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Steven T. Hatton
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 Posted: Mon Feb 20, 2006 10:06 am Post subject: ifstream binary read into unsigned char b[LENGTH] |
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What is the best way to read data from a file into a fixed size array of
unsigned char? This is a file holding only a SHA1 digest. What I would
really like to do is initialize the ifstream with the buffer allocated to
hold the data, seekg(ios_base::end) and magically have the data appear in
the buffer. I believe something close to this can be done.
I might allocate the array:
const unsigned LENGTH = 256;
unsigned char b[LENGTH];
Then cast it to a std::streambuf
std::streambuf sb(b[LENGTH]);
But can I construct std::ifstream that uses that same memory to hold the
data from its file? How to I prevent reading past the end of my buffer, or
the end of the file data?
--
NOUN:1. Money or property bequeathed to another by will. 2. Something handed
down from an ancestor or a predecessor or from the past: a legacy of
religious freedom. ETYMOLOGY: MidE legacie, office of a deputy, from OF,
from ML legatia, from L legare, to depute, bequeath. www.bartleby.com/61/ |
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Rolf Magnus
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| Posted: Mon Feb 20, 2006 1:06 pm Post subject: Re: ifstream binary read into unsigned char b[LENGTH] |
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Steven T. Hatton wrote:
| Quote: | What is the best way to read data from a file into a fixed size array of
unsigned char? This is a file holding only a SHA1 digest. What I would
really like to do is initialize the ifstream with the buffer allocated to
hold the data, seekg(ios_base::end) and magically have the data appear in
the buffer.
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Why should a seek read any data?
| Quote: | I believe something close to this can be done.
I might allocate the array:
const unsigned LENGTH = 256;
unsigned char b[LENGTH];
Then cast it to a std::streambuf
std::streambuf sb(b[LENGTH]);
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I don't think that you can be sure that the buffer will contain exactly the
data you want. After all, it's a buffer that's meant for internal use.
| Quote: | But can I construct std::ifstream that uses that same memory to hold the
data from its file?
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The purpose of streams is text formatting. If you want to read binary data,
just use a filebuf directly.
| Quote: | How to I prevent reading past the end of my buffer, or the end of the file
data?
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Your idea seems overly complicated to me. Just use the filebuf::sgetn()
function to read as many bytes as you want and you're done. |
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Steven T. Hatton
Guest
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| Posted: Wed Mar 08, 2006 12:06 am Post subject: Re: ifstream binary read into unsigned char b[LENGTH] |
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Rolf Magnus wrote:
| Quote: | Steven T. Hatton wrote:
What is the best way to read data from a file into a fixed size array of
unsigned char? This is a file holding only a SHA1 digest. What I would
really like to do is initialize the ifstream with the buffer allocated to
hold the data, seekg(ios_base::end) and magically have the data appear in
the buffer.
Why should a seek read any data?
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As I understand things, that will (usually?) force the file to be completely
read into memory.
| Quote: | Your idea seems overly complicated to me. Just use the filebuf::sgetn()
function to read as many bytes as you want and you're done.
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Will that copy the bytes from one location in memory to another? That's
what I want to avoid. This is basically the same thing as move semantics.
Rather than copying the contents of the buffer, I simply want to take
ownership of it.
--
NOUN:1. Money or property bequeathed to another by will. 2. Something handed
down from an ancestor or a predecessor or from the past: a legacy of
religious freedom. ETYMOLOGY: MidE legacie, office of a deputy, from OF,
from ML legatia, from L legare, to depute, bequeath. www.bartleby.com/61/ |
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