Replace write in fstream

**Jon Slaughter** · Jul 22 '05, 11:09 PM

Re: Replace write in fstream

"Jon Slaughter" <nobody@nowhere .com> wrote in message
news:10t4c6s9hl sts51@corp.supe rnews.com...[color=blue]
> I'm trying to replace some bytes in a file using fstream but all I seem to
> be able to do is append or 0 the file then write... Basicaly I just need
> to replace the first 512 bytes of the file with some buffer(but eventually
> I will need to replace random blocks in the file at different locations).
> Is there anyway to do this efficiently using fstream? (I know I can
> duplicate the file, but it seems a waste to rewrite the same data for no
> reason).
>
> Thanks.
>[/color]

my code:

fstream VM_Drive(argv[2], ios::out | ios::binary);

VM_Drive.seekp( 0, ios_base::beg);

char buf[512];

for(int i = 0; i<512;i++) buf[i] = 2;

VM_Drive.write( buf, 512);

VM_Drive.close( );

I've tried ios:app which will append the data, but won't replace.

the ios::out seems to be working exactly as ios::trunc. It resizes the file
to 0 when I open.

**Jonathan Turkanis** · Jul 22 '05, 11:09 PM

Re: Replace write in fstream

Jon Slaughter wrote:[color=blue]
> "Jon Slaughter" <nobody@nowhere .com> wrote in message
> news:10t4c6s9hl sts51@corp.supe rnews.com...[color=green]
>> I'm trying to replace some bytes in a file using fstream but all I
>> seem to be able to do is append or 0 the file then write... Basicaly
>> I just need to replace the first 512 bytes of the file with some
>> buffer(but eventually I will need to replace random blocks in the
>> file at different locations). Is there anyway to do this efficiently
>> using fstream? (I know I can duplicate the file, but it seems a
>> waste to rewrite the same data for no reason).
>>
>> Thanks.
>>[/color]
>
> my code:
>
> fstream VM_Drive(argv[2], ios::out | ios::binary);[/color]

try ios::in | ios::out | ios::binary

Jonathan

**Evan Carew** · Jul 22 '05, 11:09 PM

Re: Replace write in fstream

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Hash: SHA1

Jon,

Jon Slaughter wrote:[color=blue]
> "Jon Slaughter" <nobody@nowhere .com> wrote in message
> news:10t4c6s9hl sts51@corp.supe rnews.com...
>[color=green]
>>I'm trying to replace some bytes in a file using fstream but all I seem to
>>be able to do is append or 0 the file then write... Basicaly I just need
>>to replace the first 512 bytes of the file with some buffer(but eventually
>>I will need to replace random blocks in the file at different locations).
>>Is there anyway to do this efficiently using fstream? (I know I can
>>duplicate the file, but it seems a waste to rewrite the same data for no
>>reason).
>>
>>Thanks.
>>[/color]
>
>
> my code:
>
> fstream VM_Drive(argv[2], ios::out | ios::binary);[/color]
This is fine as far as it goes, however, if you have a look at
streambuf.h in your STL, you will see a flag called nocreate, and
another one called noreplace. Both of these do about what you would
expect. As for the binary flag, this is only needed on those platforms
which make a distinction between the two types of files (i.e. on MS
windows).[color=blue]
>
> VM_Drive.seekp( 0, ios_base::beg);
>
> char buf[512];
>
> for(int i = 0; i<512;i++) buf[i] = 2;
>
> VM_Drive.write( buf, 512);
>
> VM_Drive.close( );
>[snip][/color]

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**Jon Slaughter** · Jul 22 '05, 11:09 PM

Re: Replace write in fstream

"Jonathan Turkanis" <technews@kanga roologic.com> wrote in message
news:33erg8F3tk ns3U1@individua l.net...[color=blue]
> Jon Slaughter wrote:[color=green]
>> "Jon Slaughter" <nobody@nowhere .com> wrote in message
>> news:10t4c6s9hl sts51@corp.supe rnews.com...[color=darkred]
>>> I'm trying to replace some bytes in a file using fstream but all I
>>> seem to be able to do is append or 0 the file then write... Basicaly
>>> I just need to replace the first 512 bytes of the file with some
>>> buffer(but eventually I will need to replace random blocks in the
>>> file at different locations). Is there anyway to do this efficiently
>>> using fstream? (I know I can duplicate the file, but it seems a
>>> waste to rewrite the same data for no reason).
>>>
>>> Thanks.
>>>[/color]
>>
>> my code:
>>
>> fstream VM_Drive(argv[2], ios::out | ios::binary);[/color]
>
> try ios::in | ios::out | ios::binary
>
> Jonathan
>
>[/color]

that doesn't work... it still recreates the file.

**Jon Slaughter** · Jul 22 '05, 11:09 PM

Re: Replace write in fstream

"Evan Carew" <tempcarew@pobo x.com> wrote in message
news:10t5f1gftt 42787@corp.supe rnews.com...[color=blue]
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Jon,
>
> Jon Slaughter wrote:[color=green]
>> "Jon Slaughter" <nobody@nowhere .com> wrote in message
>> news:10t4c6s9hl sts51@corp.supe rnews.com...
>>[color=darkred]
>>>I'm trying to replace some bytes in a file using fstream but all I seem
>>>to be able to do is append or 0 the file then write... Basicaly I just
>>>need to replace the first 512 bytes of the file with some buffer(but
>>>eventually I will need to replace random blocks in the file at different
>>>locations) . Is there anyway to do this efficiently using fstream? (I know
>>>I can duplicate the file, but it seems a waste to rewrite the same data
>>>for no reason).
>>>
>>>Thanks.
>>>[/color]
>>
>>
>> my code:
>>
>> fstream VM_Drive(argv[2], ios::out | ios::binary);[/color]
> This is fine as far as it goes, however, if you have a look at streambuf.h
> in your STL, you will see a flag called nocreate, and another one called
> noreplace. Both of these do about what you would expect. As for the binary
> flag, this is only needed on those platforms which make a distinction
> between the two types of files (i.e. on MS windows).[/color]

I don't understand how these flags are useful? I am trying to open a
pre-existing file and overwrite only part of it. But my code truncates the
file for no reason. (if the file is 1024 bytes and I write 512 bytes, the
"new" file is 512 bytes... and I don't want to recreate the file because it
can be up to several gigs). I'm not sure whats going on, but it seems
ios::out is acting just like ios::trunc.

(and yes, I'm using windows)
[color=blue][color=green]
>>
>> VM_Drive.seekp( 0, ios_base::beg);
>>
>> char buf[512];
>>
>> for(int i = 0; i<512;i++) buf[i] = 2;
>>
>> VM_Drive.write( buf, 512);
>>
>> VM_Drive.close( );
>>[snip][/color]
>
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.0.6 (GNU/Linux)
> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org
>
> iD8DBQFB0rwupxC QXwV2bJARAhmYAJ 9h68lCfdSPEENCt tHL1JMeXe7MAwCZ AeVr
> kCZ5Xn7M2F0hyFO wLsfYEms=
> =zimQ
> -----END PGP SIGNATURE-----[/color]

**Jonathan Mcdougall** · Jul 22 '05, 11:09 PM

Re: Replace write in fstream

Evan Carew wrote:[color=blue]
> Jon,
>
> Jon Slaughter wrote:[color=green][color=darkred]
>>>I'm trying to replace some bytes in a file using fstream but all I seem to
>>>be able to do is append or 0 the file then write... Basicaly I just need
>>>to replace the first 512 bytes of the file with some buffer(but eventually
>>>I will need to replace random blocks in the file at different locations).
>>>Is there anyway to do this efficiently using fstream? (I know I can
>>>duplicate the file, but it seems a waste to rewrite the same data for no
>>>reason).[/color]
>>
>>my code:
>>
>>fstream VM_Drive(argv[2], ios::out | ios::binary);[/color]
>
> This is fine as far as it goes, however, if you have a look at
> streambuf.h in your STL, you will see a flag called nocreate, and
> another one called noreplace. Both of these do about what you would
> expect. As for the binary flag, this is only needed on those platforms
> which make a distinction between the two types of files (i.e. on MS
> windows).[/color]

nocreate and noreplace are non standard flags,
prefer not to use them if you can.

Jonathan