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fflush not writing to file

Posted on 2009-05-19
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Last Modified: 2012-05-07
Hello all,
I have a C++ class that I am using to act as a real time logger for a generic server application.  When the WriteLogEntry function is called it writes the log entry to the file stream.  After writing it out, I call fflush to move it into the file on the disk, however the data is not being written to the disk until fclose is called.  

My understanding of fflush was that it flushed the stream to the end device.  Is this a correct understanding?  If so what is going wrong here?  If not, how to I get the data written to disk without having to call fclose?  I need to be able to use the log file as a diagnostic tool without shutting down the server to get the data to the log.

Thanks much.
**************************
ServerLog.h
**************************
#ifndef SERVER_LOG_H
#define SERVER_LOG_H
 
#include <stdio.h>
#include <boost/algorithm/string.hpp>
 
using namespace std;
using namespace boost;
 
class ServerLog
{
public:
	static ServerLog* Instance();
	void WriteLogEntry(string entryData);
	~ServerLog(void);
 
protected:
	ServerLog(void);
 
private:
	static ServerLog* instance;
	static FILE* serverLogFile;
};
 
#endif /* SERVER_LOG_H */
 
**************************
ServerLog.cpp
**************************
#include "ServerLog.h"
 
FILE* ServerLog::serverLogFile = 0;
ServerLog* ServerLog::instance = 0;
 
ServerLog* ServerLog::Instance()
{
	if( instance == 0 )
	{
		instance = new ServerLog();
	}
	return instance;
}
 
ServerLog::ServerLog(void)
{
	serverLogFile = fopen("lcs_system.log", "a");
	int result = fflush(serverLogFile);
}
 
ServerLog::~ServerLog(void)
{
	fclose(serverLogFile);
}
 
void ServerLog::WriteLogEntry(string entryData)
{
	// todo: check write - throw exception if not correct
	int bytesWritten = fwrite(entryData.c_str(), sizeof(char), entryData.length(), serverLogFile);
}

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Question by:edc
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evilrix earned 2000 total points
ID: 24426916
You have to call fflush after every write, not just when you open the file. Everytime you call it the write buffer will be flushed. In your code example you only flush when you open the file, what about in WriteLogEnrty()?
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by:evilrix
ID: 24426925
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Author Comment

by:edc
ID: 24426937
I don't freakin' believe I did that.  Thanks evilrix for pointing that out.  I'm going to go feel stupid for a few days :o).
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LVL 40

Expert Comment

by:evilrix
ID: 24426949
>> I don't freakin' believe I did that.
Sometime it just needs anotehr pair of eye -- I do things like that all the time :)

>> Thanks evilrix for pointing that out.
Very welcome
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