ifstream to FILE*

[vermarajeev]

Hi,
Is there a way to convert ifstream to FILE*;

Something like this

ifstream ifs( " test.txt ");
if( !ifs.is_opsn() )
   return 0;
 
FILE* f = ifs; //how can I convert ifs to FILE*
 
 

Thanks

[AdrianH]

Hi,
Is there a way to convert ifstream to FILE*;

Something like this

ifstream ifs( " test.txt ");
if( !ifs.is_opsn() )
   return 0;
 
FILE* f = ifs; //how can I convert ifs to FILE*
 
 

Thanks

Convert it to a file descriptor using .fd() and then to a FILE stream using fdopen().

See here for more detailed information.


Adrian

[weaknessforcats]

Why???

an ifstream is already a FILE*. If you look deep enough in the template you will find it.

The whole point of streams is to hide the FILE* so the stream can be used with the various operators in an object-oriented fashion.

[AdrianH]

Why???

an ifstream is already a FILE*. If you look deep enough in the template you will find it.

The whole point of streams is to hide the FILE* so the stream can be used with the various operators in an object-oriented fashion.

There are many reasons, including (but not limited to) interfacing with legacy code.


Adrian

[weaknessforcats]

Good point. I forgot about the legacy code. Time for more coffee.

[Savage]

Good point. I forgot about the legacy code. Time for more coffee.

Ha,ha.memory leak I would say.

PS:Subscribing(Intresting thread)

Savage

[vermarajeev]

Convert it to a file descriptor using .fd() and then to a FILE stream using fdopen().

See here for more detailed information.


Adrian

Did you try the code, it dont work for me. I tried this

int main ( )
{
    fstream fs( "test.txt", ios::in );
    if( fs.is_open() )
      return 0;        
    FILE* f = fs.fd(); 
    return 0;
}

I get this error

error C2039: 'fd' : is not a member of 'std::basic_fstream<_Elem,_Traits>'

I need FILE pointer from a C++ stream because, I need to lock the file and I came across this function

void _lock_file(
   FILE* file
);

which takes FILE* but in my application I make use of C++ stream to read and write to a file which I cannot change.

Can you correct me if I'm doing something wrong above....
Thanks for your understanding

[vermarajeev]

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;  
      } 

[vermarajeev]

Sorry, there are #endif missing

[AdrianH]

It would appear that the functions I linked to you are not standard. Sorry.


Adrian