std::ifstream::readsome not reading requested number of chars from stream.

[Howind, Axel]

Hi,

I'm having some trouble with std::ifstream. I would expect
std::ifstream::readsome to extract the requested number of bytes/chars
from the input stream unless end of stream is reached. However, reading
64 chars at a time, readsome only returns 63 chars when reaching 8192
bytes total. This does not occur with stringstreams. I can reproduce
this with different systems/compiler versions (gcc 3.4.6 on CentOS 4.4
and Redhat EL 4 as well as gcc 4.1/Ubuntu 7.04).

Seems like readsome just reads until the internal buffer is empty and in
this case returns less than the requested count of chars. I hope the
attached code makes clear what I mean. I would expect the loop only to
be exited when all data is read or an error occured.

Is this correct behaviour, or is this a bug in libstd++?

Thanks,
Axel


    $ g++ bug.cpp -o bug
    $ ./bug 0
    n = 0
    read 0 bytes total
    tellg: 0
    $ ./bug 123
    n = 123
    read 123 bytes total
    tellg: 123
    $ ./bug 8191
    n = 8191
    read 8191 bytes total
    tellg: 8191
    $ ./bug 8192
    n = 8192
    read 8191 bytes total
    tellg: 8191
    $ ./bug 10000
    n = 10000
    read 8191 bytes total       # why not 10000 here?
    tellg: 8191
    $ wc -c test.txt 
    10000 test.txt

Code:
bug.cpp ----8<----8<----8<----

#include <iostream>
#include <fstream>
int main(int argc, char** argv)
{
  int n = 1024;
  if ( argc == 2 )
    n = std::atoi(argv[1]);

  std::cout << "n = " << n << "\n";

  std::ofstream out ( "test.txt" );
  for ( int i=0; i<n; i++ )
    out.put ( '\0' );
  out.close();

  char buffer[64];
  std::ifstream in ( "test.txt" );
  int bytes, bytes_total = 0;
  while ( ( bytes = in.readsome ( &buffer[0], 64 ) ) == 64 )
  {
    bytes_total += bytes;
  }
  bytes_total += bytes;

  std::cout << "read " << bytes_total << " bytes total\n";
  std::cout << "tellg: " << in.tellg() << "\n";
}

---->8---->8---->8---->8----