C++ :: Multithreading - How To Modify Shared Object
Aug 24, 2014
class MyOwner {
...
int m_count;
bool b_locked;
};
[Code] ....
I am using an API where I create many MyObjects on the heap, and the API uses a separate thread to send messages to each object. Each MyObject contains a pointer to MyOwner.
Suppose I want to keep a count of all messages received in all MyObjects: How can I do this in a thread safe way?
I am assuming that the code I have written above will not be safe--at the very least it seems that it would potentially miss message counts when it was locked--not the worse case scenario for me. I am most concerned about not causing the application to crash.
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Nov 11, 2013
i'm writing a lexer, and i want to use boost::regex for it. im sure im using it right (just in case though)
while(!this->Source.empty())
{
if(regex_search(Start, End, Match, regex(""[^"]*""), Flags))
[code].....
someone told me to download the latest source and build that so i did, by unzipping it, cding to the source, running ./bootstrap.sh, ./b2, and finally ./b2 install. when i compile i get no errors but when i run it i get ./jade: error while loading shared libraries: libboost_regex.so.1.55.0: cannot open shared object file: No such file or directory
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Apr 14, 2014
I'm having a problem trying to modify my patient's data. It seems to work, but once the block of code ends it does not save to the linked list.
Problem located in case M.
linked list template header: [URL] ...
patient header: [URL] ...
patient implementation: [URL] ...
#include <iostream>
#include "listTemplate.h"
#include "PatientRecord.h"
using namespace std;
char menu() {
char input
[Code]...
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Jan 11, 2013
I have one input file having almost trillions of records.
Input:
Key Value
a XYZ
b xyz
. ...
. ...
How to store the above data in multithreaded map and retrieve in the same manner?
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Feb 27, 2014
I have come across below code to demenostrate the use of semaphores
#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
#include <stdlib.h>
#define NITER 1000000
int count = 0;
void * ThreadAdd(void * a)
[Code] ....
Why the question of syncronising threads and mutexes and semaphores come into the picture when we join threads?
Like in the above example we have joined the first thread and then second thread so that main process will pause for the first thread to finish as its joined and then main process resumes and gives control to second thread and pauses till its finished as this thread is also joined.
Then where is question of simultaneous access?
First thread does its job and ends itself.
Second thread does its job and ends itself.
I wonder why there is need to use semaphores or mutexes in this example? I am very new to Multithreading and this question bothers me as to why we have to worry about simeltaneous access when we can join threads and can make control wait till that thread ends?
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May 11, 2014
I was writing a program and it started crashing on exit (segment fault), after the 'return 0' in main(). I figure it's an std destructor.
I started with the program I was writing and just stripped out as much as I could, while making sure the crash persisted. If I remove any of the remaining code the crash disappears, even the seemly unrelated or scoped code.
#include <iostream>
#include <fstream>
#include <condition_variable>
#include <mutex>
#include <thread>
/* Call stack results:
[Code] ....
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Feb 13, 2013
Simultaneous Reading and write from and to a text file using Multithreading in c/C++
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Nov 1, 2013
My below code is createing databases(database here is a kd tree) and indexing one image per database. I have two classes LastDatabaseTndexingPolicy and another forwardingDatabaseaccessor.cpp .
I am calling the GetDatabaseToAccess() function from forwardingDatabaseAccessor.cpp class .GetDatabaseToAccess() function is present in LastDatabaseTndexingPolicy class and it returns the database created as a pointer and using that database pointer we call another function which actually indexes the image to the database .
Now my issue is i am not able to have multiple threads act on the following functions as DatabaseAccessor_ptr db which is in the following file is coupled with two functions and however i put locks in the LastDatabaseTndexingPolicy file as below i end up getting synchronization issue ..........
Hence now i am using a single lock in forwardingDatabaseAccessor.cpp and serializing the code,. How can i change my design to parallelize this code .........
In ForwardingDatabaseAccessor.cpp we are calling function from LastDatabaseTndexingPolicy as shown below:-
DatabaseAccessor_ptr db is something which needs to be synchroinized. I tried createing 256 databases with one image each and when i run this code i ended up creating 175 databses and though i was restricting in code with locks that every database has only one image i ended up having two images in single database ..... ideally i had to get only one image per database but i got two images in few of them hence instead of 256 database this code created 175 or so databases.
indexing::IndexReturnValue ForwardDatabaseAccessor::index(cv::Mat image,
const std::string& imageName, features::Camera::Type indexCameraType,
features::Camera::Type recogCameraType) {
DatabaseAccessor_ptr db = this->IndexingPolicy_ptr->GetDBToIndex();
[Code] .....
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Sep 19, 2014
Is their a way to completely drop a shared_ptr. Then it will delete the memory and NULL all pointers linking to it?
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Apr 13, 2014
Say now I have a dll, loaded and run by a 32bit program. One of the things that I access from the program in the dll are several 1024x1024 int buffers. However I would like to put some data into those buffers from a external process. And I would like a separate thread in that external process for each buffer. Is there any way I can make that memory space accessible to the external process so I can use my own multi threaded memory transfers to pass that data over provided I ensure that the original process doesn't try to do anything with that data? And I would like to do this without resorting to the Read/WriteProcessMemory functions which are not threadsafe. In short, I want to set up direct memory access between the 2 programs without creating any intermediate shared memory buffers ie I want to set permissions for an existing memory space. Is this possible?
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Feb 9, 2013
I am not so experianced with c++ myself, but I need to evaluate if a certain idea might work.
I am working with a system for automation purposes that is running on a realtime OS in parallel to windows. Windows and the RTOS exchange data via shared memory. The application in the RTOS is compiled in C++. Now I would like to be able to influence the some data manipulation tasks in the RTOS application without changing the code of the RTOS application. So a concept like calling a dll.
My idea was to create a class with virtual methods in the RTOS application. The objects that are used should then be created on the Windows side with the same class prototype, but specific implementation of the virtual methods. The objects should then be moved to the shared memory, where they are used by the RTOS application.
Is something like this possible or am I completly on the wrong path?
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May 27, 2014
In interprocess communication(IPC) when processe have to share data among each other,why cant they all connect to one single file and share data with basic file handling functions such as read and write?
Why do we need
shared memory(shmget shmat(),shmdt()..etc)
and
mapped memory(mmap(),munmap()..etc)
concepts?
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Jul 24, 2013
All i want to do is modify a header of a file(.exe file). I just want to do if for fun and see what I can do with it.
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Jun 17, 2014
I have a std::vector<int> and I want to modify subset of the elements. I thought I might be able to use:
std::vector<int> a = { 1,2,3,4,5,6,7,8,9 };
std::vector<int> b(&a[3],&a[7]);
for(auto& each : b) {
each++;
}
for(auto& each : a) {
std::cout << each << "
";
}
but that didn't work. The elements of 'a' retain their original values. Then, I thought, "Ooo, maybe I could make 'b' a reference." Nope. What approach would be to access a subset of a vector for potential alteration?
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Jun 10, 2014
I'm trying to modify a library of a code and do not know exactly how to do it.I'm modifying the .cpp file and I know (as I have read on the internet) that I should compile it to obtain the .a file, because the main.cpp uses the .a file.I have written in the terminal the following command (founded on the internet):gcc -o analysis.hpp -c analysis.cpp but I still get an error.
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Apr 7, 2013
After the data is in the array, prompt the user to modify the array by inputting a species name and a count. If the species is not in the array, print a message indicating this and add the species to the end of the array. If the species is in the array, change the count in the array to the new count. Allow the user to input any number of changes.
This is what I have so far:
//Description of program:Manages a list of bird species and counts stored in an array of structs.
#include <fstream>
#include <algorithm>
#include <stdio.h>
#include <string>
#include <string.h>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <cstdlib>
using namespace std;
#define ARRAY_SIZE 200
[Code] ....
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Apr 22, 2014
My problem is that I have to modify only some specific details in a txt file (actually it is a wrl file but it can be considered as a txt file). There is a lot of text there but I just need to modify the point coordinates and leave the rest of the text as it is. Reading the "sample file" at some stage we will see some blocks that look like the below:
coord Coordinate {
point [
# bottom
-1.0 -1.0 1.0, #vertex 0
1.0 -1.0 1.0, #vertex 1
[Code] ....
where the numbers are (X,Y,Z) coordinates. What I have to do is to change those figures for (X,0,sqrt(X^2+Z^2)) points. Therefore the expected result (test file) will be as follows:
coord Coordinate {
point [
# bottom
-1.0 0.0 1.4142, #vertex 0
1.0 0.0 1.4142, #vertex 1
[Code] .....
So in my opinion it can be good to have two files: sample.txt and test.txt where the sample file will never be modified whereas the test.txt will be the same as sample.txt but including the modifications as explained above. The idea can be to read the sample.txt, store the info and then start copying each line in the test file until we get to the points coordinates (which needs to be modified) and continuying with this process until the next set of coordinates.
The problem is that I do not know how to change x,y,z coordinates from the sample.txt to (X,0,sqrt(X^2+Z^2)) and to copy them in the test.txt. Please see some code below and the sample.txt file attached.
//Read a Text File///////////////////////////////////////////////////////////////////////////////////////////////////
using System;
using System.IO;
namespace readwriteapp {
class Class1
[code]....
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Jan 16, 2014
I have one requirement to store an array of structure at shared memory. Also the shared memory should have one counter to store number of elements in the array.
I tried to look at some placed but didn't find anything relevant.
So my first question, is it possible that we can store two things on same shared memory. And second if not then how to achieve the same?
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Dec 8, 2014
I am trying to initialize a 2D char array of strings into POSIX shared memory to be shared between 3 other processes. There are plenty of tutorials on how to use a pointer to share a single string or an integer between processes, but I could find no examples on how to initialize 1D or 2D arrays using mmap(). I have posted what I have so far below. It is the first program, which creates the shared memory object and initialize the array char files[20][2][100] with the value files[0][0][0] = ' '. What is the proper method to initialize and share an array in C?
(program initializes shared memory object and array)
Code:
#include<fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/mman.h>
int main (int argc, char *argv[]){
[Code] ......
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Jun 27, 2014
I have trouble reading the last element in a struct.I do get the correct value for the first two elements. In my example that is:
a = 11 and c = H
however I get:
b = 0
but I am expecting b=2
I got two files to handle this.The first one is writing data to memory -
Code:
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
}
[code]....
why I cannot get correct value for the third element in my struct?In the second file where I'm reading from memory I allocate some space. Is this incorrect in some way? I'm running this on a Linux machine.
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Jul 11, 2014
I was trying to put mutex lock on shared memory but my code is not working. What I want is that if some process is putting something on shared memory segment, other process should not be able to access that segment. For testing , I create a server program which put data in shared memory and client program which access the data.
To test it, I put break point after:
pthread_mutex_lock(&(init_lock));
But I see that client program was able to access shared memory.
Below is code:
Server: Code: pthread_mutex_t init_lock = PTHREAD_MUTEX_INITIALIZER;
main()
{
char c;
[Code].....
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Aug 18, 2014
I want to know:
1. Pros and cons of shared libraries
2. How to create them(any tutorial link or book will be good)
3. Compare static libraries to dynamic ones
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Jan 10, 2013
A test program of mine loads a shared library (.so file). A function call in the shared library throws an exception that I am trying to catch in the main function of my test program. (I know that exception is being thrown for sure, I wrote the library to do that.)
However in the main program, the exception is not being caught. The flow of program goes past the catch block like no error has occurred. I am using g++ and I load the shared file using -l option. Only trying to load the program statically I got the following error:
/usr/bin/ld: cannot find -lmy-shared-library
collect2: ld returned 1 exit status
Why the exception is not getting caught.
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Aug 13, 2013
I want to create a simple shared library that exports a simple C API, just like llvm-clang.
The library should be compilable with any compiler and be used by a variety of languages, again just like clang.
For visual studio that is:
Code:
extern "C" __declspec(dllexport) void __cdecl myfunc(int arg);
The __cdecl is the issue here: i cannot seem to find a compiler agnostic way of specifying the use of cdecl.
So i went to see how my favorite C API libraries (ie clang) handle it.
none of them specify a calling convention at all!
Code:
#define CINDEX_LINKAGE __declspec(dllexport)
CINDEX_LINKAGE void clang_disposeIndex(CXIndex index); //exported
how does that work? How does the compiler know to use __cdecl and how would i link to it dynamically without that knowledge?
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Apr 12, 2013
I need to list out all the shared folders(directory) in the system along with their path . i want a command or an api in MFC, for listing out the shared directories.
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May 27, 2013
I have made a Student record system in c++ using the concept of linked list. The program will take the student id, name and marks for 3 different exams. Then it will calculate the aggregate percentage based on those exams marks and display the relevant message according to the aggregate percentage. All is going well. But there is only one problem. When I modify the student record especially the marks, it doesn't change the aggregate percentage of that specific record. It shows the old one. Similarly the relevant message doesn't change too.
Code:
struct student{
int id;
char name[MAX];
string status;
double aggr;
int matric_marks, inter_marks, entryTest_marks;
[Code] .....
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