[英]Multi-threaded file reading produces the same result for each thread
基本上,標題中是我遇到的問題,我試圖創建一個多線程應用程序來讀取和總結文件的內容,這可以在一個線程中正常工作。 但是,當引入更多時,它們的輸出相同。 我該如何解決?
編碼
void *sumThread(void *);
pthread_mutex_t keepOut = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t keepOutSum = PTHREAD_MUTEX_INITIALIZER;
int counter = 0, line_count = 0;
char* loc;
double total = 0;
void split(const string& s, char c, vector<string>& v)
{
string::size_type i = 0;
string::size_type j = s.find(c);
while (j != string::npos)
{
v.push_back(s.substr(i, j - i));
i = ++j;
j = s.find(c, j);
if (j == string::npos)
v.push_back(s.substr(i, s.length()));
}
}
int main(int argc, char* argv[])
{
if (argc < 2)
{
cerr << "Usage: " << argv[0] << " filename" << endl;
return 1;
}
string line;
loc = argv[1];
ifstream myfile(argv[1]);
myfile.unsetf(ios_base::skipws);
line_count = std::count(std::istream_iterator<char>(myfile),
std::istream_iterator<char>(),
'\n');
myfile.clear();
myfile.seekg(-1, ios::end);
char lastChar;
myfile.get(lastChar);
if (lastChar != '\r' && lastChar != '\n')
line_count++;
myfile.setf(ios_base::skipws);
myfile.clear();
myfile.seekg(0, ios::beg);
pthread_t thread_id[NTHREADS];
for (int i = 0; i < NTHREADS; ++i)
{
pthread_create(&thread_id[i], NULL, sumThread, NULL);
}
for (int i = 0; i < NTHREADS; ++i)
{
pthread_join(thread_id[i], NULL);
}
cout << setprecision(2) << fixed << total << endl;
return 0;
}
void *sumThread(void *)
{
pthread_mutex_lock(&keepOut);
int threadNo = counter;
counter++;
pthread_mutex_unlock(&keepOut);
ifstream myfile(loc);
double runningTotal = 0;
string line;
if (myfile.is_open())
{
for (int i = threadNo; i < line_count; i += NTHREADS)
{
vector < string > parts;
getline(myfile, line);
// ... and process out the 4th element in the CSV.
split(line, ',', parts);
if (parts.size() != 3)
{
cerr << "Unable to process line " << i
<< ", line is malformed. " << parts.size()
<< " parts found." << endl;
continue;
}
// Add this value to the account running total.
runningTotal += atof(parts[2].c_str());
}
myfile.close();
}
else
{
cerr << "Unable to open file";
}
pthread_mutex_lock(&keepOutSum);
cout << threadNo << ": " << runningTotal << endl;
total += runningTotal;
pthread_mutex_unlock(&keepOutSum);
pthread_exit (NULL);
}
樣品輸出
2: -46772.4
0: -46772.4
1: -46772.4
3: -46772.4
-187089.72
每個線程應該讀取並總結文件中的數字,然后在完成后將它們加在一起。 但是,即使threadNo變量與輸出中指示的明顯不同,所有線程似乎都返回相同的數字。
您的問題在這里:
for (int i = threadNo; i < line_count; i += NTHREADS) {
vector<string> parts;
getline(myfile, line);
getline()不知道i
的值,因此它仍從文件中讀取相鄰的行,而不會跳過任何行。 因此,所有線程都讀取文件的相同的前幾行。
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