Writing (logging) into same file from different threads , different functions? - c++

Question

In C++ is there any way to make the writing into file thread safe in the following scenario ?
void foo_one(){
lock(mutex1);
//open file abc.txt
//write into file
//close file
unlock(mutex1);
}
void foo_two(){
lock(mutex2);
//open file abc.txt
//write into file
//close file
unlock(mutex2);
}
In my application (multi-threaded) , it is likely that foo_one() and foo_two() are executed by two different threads at the same time .
Is there any way to make the above thread safe ?
I have considered using the file-lock ( fcntl and/or lockf ) but not sure how to use them because fopen() has been used in the application ( performance reasons ) , and it was stated somewhere that those file locks should not be used with fopen ( because it is buffered )
PS : The functions foo_one() and foo_two() are in two different classes , and there is no way to have a shared data between them :( , and sadly the design is such that one function cannot call other function .

Answer 1

Add a function for logging.
Both functions call the logging function (which does the appropriate locking).
mutex logMutex;
void log(std::string const& msg)
{
RAIILock lock(logMutex);
// open("abc.txt");
// write msg
// close
}

Answer 2

If you really need a logger, do not try doing it simply by writing into files and perhaps use a dedicated logger, thus separating the concerns away from the code you're writing. There's a number of thread-safe loggers: the first one that comes to mind: g2log. Googling further you'll find log4cplus, a discussion here, even a minimalist one, +1

Answer 3

If the essence of functions foo_one() and foo_two() are only to open the file, write something to it, and close it, then use the same mutex to keep them from messing each other up:
void foo_one(){
lock(foo_mutex);
//open file abc.txt
//write into file
//close file
unlock(foo_mutex);
}
void foo_two(){
lock(foo_mutex);
//open file abc.txt
//write into file
//close file
unlock(foo_mutex);
}
Of course, this assumes these are the only writers. If other threads or processes write to the file, a lock file might be a good idea.

Answer 4

You should do this, have a struct with a mutex and a ofstream:
struct parser {
ofstream myfile
mutex lock
};
Then you can pass this struct (a) to foo1 and foo2 as a void*
parser * a = new parser();
initialise the mutex lock, then you can pass the struct to both the functions.
void foo_one(void * a){
parser * b = reinterperet_cast<parser *>(a);
lock(b->lock);
b->myfile.open("abc.txt");
//write into file
b->myfile.close();
unlock(b->mutex);
}
You can do the same for the foo_two function. This will provide a thread safe means to write to the same file.

Answer 5

Try this code. I've done this with MFC Console Application
#include "stdafx.h"
#include <mutex>
CWinApp theApp;
using namespace std;
const int size_ = 100; //thread array size
std::mutex mymutex;
void printRailLock(int id) {
printf("#ID :%", id);
lock_guard<std::mutex> lk(mymutex); // <- this is the lock
CStdioFile lastLog;
CString logfiledb{ "_FILE_2.txt" };
CString str;
str.Format(L"%d\n", id);
bool opend = lastLog.Open(logfiledb, CFile::modeCreate | CFile::modeReadWrite | CFile::modeNoTruncate);
if (opend) {
lastLog.SeekToEnd();
lastLog.WriteString(str);
lastLog.Flush();
lastLog.Close();
}
}
int main()
{
int nRetCode = 0;
HMODULE hModule = ::GetModuleHandle(nullptr);
if (hModule != nullptr)
{
if (!AfxWinInit(hModule, nullptr, ::GetCommandLine(), 0))
{
wprintf(L"Fatal Error: MFC initialization failed\n");
nRetCode = 1;
}
else
{
std::thread threads[size_];
for (int i = 0; i < size_; ++i) {
threads[i] = std::thread(printRailLock, i + 1);
Sleep(1000);
}
for (auto& th : threads) { th.hardware_concurrency(); th.join(); }
}
}
else
{
wprintf(L"Fatal Error: GetModuleHandle failed\n");
nRetCode = 1;
}
return nRetCode;
}
Referance:
http://www.cplusplus.com/reference/mutex/lock_guard/
http://www.cplusplus.com/reference/mutex/mutex/
http://devoptions.blogspot.com/2016/07/multi-threaded-file-writer-in-c_14.html