How much was read during istream::read ?

[Steve]

Hi,

I'm trying to convert a file reading loop into one using streams. The BSD OS
read API returns the number of bytes read, but istream::read returns itself.
How can I find out the number of bytes actually read?

What the code fragment should do is read up to 1000 bytes into a buffer, or
finish early if reading failed. Just your average read loop.

I have: (this is a simplified version; I know there's no detailed error
checking!)

char buffer[1000];
int bytestoread = 1000;
int totalbytes = 0;

int fd = ... ; // a file descriptor

while( bytestoread )
{
int bytesread = read( fd, buffer, bytestoread );
if( bytesread <= 0 )
break;
buffer += bytesread;
bytestoread -= bytesread;
totalbytes += bytesread;
}

And I want:

char buffer[1000];
int bytestoread = 1000;
int totalbytes = 0;

std::istream& is( ... ); // an istream

while( bytestoread )
{
is.read( buffer, bytestoread ); // << PROBLEM

if( bytesread <= 0 )
break;
buffer += bytesread;
bytestoread -= bytesread;
totalbytes += bytesread;
}
The problem is how can I find out how many bytes were really read? And if
there is, does the mechanism work the same as the OS read API? - ie. Zero to
indicate end, negative for error, etc.

(What I am actually trying to do is interface to libxml2 and get some XML to
be parsed from an istream using xmlCtxtReadIO).
Thanks for any help.

--
Regards,
Steve.

[Buster]

Steve wrote:

Hi,

I'm trying to convert a file reading loop into one using streams. The BSD OS
read API returns the number of bytes read, but istream::read returns itself.
How can I find out the number of bytes actually read?
Use istream::readsome.
What the code fragment should do is read up to 1000 bytes into a buffer, or
finish early if reading failed. Just your average read loop.

That's a one-liner.

--
Regards,
Buster.

[John Harrison]

"Steve" <po********@127.0.0.1> wrote in message
news:BCC8CFEE.7183B%po********@127.0.0.1...

Hi,

I'm trying to convert a file reading loop into one using streams. The BSD OS read API returns the number of bytes read, but istream::read returns itself. How can I find out the number of bytes actually read?
gcount()

It's a clunky part of the iostream API I think, but that's the way you do
it.


The problem is how can I find out how many bytes were really read? And if
there is, does the mechanism work the same as the OS read API? - ie. Zero to indicate end, negative for error, etc.

gcount() returns the number of bytes read, that is never negative.

john

[John Harrison]

"Buster" <no***@nowhere.com> wrote in message
news:c7**********@news5.svr.pol.co.uk...

Steve wrote:

Hi,

I'm trying to convert a file reading loop into one using streams. The BSD OS read API returns the number of bytes read, but istream::read returns itself. How can I find out the number of bytes actually read?

Use istream::readsome.

readsome only reads characters that are immediately available from the
buffer.

john

[Buster]

John Harrison wrote:

"Buster" <no***@nowhere.com> wrote

Use istream::readsome.

readsome only reads characters that are immediately available from the
buffer.

Yes. Thanks a lot; apologies to OP.

--
Regards,
Buster.

[Steve]

On 13/5/04 7:18 am, in article 2g************@uni-berlin.de, "John Harrison"
<jo*************@hotmail.com> wrote:

"Steve" <po********@127.0.0.1> wrote in message
news:BCC8CFEE.7183B%po********@127.0.0.1...

Hi,

I'm trying to convert a file reading loop into one using streams. The BSD

OS

read API returns the number of bytes read, but istream::read returns

itself.

How can I find out the number of bytes actually read?

gcount()

It's a clunky part of the iostream API I think, but that's the way you do
it.

The problem is how can I find out how many bytes were really read? And if
there is, does the mechanism work the same as the OS read API? - ie. Zero

to

indicate end, negative for error, etc.

gcount() returns the number of bytes read, that is never negative.

john

Ah, OK, got it. Thanks for that.
Steve.

[Steve]

On 13/5/04 7:29 am, in article c7**********@news6.svr.pol.co.uk, "Buster"
<no***@nowhere.com> wrote:

John Harrison wrote:

"Buster" <no***@nowhere.com> wrote

Use istream::readsome.

readsome only reads characters that are immediately available from the
buffer.

Yes. Thanks a lot; apologies to OP.

No problem.
That's the mistake I made on my first attempt! :)

Cheers,
Steve.