var;
PG_TRY();
{
var = 5;
elog(ERROR, "testing");
}
PG_CATCH();
{
elog(INFO, &...
- 41
1
vote
2
answers
199
views
Java readwrite concurrency: readwrite lock vs lock + volatile
Assuming that readers = 10 x writers, which of the following solutions is better in terms of throughput. Which solution is better to use in production code?
Using single lock for set operation and ...
- 13