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

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
}