Thread: problem with streambuf

Hello experts,

i have one query which i am not able to solve please suggest me my problem is i have to open a filedescriptor and have to store the contents in streambuffer,

i,e

Code:

int main () {

 char ch;
 streambuf * pbuf;size_t size;
  ifstream istr(const char *)fd);
  pbuf = istr.rdbuf();

  while (pbuf->sgetc()!=EOF)
  {
     ch = pbuf->sbumpc();
     cout << ch;
  }

  istr.close();

  return 0;
}

but here the line

Code:

ifstream istr(const char *)fd);

is not properly interpreting thats why there is no out put

if i simply give

Code:

ifstream istr(fd);

it gives me error as this method does not exist,
please helop me what to do but i want the result in <b>streambuffer</b> because this is the input for some other module in my project pleasesuggest what are the different way i can get so the i get the contents in <b>streambuffer.</b>

thanks in advance.

There is no standard C++ method of reading from a UNIX fd. You need to use the native read() function and read directly into the streambuf's buffer.

I don't use streambufs enough to tell you how to do that. Somebody else might be able to answer better.

thank u for reply,

but please help me any experts who knows the answer as i am struck at this point very badly , any way to open the file descriptor so that the contents should gt stored in streambuffer, please help me.

please any experts help me.............................

I did this awhile ago to help someone out, but I make no claim as to its robustness. It was more of an example than anything. But, here you go:

Code:

#ifndef H_FDIO
#define H_FDIO 1

#include <algorithm>                                    // std::min
#include <cerrno>                                       // errno, EINTR
#include <cstdio>                                       // BUFSIZ, EOF
#include <cstring>                                      // std::memcpy
#include <istream>                                      // std::streambuf, std::[i,o,io]stream

extern "C" {
#include <unistd.h>                                     // read, write
}

namespace fdio {
        // derived streambuf class, full support for buffered reading and writing
        class streambuf : public std::streambuf {
        protected:
                int fd;                                 // file descriptor

                enum { BACKSIZE = 8 };                  // putback area size
                enum { PUTSIZE = BUFSIZ };              // put area size
                enum { GETSIZE = BUFSIZ };              // get area size

                char pbuf[PUTSIZE];                     // put area

                char gbuf[GETSIZE + BACKSIZE];          // get and putback areas
                char *const gbase;                      // get area base pointer

                // -- flush: output to destination.
		//           returns number of characters sent,
		//           or EOF upon failure
                int flush() {
                        // calculate # characters to send
                        std::ptrdiff_t n = pptr() - pbase();

                        // send characters to destination
                        if ( write(fd, pbase(), n) != n )
                                return EOF;

                        // reset put pointer
                        pbump(-n);
                        return n;
                }

                // -- overridden virtual functions
                virtual int sync() {
                        return flush() == EOF ? -1 : 0;
                }

                virtual int overflow(int c) {
                        if ( c != EOF ) {
                                *pptr() = c;            // use last free space (see ctor)
                                pbump(1);
                        }
                        return flush() == EOF ? EOF : c;
                }

                virtual int underflow() {
                        if ( gptr() < egptr() )         // characters available in get area?
                                return *gptr();

                        // set putback area
                        std::ptrdiff_t nback = std::min<std::ptrdiff_t>(gptr() - eback(), BACKSIZE);
                        std::memcpy(gbase - nback, gptr() - nback, nback);

                        // read new characters
                        ssize_t nread = read(fd, gbase, GETSIZE);
                        if ( nread <= 0 )
                                return EOF;

                        // reset get area pointers
                        setg(gbase - nback, gbase, gbase + nread);
                        return *gptr();
                }

        public:
                // -- ctor
                streambuf(int file_descriptor)
                        : fd    (file_descriptor)
                        , gbase (gbuf + BACKSIZE)
                {
                        setg(gbase, gbase, gbase);      // trigger underflow call on first read
                        setp(pbuf, pbuf+PUTSIZE-1);     // one space left for overflow
                }

                // -- xtor
                virtual ~streambuf() {
                        sync();
                }
        };

        // derived ostream class
        class ostream : public std::ostream {
        public:
                ostream(int fd)
                        : std::ostream  (0)
                        , sbuf          (fd)
                { init(&sbuf); }

        protected:
                fdio::streambuf sbuf;
        };

        // derived istream class
        class istream : public std::istream {
        public:
                istream(int fd)
                        : std::istream  (0)
                        , sbuf          (fd)
                { init(&sbuf); }

        protected:
                fdio::streambuf sbuf;
        };

        // derived iostream class
        class iostream : public std::iostream {
        public:
                iostream(int fd)
                        : std::iostream (0)
                        , sbuf          (fd)
                { init(&sbuf); }

        protected:
                fdio::streambuf sbuf;
        };
}

#endif // H_FDIO

And a cheap example program to exercise and show it's capabilities:

Code:

#include "fdio.h"

void hello(fdio::streambuf&);
void world(fdio::streambuf&);

int main() {
    fdio::streambuf out(1);
    hello(out);
    world(out);
    out.pubsync();
}

void hello(fdio::streambuf& out) {
    out.sputc('H');
    out.sputc('e');
    out.sputn("llo ", 4);
}

void world(fdio::streambuf& out) {
    out.sputc('W');
    out.sputn("orld!", 5);
    out.sputc('\n');
}

Like I said....no warranties here. I just pulled it out of some old stuff. You can use the [i,o,io]stream classes if you want formatted I/O as well. Good Luck!

I'd recommend Josuttis' book on the C++ STL for more about using streambuf and friends. I have another book I actually like better, called Standard C++ IOStreams, but that's alot to chew at first.

thank you very much for u r valueble inputs,

here is the another question like:

am trying this way also

Code:

streambuf * pbuf;
  ifstream istr; 
pbuf=istr.rdbuf()->open(fd);

this code is successfull in some times and fails some time can anyone
tell me what is the main reason.

Originally Posted by vin_pll

thank you very much for u r valueble inputs,

here is the another question like:

am trying this way also

Code:

streambuf * pbuf;
  ifstream istr; 
pbuf=istr.rdbuf()->open(fd);

this code is successfull in some times and fails some time can anyone
tell me what is the main reason.

Without anymore information, I would have to guess that you are using open wrong. I don't remember ever seeing an open member function that accepts an already open file descriptor. You will need to explain what happens when the code fails, but make sure that the open member function is okay with accepting an already open file descriptor.

thank you whoie,
for u r inputs when code fails it return 0 and if successfully opens it gives me no of characters in the fd.

again ,as that of u r previous example you sent, i can do

Code:

ifstream f(fd);

but i want the output in

Code:

streambuf *s

which is not acceptable there please if you can simply it will be great favour even i am also trying hard for that ,
any how thanks in adavance for helping me this much.

Hello whoie,

thank you very much for u r fdio.h , iwas able to solve that problem ,
now my streams are reading the file descriptors successfully.

Originally Posted by vin_pll

Hello whoie,

thank you very much for u r fdio.h , iwas able to solve that problem ,
now my streams are reading the file descriptors successfully.

A word of caution from someone whose had to compile across multiple platforms: The code you have right now may not run on any other platforms that the current one you're using to work on your current project.

thank u for u r inputs,

even my task is to run this code platform independently , as u said code will not work please may i know the reason for this and what can be the alternate solution for this.

expecting your response.