I was implementing a POSIX interface for my library, and i discovered this solution to the stdin/errno problem that i find quite elegant:

* Like Walter already did for errno, write a little C file that accesses all C globals that are difficult to get otherwise:

#####
#include <errno.h>
#include <stdio.h>

int indigo_getErrno()
{
  return errno;
}

void indigo_setErrno(int value)
{
  errno = value;
}

FILE* indigo_getStdin()
{
  return stdin;
}

FILE* indigo_getStdout()
{
  return stdout;
}

FILE* indigo_getStderr()
{
  return stderr;
}
#####

* Link the object into Phobos/Ares, of course.

* Provide these declarations in std.c.stdio / std.c.stdlib:

#####
extern (C)
{
  private int indigo_getErrno();
  private void indigo_setErrno(int value);
  private FILE* indigo_getStdin();
  private FILE* indigo_getStdout();
  private FILE* indigo_getStderr();

  public alias indigo_getErrno errno;
  public alias indigo_setErrno errno;
  public alias indigo_getStdin stdin;
  public alias indigo_getStdout stdout;
  public alias indigo_getStderr stderr;
}
#####

* Now what's the effect of it? You can write things like this:

#####
private import std.c.stdio;
private import std.c.stdlib;
private import std.string;

int main()
{
  errno = 5;
  fprintf(stderr, "Error %i is: %s\n", errno, strerror(errno));
}
#####

Just like you would do in C!

Ciao
uwe