Re: Analysis of libstdc++/2071 test case

[Peter Schmid <schmid at snake dot iap dot physik dot tu-darmstadt dot de>]

First of all, I like to thank you for your excellent analysis of the
problem.  I can confirm that reading from cin produces illegal seeks. 
The program, 

#include <iostream>

using namespace std;
int main()
{
        int i;
        cin >> i;
        cout << "i == " << i << endl;
}

included in your email, generates 18 illegal seeks, like:
_llseek(0, -5, 0xbffff62c, SEEK_CUR)    = -1 ESPIPE (Illegal seek).

If I add some error checking to your second code, check the value of
errno, and run the resulting program compiled by gcc version 3.1
20010508 (experimental) on i686-pc-linux-gnu (glibc 2.2).

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

int main ()
{
  char buf;
  char buf2;

  if (fread (&buf, 1, 1, stdin) != 1)
      perror(NULL);
  if (fseek (stdin, 0, SEEK_CUR) == -1)
      perror(NULL);
  if (fread (&buf2, 1, 1, stdin) != 1)  
      perror(NULL);;

  printf ("%c %c %d\n", buf, buf2, errno);
  return 0;
}

The output is as follows:
./a.out
1234
Illegal seek
1 2 29

If I replace the fseek with an lseek on STDIN_FILENO, the same error
message occurs.  

I believe that seeking on the stdin file descriptor is not
allowed. Therefore, the result of a program run of the above program is
not well defined.       

I see no reason why the call to fseek is not dropped when reading from
stdin.

Hope this helps,

Peter Schmid