Boost logo

Boost Users :

Subject: Re: [Boost-users] [filesystem] Problem mixing read and write on io fstream
From: Germain BARRET (germain.barret_at_[hidden])
Date: 2008-10-08 12:18:08

I have something like your problem...

First I have a question, when, you say :
 1) write some data
 2) read the data
 3) overwrite old data //this fails

In step 3, does it mean you replace ponctualy some datas int the stream ?
Or do you replace the whole datas ?

In my case, I'm making a http resume... I what to seek to the end of the
stream, but by me :
making a seekp(sizeOfFile) erase the beginning of my file and replace it by
0 (zero) ....

Why ?

Is that not possible to go further the EOF ?

2008/8/22 Sachin Garg <schngrg_at_[hidden]>

> I have a program which opens a boost fstream in binary input+output
> mode, creating the file if it doesn't exists. But writing doesn't
> works after reading, it must be something obvious that I am not aware
> of. filename is a wpath.
>,ios::in | ios::out | ios::binary | ios::trunc)
> The program flow is
> 1) write some data
> 2) read the data
> 3) overwrite old data //this fails
> I am aware that mixing reading and writting is complicated due to
> buffering and as far as I know the solution is to do a seekg or seekp
> when changing between reading and writting, I am doing this between 1,
> 2 and 3.
> In 'my' program, writting in step 3 fails. I wrote a simple program
> that does just the above 3 steps with seeks and it works but in my
> program, it sets the failbit.
> After lots of debugging in internal fstream code I figured that an
> internal flag _IOREAD gets set somehow in step 2 which indicates that
> the stream is now readonly and causes subsequent writes to fail. It
> also seems that _IOREAD is set anytime any read operation is done on a
> read+write file, is this correct? And now how do I 'unset' this flag?
> I tried calling f.clear and f.seekp but that doesnt helps.
> What might be special in my program that causes this read-mode flag to
> remain set while this doesn't happens in a barebone/simple test
> program. I am not doing anything except f.seekg, f.get,
> Sachin Garg
> ps. If it matters, I am using visual studio 9.0. Problem might not be
> boost specific as boost only uses stl fstream but any help will be
> really useful.
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]

Boost-users list run by williamkempf at, kalb at, bjorn.karlsson at, gregod at, wekempf at