LinuxQuestions.org - Redirecting non-standard file descriptors

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Redirecting non-standard file descriptors

On a normal executable, the system will prepare file descriptors 0, 1, and 2 for stdin, stdout and stderr. This allows for shell commands such as:

Code:

myBin < myInputFile > myOutputFile 2> myErrorLogFile

I have several questions regarding non-standard file descriptors. For example, is this valid (I believe I have heard of programs doing this):

Code:

myBin 3< WeirdInputFile

(Does not produce errors in the shell)

How would you access such a thing in C++ (or C for that matter)? How can you query the OS to see if a given file descriptor (here 3) is open already when execution starts? I would prefer to work with <iostream> classes, but am willing to deal with C functions if I need to.

Background: I am working on a project for school. Besides being curious about this topic, I think this would simplify my testing code. The instructor is emphasizing drivers and stubs, and stubs would be much easier if each could read from it's own stream. Something like:

Code:

myBin < test42.in > test42.myout 2>test42.myerr 3< test42.firststub 4< test42.secondsstub 5< test42.thirdstub

diff test42.out test1.myout

diff test42.err test1.myerr

(And no, I think I will be using one at a time in addition to stdin for almost every possible case, but may want more once or twice.)

Any clues would be appreciated.
(And before you say it, yes I could use normal file IO but that wouldn't satisfy my curiosity. And named pipes are cool too.)

If there are any C++ experts out there, please jump right in, because when it comes to C++, I am but dust and ashes. But maybe I can help anyway.

This post is divided into two parts: the C part (for whose accuracy I can vouch), and the C++ part.

=== Part 1: the C part.

In C, you can read using low-level system calls open(), close(), read(), and so on. Or you can use high-level library calls fopen(), fclose(), fgets(), and so on. (Don't ever, ever use gets().)

The following program illustrates doing what you want for unit 3:

Code:

#define UNIT_NUMBER 3



#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>



int main(int    argc,

        char **argv

        )

{

  int  read_result;



  char buffer[48];



  FILE *phyle;



  if(argc<2)

  {

    read_result=read(UNIT_NUMBER,buffer,sizeof(buffer)-1);



    if(read_result==-1)

    {

      fprintf(stderr,"for read(), unit %d does not seem to be assigned\n",UNIT_NUMBER);



      exit(1);

    }



    buffer[read_result]=0;



    printf("The first %d bytes are these:\n%s\n",

          sizeof(buffer)-1,

          buffer

          );



    return 0;

  }

  else

  {

    phyle=fdopen(UNIT_NUMBER,"r");



    if(phyle==NULL)

    {

      fprintf(stderr,"for fdopen(), unit %d does not seem to be assigned\n",UNIT_NUMBER);



      exit(1);

    }



    if(fgets(buffer,

            sizeof(buffer),

            phyle

            )

      ==NULL

      )

    {

      fprintf(stderr,"fgets() failed\n");



      exit(1);

    }



    printf("the first line is this:\n%s",

          buffer

          );



    return 0;

  }



} /* main() */

Running that code would work like this:

Code:

evans:~$ cc tuesday.c -o tuesday

evans:~$ tuesday

for read(), unit 3 does not seem to be assigned

evans:~$ tuesday x

for fdopen(), unit 3 does not seem to be assigned

evans:~$ tuesday 3< tuesday.c

The first 47 bytes are these:

#define UNIT_NUMBER 3



#include <stdio.h>

#incl

evans:~$ tuesday x 3< tuesday.c

the first line is this:

#define UNIT_NUMBER 3

evans:~$

=== Part 2: the C++ part.

For C++, I googled:

Code:

C++ "file descriptor"

and got oodles of responses. In the first few, this seemed most promising:

Code:

http://wwwwbs.cs.tu-berlin.de/user-taipan/kraxel/gnuinfo/iostream/Files.html

Hope this helps.

It does help. Thank you for taking the time to respond. I'm still looking for the C++ way to do it. Now at least I know I have a way that I can imitate.

Thanks again.

I have since edited my first response to include a possible C++ approach. We missed by minutes. Sigh.

Quote:

The GNU C++ Iostream Library - Files
...
Constructor ifstream::ifstream (int fd)
Make an ifstream for reading from a file that was already open, using file descriptor fd. (This constructor is compatible with other versions of iostreams for posix systems, but is not part of the ansi working paper.)
...
Constructor ofstream:: ofstream (int fd)
Make an ofstream for writing to a file that was already open, using file descriptor fd.

I don't know why I didn't try this earlier. Note that it is not part of the ANSI standard. That would explain why these didn't show up on the other reference pages that I have been using.

Maybe it's too late at night, and I'm not thinking clearly, but I can't get it to compile. Here's my code:

Code:

#include <iostream>



using namespace std;



int main()

{

        istream thirdin(3);



        string buffer;



        while (thirdin >> buffer)

        {

                cout << buffer << endl;

        }

}

And my compile error:

Code:

$ g++ fdStream.cpp

fdStream.cpp: In function ‘int main()’:

fdStream.cpp:7: error: invalid conversion from ‘int’ to ‘std::basic_streambuf<char, std::char_traits<char> >*’

fdStream.cpp:7: error:  initializing argument 1 of ‘std::basic_istream<_CharT, _Traits>::basic_istream(std::basic_streambuf<_CharT, _Traits>*) [with _CharT = char, _Traits = std::char_traits<char>]’

I'm hoping that I look at this again tomorrow and laugh at a simple mistake. As it stands, it looks like that constructor does not exist!

For reference

Code:

$ g++ -v

Using built-in specs.

Target: i486-linux-gnu

Configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --program-suffix=-4.1 --enable-__cxa_atexit --enable-clocale=gnu --enable-libstdcxx-debug --enable-mpfr --with-tune=i686 --enable-checking=release i486-linux-gnu

Thread model: posix

gcc version 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)

I looked at more of the google results. I don't have the time to learn more about C++ at this point (I wish I did), but does this link help?

Code:

http://docs.sun.com/source/819-3704/bas_4928.htm

Since you are using the latest compiler you will have less luck with backward compatibility when using non-standard code.

Hya,

May be a little bit off topic. However,

Code:

$thisIsMyprogram 3< thisFileIsWhatIWantToFeedThrough3

$

works with bash, but not tcsh (csh....) At least, with my Penguin, same Penguin, only shell is different, exact same binary.

Happy Penguins!

Quote:

Originally Posted by wjevans_7d1@yahoo.co

If there are any C++ experts out there, please jump right in, because when it comes to C++, I am but dust and ashes. But maybe I can help anyway.

Wow, that is one of the most useful things I've ever encountered. Do you happen to know of it's safe to dup2 to an arbitrary int greater than 2 (provided no other files are open)? That would save me a huge hassle when vfork'ing with alternate file descriptors.

Here is what I am thinking:

Code:

//old



char *command[] = { "program", "program", NULL };



int output_temp = dup(STDOUT_FILENO);

int input_temp  = dup(STDIN_FILENO);



int out_pipes[2];

int in_pipes[2];



pipe(out_pipes);

pipe(in_pipes);



dup2(out_pipes[1], STDOUT_FILENO); //normally need some sort of I/O control starting here

dup2(in_pipes[0], STDIN_FILENO);



if (!vfork()) execv(command[0], command);



dup2(output_temp, STDOUT_FILENO);

dup2(input_temp, STDIN_FILENO);

Code:

//new



char *command[] = { "program", "program", NULL };



int out_pipes[2];

int in_pipes[2];



pipe(out_pipes);

pipe(in_pipes);



dup2(out_pipes[1], 4);

dup2(in_pipes[0], 3);



if (!vfork()) execv(command[0], command); //then command dup2's 3 -> STDIN_FILENO, etc.

Sorry, I haven't tested it since I am Windows'ing it up right now. I will later. Thanks.
ta0kira

ta0kira, let's discuss that after we've solved gd2shoe's problem. Either that, or post it in a different thread. It's amazing how easy it is for well-meaning people (including me) to hijack a thread without meaning to.

gd2shoe, how's it going? Found an answer yet?