How to resume appending data to a file at a specific position? (std::ostream, streampos, tellp/seekp)

Question

I'm trying to append some data to a file, but in some cases want to skip back a bit from the end to overwrite the tail end of the file. However, neither seekp( pos ) nor seekp( offset, relative ) is having any effect for me (except complaining when using a negative offset). Am I using them incorrectly or are they broken?

A little example follows. Compiler: gcc version 4.4.4 (Debian 4.4.4-6)

    #include <fstream>
    #include <sstream>
    #include <boost/date_time/posix_time/posix_time.hpp>
    using namespace std;
    using namespace boost::posix_time;

    int main(int nargs, char** pargs){
    if( nargs < 2 || nargs > 3 ){
     cerr<<"Usage: "<<pargs[0]<<" file [pos]"<<endl;
     return 1;
 }

 const char* fname = pargs[1];
 ofstream f( fname, ios::binary|ios::out|ios::ate );
 if( !f.good() ){
     cerr<<"Unable to open file!"<<endl;
     return 1;
 }

 if( nargs == 3 ){
     istringstream offss(pargs[2]);
     streamoff off;
     offss >> off;
     cout <<"Seeking to: "<<off<<endl;
     f.seekp( off, ios_base::end ); // using beg or cur instead doesn't help, nor does: seekp( off )
     if( f.fail() ){
  cerr<<"Unable to seek!"<<endl;
  f.clear(); // clear error flags
     }
 }

 f<<"[appending some data at "<<second_clock::local_time()<<"]\n";

 return 0;
    }

Now, if I seek using a 0 offset, it should place the output-position at the end of the file and writes should append, right? Well, it's having no effect for me (osf was not previously empty):

> ./ostream_app_pos osf 0
Seeking to: 0
> cat osf
[appending some data at 2010-Jul-21 11:16:16]

The usual way of appending is to use ios::app. In this case, appending works, but trying to seek with a neg/pos offset has no effect, since (from gcc doc):

ios::app Seek to end of file before each write.

I also tried using neither ios::ate nor ios::app (presumably truncate mode), to the same effect as ios::ate.

Sorry if this reads rather like a bug report, but I wanted to check whether there's something I've got wrong here in the usage of seekp and get an idea of whether it's compiler-specific.

Answer 1

You need to open the file with both input and output attributes.
The following code doesn't have the usual error handling, it is just to illustrate a technique.

#include <iostream>
#include <fstream>

int main()
{
    const char *szFname = "c:\\tmp\\tmp.txt";
    std::fstream fs(szFname, 
                    std::fstream::binary | 
                    std::fstream::in     | 
                    std::fstream::out);
    fs.seekp(13, std::fstream::beg);
    fs << "123456789";

    return 0;
}

================================================

C:\Dvl\Tmp>type c:\tmp\tmp.txt
abdcefghijklmnopqrstuvwxyz
C:\Dvl\Tmp>Test.exe
C:\Dvl\Tmp>type c:\tmp\tmp.txt
abdcefghijklm123456789wxyz
C:\Dvl\Tmp>