[SOLVED] How to convert C file descriptors to C++ fstream?

[majoron]

Hi!
Is there a way to create a fstream object with a file descriptor as argument?
One can create in standard C++ a fstream object like this:

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istream infile( "foo.bar" );

but I tried (fds is a file descriptor)

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istream infile( fds );

and it does not work.
BTW, fds is a file descriptor associated with a pipe (so I'm trying to read from a pipe, and that's why I don't have the name of the file).

However, I have seen that there are some implementations which allow it (see this, overload 3 in ifstream constructor). I know, this is AIX, but is it not possible to have it with gcc?

This is what I have:

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$ g++ -v
Leyendo especificaciones de /usr/lib/gcc/i686-pc-linux-gnu/3.4.5/specs
Configurado con: /var/tmp/portage/gcc-3.4.5-r1/work/gcc-3.4.5/configure --prefix=/usr --bindir=/usr/i686-pc-linux-gnu/gcc-bin/3.4.5 --includedir=/usr/lib/gcc/i686-pc-linux-gnu/3.4.5/include --datadir=/usr/share/gcc-data/i686-pc-linux-gnu/3.4.5 --mandir=/usr/share/gcc-data/i686-pc-linux-gnu/3.4.5/man --infodir=/usr/share/gcc-data/i686-pc-linux-gnu/3.4.5/info --with-gxx-include-dir=/usr/lib/gcc/i686-pc-linux-gnu/3.4.5/include/g++-v3 --host=i686-pc-linux-gnu --build=i686-pc-linux-gnu --disable-altivec --enable-nls --without-included-gettext --with-system-zlib --disable-checking --disable-werror --disable-libunwind-exceptions --disable-multilib --disable-libgcj --enable-languages=c,c++,f77 --enable-shared --enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu
Modelo de hilos: posix
gcc versión 3.4.5 (Gentoo Hardened 3.4.5-r1, ssp-3.4.5-1.0, pie-8.7.9)

(sorry for the spanish)

TIA and Regards

[rokstar83]

I think you are looking for the attach method.

istream infile;

infile.attach(fs);

You may need to make sure that the correct end of the pipe is closed, though i'm not sure.

[majoron]

Thanks for the reply.
However it does not seem to work.
The compiler complains saying that there is no "attach" member in std::istream (neither in std::ifstream). In fact, I cannot find such method in the whole set of C++ headers.
This is the code I tried, just in case you feel curiosity:

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#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <fstream>
#include <iostream>
#include <string>

using namespace std;

int main() {
  int fds[2];
  pid_t pid;

  pipe( fds );

  pid = fork();

  if( pid == (pid_t) 0 ) {
    close( fds[0] );
    dup2( fds[1], STDOUT_FILENO );
    execlp( "ls", "ls", 0 );
  }
  else {
    //    FILE* stream;
    close( fds[1] );
    ifstream chorro;
    chorro.attach( fds[ 0 ] );
    //    stream = fdopen( fds[0], "r" );
    char* bufar = (char*) malloc( 1024 );
    string prev = "";
    string actu = "";
    //    while( fscanf( stream, "%s", bufar ) ) {
    while( chorro >> bufar ) {
      actu = bufar;
      if( actu == prev ) break;
      cout << "fichereres: " << actu << endl;
      prev = actu;
    }

    free( bufar );
  }
  return 0;
}

(the commented code corresponds to the lines in C which make the analogous work)

Regards

[majoron]

Hi again!
I was browsing in the deep sea of libstdc++ documentation pages, and I finally found a solution.

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#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <fstream>
#include <iostream>
#include <string>
#include <ext/stdio_filebuf.h>

using namespace std;

int main() {
  int fds[2];
  pid_t pid;

  pipe( fds );

  pid = fork();

  if( pid == (pid_t) 0 ) {
    close( fds[0] );
    dup2( fds[1], STDOUT_FILENO );
    execlp( "ls", "ls", 0 );
  }
  else {
    close( fds[1] );

    __gnu_cxx::stdio_filebuf<char> churro( fds[ 0 ], ios_base::in );
    istream cherro( &churro );

    char* bufar = (char*) malloc( 1024 );
    string prev = "";
    string actu = "";

    while( cherro >> bufar ) {
      actu = bufar;
      if( actu == prev ) break;
      cout << "fichereres: " << actu << endl;
      prev = actu;
    }

    free( bufar );
  }
  return 0;
}

It basically summarizes in: 1) define a stdio_filebuf object out of a file descriptor; and 2) define a stream object (in this case was istream) from the previously defined stdio_filebuf. Once you carry out this latter step you can read from the file using streams technology, which is basically what I was looking for.

I don't know how elegant is this procedure, but I don't know any other option...

Best regards

[rokstar83]

I'm glad you found the solution but I think I figured out why it doesn't work. I think that fstream is the class that has the attach method. The derived classes, ifstream and ofstream should all have the appropriate classes. I think something like this would work.

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#include <fstream>
#include <stdio.h>
#include <string>
#include <iostream>

using namespace std;

int main() {
   int fd = open("somefile");
   string foo;
   ifstream infile;
   infile.attach(fd);
   foo << infile;
   cout << foo;  
}[\code]

[majoron]

But, can you compile the code you wrote?
I cannot. It complains saying that 'struct std::ifstream' does not have a method called 'attach'.
I also tried doing this:

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  fstream infile;

and the same error occurs (substituting 'ifstream' by 'fstream').
Regards

[01mf02]

I looked in my Stroustrup, man pages and online documentation, but there is no attach() function. I'm sorry for you.


[EDIT]
The method that works for you is extremely unelegant (never use __ stuff!). I would really try not to use it.

[rokstar83]

http://osr5doc.sco.com:1997/cgi-bin/man ... am+C%2B%2B

is a link to some man page that I saw that on. Guess its not the right man page, SCO specific?? At any rate, the man page on my machine shows a basic constructor

explicit basic_fstream(int fd);
basic_fstream(int fd, char_type *buf, int len);

for the fstream object. I'm not sure why you are not able to access them.

[majoron]

The method that works for you is extremely unelegant (never use __ stuff!). I would really try not to use it.

Thanks for the comment, but is there a better way to do it? How?

[markkuk]

Standard C++ doesn't provide any way to connect streams to UNIX file descriptors. Any solution to this problem will be implementation-specific.

[ciaranm]

Standard C++ doesn't provide any way to connect streams to UNIX file descriptors. Any solution to this problem will be implementation-specific.

Although... There is a portable way of doing it, assuming you're also on a POSIX system. See the Streams chapter in The C++ Standard Library by Josuttis for code and an explanation. You could also take a look at the Paludis PStream class, which does what you're after to a popen()ed process.

Header
Implementation
Simple test cases

[majoron]

Standard C++ doesn't provide any way to connect streams to UNIX file descriptors. Any solution to this problem will be implementation-specific.

I see... I had to have figured it out because the Stroustrup book does not comment on it.
Anyway, I'm just interested in the gnu implementation, so what is the correct way to do that whitin this implementation? Or should I forget about it, and use file descriptors?

[ciaranm]

Standard C++ doesn't provide any way to connect streams to UNIX file descriptors. Any solution to this problem will be implementation-specific.

I see... I had to have figured it out because the Stroustrup book does not comment on it.
Anyway, I'm just interested in the gnu implementation, so what is the correct way to do that whitin this implementation? Or should I forget about it, and use file descriptors?

Wrong book. Look in Josuttis instead, and write portable code. GNU internals can and do change.

[majoron]

Although... There is a portable way of doing it, assuming you're also on a POSIX system. See the Streams chapter in The C++ Standard Library by Josuttis for code and an explanation.

Thanks for the advise but I do not have this book . I only have the one by Stroustrup and "C++ primer" by Lippman&Lajoie. So if you could give me a clue about that POSIX standard way of doing it, it would be great.

You could also take a look at the Paludis PStream class, which does what you're after to a popen()ed process.

Is this Paludis the same that "emerge -s paludis" tells about? or do I have to install it without emerge?

Wrong book. Look in Josuttis instead, and write portable code. GNU internals can and do change.

Wrong book? Why? I know it is not a primer book but looks a pretty complete reference for C++ standards, doesn't it?
About portable code... yes that would be great, but if I cannot do it or if I need to install a non-standard library for making it work, it is more or less the same as writting non-portable code, isn't it?
BTW, I only need to make that code working in systems whith the GNU compiler.

Again, thanks for the help.

[ciaranm]

You could also take a look at the Paludis PStream class, which does what you're after to a popen()ed process.

Is this Paludis the same that "emerge -s paludis" tells about? or do I have to install it without emerge?

You can install it via emerge. Or you can peek at the SVN tree online using the links I provided. The latter's probably easier.

Wrong book. Look in Josuttis instead, and write portable code. GNU internals can and do change.

Wrong book? Why? I know it is not a primer book but looks a pretty complete reference for C++ standards, doesn't it?

TC++PL doesn't go into much depth on the standard library. TC++SL is the de facto reference for that.

About portable code... yes that would be great, but if I cannot do it or if I need to install a non-standard library for making it work, it is more or less the same as writting non-portable code, isn't it?

The Paludis code is easily rip-off-able, and it's GPL2ed. Just hack out the relevant bits.

BTW, I only need to make that code working in systems whith the GNU compiler.

That's the kind of attitude that leads to all sorts of things not working every time a new GCC major release is made.

[majoron]

Thanks for the insightful comments; I still have a lot to learn about C++...