How to safely operate on parameters in threads, using C++ & Pthreads?
- by ChrisCphDK
Hi.
I'm having some trouble with a program using pthreads, where occassional crashes occur, that could be related to how the threads operate on data
So I have some basic questions about how to program using threads, and memory layout:
Assume that a public class function performs some operations on some strings, and returns the result as a string. The prototype of the function could be like this:
std::string SomeClass::somefunc(const std::string &strOne, const std::string &strTwo)
{
//Error checking of strings have been omitted
std::string result = strOne.substr(0,5) + strTwo.substr(0,5);
return result;
}
Is it correct to assume that strings, being dynamic, are stored on the heap, but that a reference to the string is allocated on the stack at runtime?
Stack: [Some mem addr] pointer address
to where the string is on the heap
Heap: [Some mem addr] memory
allocated for the initial string which
may grow or shrink
To make the function thread safe, the function is extended with the following mutex (which is declared as private in the "SomeClass") locking:
std::string SomeClass::somefunc(const std::string &strOne, const std::string &strTwo)
{
pthread_mutex_lock(&someclasslock);
//Error checking of strings have been omitted
std::string result = strOne.substr(0,5) + strTwo.substr(0,5);
pthread_mutex_unlock(&someclasslock);
return result;
}
Is this a safe way of locking down the operations being done on the strings (all three), or could a thread be stopped by the scheduler in the following cases, which I'd assume would mess up the intended logic:
a. Right after the function is called, and the parameters: strOne & strTwo have been set in the two reference pointers that the function has on the stack, the scheduler takes away processing time for the thread and lets a new thread in, which overwrites the reference pointers to the function, which then again gets stopped by the scheduler, letting the first thread back in?
b. Can the same occur with the "result" string: the first string builds the result, unlocks the mutex, but before returning the scheduler lets in another thread which performs all of it's work, overwriting the result etc.
Or are the reference parameters / result string being pushed onto the stack while another thread is doing performing it's task?
Is the safe / correct way of doing this in threads, and "returning" a result, to pass a reference to a string that will be filled with the result instead:
void SomeClass::somefunc(const std::string &strOne, const std::string &strTwo, std::string result)
{
pthread_mutex_lock(&someclasslock);
//Error checking of strings have been omitted
result = strOne.substr(0,5) + strTwo.substr(0,5);
pthread_mutex_unlock(&someclasslock);
}
The intended logic is that several objects of the "SomeClass" class creates new threads and passes objects of themselves as parameters, and then calls the function: "someFunc":
int SomeClass::startNewThread()
{
pthread_attr_t attr;
pthread_t pThreadID;
if(pthread_attr_init(&attr) != 0)
return -1;
if(pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED) != 0)
return -2;
if(pthread_create(&pThreadID, &attr, proxyThreadFunc, this) != 0)
return -3;
if(pthread_attr_destroy(&attr) != 0)
return -4;
return 0;
}
void* proxyThreadFunc(void* someClassObjPtr)
{
return static_cast<SomeClass*> (someClassObjPtr)->somefunc("long string","long string");
}
Sorry for the long description. But I hope the questions and intended purpose is clear, if not let me know and I'll elaborate.
Best regards.
Chris
