Restrict user to open the same file

[vermarajeev]

You can get a FILE* from a C++ stream, you know... And then both flock and _lock_file will work.

How to convert C++ stream to FILE*......

[wysota]

You'll have to search through available documentation of ostream probably. There should be a way to access the buffer which you can then associate with a file descriptor or file handler or whatever.

[vermarajeev]

You'll have to search through available documentation of ostream probably. There should be a way to access the buffer which you can then associate with a file descriptor or file handler or whatever.

So far so good.
I have written a sample to lock a file on windows. On linux there are some problems. Please help me to make the below code work on linux too.

int main ( )
{
	FILE    *pFile = NULL;
    char    *fileName = "Logfile.log";    
 
    // Open the file in write mode
   #ifdef win32
    fopen_s(&pFile, fileName ,"r");
   #else
    pFile = fopen( fileName ,"r");
 
    if (!pFile)
    {
        printf("Error opening file %s!\n", fileName);
        exit(1);
    }
 
    // Lock the file.
   #ifdef win32
       _lock_file(pFile);
   #else
       //lock file on linux
 
     printf("Locking the file %s.\n", fileName);
 
	ifstream ifs( pFile );	//This doesnt work on linux why??? 
	char line[255];
	ifs.seekg( 0 );
	while( !ifs.eof() )
	{
		ifs.getline( line, sizeof( line ) );
		cout<<line;
	}
	return 0;
}

int main ( )
{
	FILE    *pFile = NULL;
    char    *fileName = "Logfile.log";    

    // Open the file in write mode
   #ifdef win32
    fopen_s(&pFile, fileName ,"r");
   #else
    pFile = fopen( fileName ,"r");

    if (!pFile)
    {
        printf("Error opening file %s!\n", fileName);
        exit(1);
    }

    // Lock the file.
   #ifdef win32
       _lock_file(pFile);
   #else
       //lock file on linux
       
     printf("Locking the file %s.\n", fileName);
	
	ifstream ifs( pFile );	//This doesnt work on linux why??? 
	char line[255];
	ifs.seekg( 0 );
	while( !ifs.eof() )
	{
		ifs.getline( line, sizeof( line ) );
		cout<<line;
	}
	return 0;
}

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Thanks

[wysota]

I don't see any "#endif" lines...

[vermarajeev]

I don't see any "#endif" lines...

Ok, I think I copied the wrong program....
There has to be #endif but still it doesnt work...

[wysota]

What errors do you get?

[veda]

Where is the error?????

[vermarajeev]

What errors do you get?

Ok, First the sample

int main (){
	FILE    *pFile = NULL;
	char    *fileName = "Logfile.log";
 
    // Open the file in read mode
	#ifdef win32
		fopen_s(&pFile, fileName ,"r");
	#else
		pFile = fopen( fileName ,"r");
	#endif
 
	if (!pFile){
		printf("Error opening file %s!\n", fileName);
		exit(1);
	}
 
	// Lock the file.
	#ifdef win32			
		_lock_file(pFile);	
	#else
		flockfile(pFile);   
	#endif	
 
    printf("Locking the file %s.\n", fileName);
	ifstream ifs( pFile );  //This doesnt work on linux why???
	char line[255];
	ifs.seekg( 0 );
	while( !ifs.eof() ){
		ifs.getline( line, sizeof( line ) );
		cout<<line;
	}	
 
	//unlock the file after the instance is closed
	return 0;
}

int main (){
	FILE    *pFile = NULL;
	char    *fileName = "Logfile.log";

    // Open the file in read mode
	#ifdef win32
		fopen_s(&pFile, fileName ,"r");
	#else
		pFile = fopen( fileName ,"r");
	#endif

	if (!pFile){
		printf("Error opening file %s!\n", fileName);
		exit(1);
	}
	
	// Lock the file.
	#ifdef win32			
		_lock_file(pFile);	
	#else
		flockfile(pFile);   
	#endif	
	
    printf("Locking the file %s.\n", fileName);
	ifstream ifs( pFile );  //This doesnt work on linux why???
	char line[255];
	ifs.seekg( 0 );
	while( !ifs.eof() ){
		ifs.getline( line, sizeof( line ) );
		cout<<line;
	}	

	//unlock the file after the instance is closed
	return 0;
}

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I get error on line 25 on linux machine. The compiler says there is no constructor defined.
There is no problem on windows.

I tried using both gcc and g++ compilers
Thanks

[wysota]

Could we see the exact message? It probably suggest other constructors.

[vermarajeev]

Could we see the exact message? It probably suggest other constructors.

Here it goes

/Desktop/fileLock/lock/src/lock.cpp:77: error: no matching function
for call to ‘std::basic_ifstream<char, std::char_traits<char>
>::basic_ifstream(FILE*&)’
/usr/lib/gcc/i386-redhat-linux/4.0.0/../../../../include/c++/4.0.0/fstream:442:
note: candidates are: std::basic_ifstream<_CharT,
_Traits>::basic_ifstream(const char*, std::_Ios_Openmode) [with _CharT =
char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i386-redhat-linux/4.0.0/../../../../include/c++/4.0.0/fstream:428:
note: std::basic_ifstream<_CharT, _Traits>::basic_ifstream() [with
_CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i386-redhat-linux/4.0.0/../../../../include/c++/4.0.0/iosfwd:89:
note: std::basic_ifstream<char, std::char_traits<char>
>::basic_ifstream(const std::basic_ifstream<char,
std::char_traits<char> >&)
*** Exited with status: 2 ***

[wysota]

Seems it won't be that easy. You'd probably have to dig into the base class and find a way to initialise the object properly. It'd be much faster if you got rid of C++ streams and used stdio or if you used Qt streaming classes and files.

[vermarajeev]

Seems it won't be that easy. You'd probably have to dig into the base class and find a way to initialise the object properly.

But the same constructor works fine on windows. According to me since it is a C++ stream, it should work.

It'd be much faster if you got rid of C++ streams and used stdio or if you used Qt streaming classes and files.

It'd be much faster if you got rid of C++ streams and used stdio or if you used Qt streaming classes and files.

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But then I need to change a lot of code then and you know it is really overhead.

There has to be some way...Isnt it???

[wysota]

Sure there is a way. Dig into ios::base and find a way to use your FILE* as the appropriate buffer for the stream. That's quite time consuming, so that's why I suggested to find an alternative. Substituting using Qt streams should be quite quick using search&replace tools.