Re: libstdc++/5180: seekp() on fstreams opened in read only mode

Re: libstdc++/5180: seekp() on fstreams opened in read only mode

[bkoz]

Synopsis: seekp() on fstreams opened in read only mode

State-Changed-From-To: feedback->closed
State-Changed-By: bkoz
State-Changed-When: Mon Apr  8 23:07:37 2002
State-Changed-Why:
    The infinte loop is user error, I'm afraid. A better way to write that loop would be:
    
      while (in.good())
        {
          string s;
          getline(in, s);
          cout << s << endl;;
        }
    
    Anyway.
    
    This bug report actually crosses the paths of two library bugs, so this is perhaps why you might see differing interpretations of this code with other compilers and toolchains. 
    
    To wit:
    
      fstream in(name, std::ios::in);
      in.seekp(0); 
      state = in.rdstate();
    
    will give different results. I think state should be failbit, and here's why:
    
        basic_ostream::seekp(pos_type __pos)
        // DR 136
        this->rdbuf()->pubseekpos(__pos, ios_base::out);
        
    then
    
      filebuf::seekpos
      // DR 171
    
    best,
    benjamin

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=5180

Re: libstdc++/5180: seekp() on fstreams opened in read only mode

[paolo]

Synopsis: seekp() on fstreams opened in read only mode

Responsible-Changed-From-To: unassigned->paolo
Responsible-Changed-By: paolo
Responsible-Changed-When: Mon Dec 24 01:45:56 2001
Responsible-Changed-Why:
    Taking care of it.
State-Changed-From-To: open->feedback
State-Changed-By: paolo
State-Changed-When: Mon Dec 24 01:45:56 2001
State-Changed-Why:
    I think the behaviour you are reporting is standard
    conforming. In fact, after seekp(0) the failbit is set
    and the standard prescribes that all IOStreams operations
    have no effect once the stream state indicates an error
    situation. Compare the behaviour of the testcase with that
    (identical) of:
    
    #include <iostream>
    #include <fstream>
    #include <string>
    
    using namespace std;
    
    int main( int argc, char** argv )
    {
    
      if( argc != 2)
        return 1;
    
      fstream in( argv[1], std::ios::in );
    
      if( !in )
        {
          cout << "cannot open file " << endl;
          return 2;
        }
    
      /*
      in.seekp(0); // <<<<<<<
    
      if (in.rdstate() & ios::failbit)
        {
          cout << "fail" << endl;
          in.clear();
        }
      */
    
      while( !in.eof() )
        {
          string s;
          //getline( in, s );
          cout << s << endl;;
        }
    
      return 0;
    };

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=5180

libstdc++/5180: seekp() on fstreams opened in read only mode

[kingleo]

>Number:         5180
>Category:       libstdc++
>Synopsis:       seekp() on fstreams opened in read only mode
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Dec 23 08:36:01 PST 2001
>Closed-Date:
>Last-Modified:
>Originator:     Martin Oberzalek
>Release:        gcc version 3.0.3
>Organization:
>Environment:
debian/linux/unstable on i386; glibc 2.2.4-7
>Description:
if a file is opened via std::fstream in read only mode,
and a seekp(0) will be done std::fstream::eof() will never
return true.
This causes the program to hang in a loop.
>How-To-Repeat:
just compile the program (no Optimation) and start it 
by setting as the first option a simple text file.
$ echo "foobar" > foo && ./test_bug foo
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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