[c++] How can I get a file descriptor from a c++ i/o stream?

chuanyung · 03-05-2004, 01:56 AM

Dear all,

Because of some reason, I must get a file descript from a c++ I/O stream,
for example: ifstream.

In VC++, I found it have a function ifstream::fd() to do this.
But in Linux, there is not a such function of ifstream class.
Is there not a such function in standard C++ ?
Or is there another way to get?

Thanks for your help.
Regards,
cylin.

itsme86 · 03-05-2004, 02:01 AM

I don't know much about C++ or i/o streams, but you might be able to use fileno(). It works on 'FILE *' objects anyway (i.e. fd = fileno(stdin)).

itsme86 · 03-05-2004, 02:17 AM

I found this in the fstream.h header file. Maybe it's useful to you:

Code:

extern "C++" {
class fstreambase : virtual public ios {
#ifdef _IO_NEW_STREAMS
    mutable filebuf __my_fb; // mutable so rdbuf() can be const
#endif
    void __fb_init ();
  public:
    fstreambase();
    fstreambase(int fd);
    fstreambase(int fd, char *p, int l); /* Deprecated */
    fstreambase(const char *name, int mode, int prot=0664);
    void close();
#ifdef _IO_NEW_STREAMS
    filebuf* rdbuf() const { return &__my_fb; }
#else
    filebuf* rdbuf() const { return (filebuf*) ios::rdbuf(); }
#endif
    void open(const char *name, int mode, int prot=0664);
    int is_open() const { return rdbuf()->is_open(); }
    void setbuf(char *ptr, int len) { rdbuf()->setbuf(ptr, len); }
    void attach(int fd);
#ifdef _STREAM_COMPAT
    int filedesc() { return rdbuf()->fd(); }
    fstreambase& raw() { rdbuf()->setbuf(NULL, 0); return *this; }
#endif
};

The section I think you'd be interested in is in bold. If it's not anything useful then I'm sorry for wasting space

zmieff · 03-05-2004, 10:50 AM

first, include the header files

Code:

#include <fstream>
#include <iostream>
using namespace std;

then, the main function:

Code:

int main(void)
{
 ofstream output;  //this builds an instance of the class ofstream
 
 output.open("\/home\/user\/test.txt"); //open/create the file
 
 output <<"This is a Test" << endl;
       
  output.close();
  return 0;

}

hope that helps

Hko · 03-05-2004, 08:23 PM

I don't know the C++ classes very well, but you may want to have a look at this info-doc:

shell$ info iostream Streambuf

lyle_s · 03-07-2004, 03:22 PM

The fd() function isn't in the C++ standard. C++ implementations can provide it as an extension, but you should avoid it because you can't count on it being there (as you're finding out).

Lyle

Tinkster · 03-08-2004, 05:41 PM

As a more specific addition to what the
others have said (and because it's good
reading ;}) ....

http://www.icce.rug.nl/documents/cplusplus/

Have a look at chapter 5

Cheers,
Tink

chuanyung · 03-08-2004, 08:55 PM

Thanks all,

I got it.

Regards,
cylin.

infamous41md · 03-08-2004, 11:23 PM

thanks for that link tinkster i 've been looking for sometihg like that, most C++ tutorials seemed to be geared torwards newbs, but that one is great for C hackers.