Comparing memory with a constant

[galapogos]

Hi,
I'm trying to compare an array of unsigned chars(basically just data
without any context) with a constant, and I'm not sure how to do that.
Say my array is array[10] and I want to compare it with the constant
0x0001020304050 6070809 where array[0] == 0x00, array[1] == 0x01, etc.
How do I do that. Obviously I can't use memcmp() with the
0x0001020304050 6070809 since the compiler will use that constant as an
address and give me a segfault. Other than doing an if (array[i] =
0x0i) for all 10 values of i, how can I do such a compare? I thought of
converting the constant to a string and doing a memcmp/strcmp using
that string but not all the bytes are printable ASCII characters.

Thanks!

[james of tucson]

Re: Comparing memory with a constant

galapogos wrote:

How do I do that. Obviously I can't use memcmp()

That's obvious but I think not for the reason you are thinking of.
Consider byte-order and alignment for instance.

Depending on how your long number was generated, you might not be able
to do what you're suggesting at all, with any portability.

You are making assumptions about the bitwise order and alignment of an
array, and also about the bitwise representation of a long value. Both
of these fall under the category of "you might get lucky."

[Ian Collins]

Re: Comparing memory with a constant

galapogos wrote:

Hi,
I'm trying to compare an array of unsigned chars(basically just data
without any context) with a constant, and I'm not sure how to do that.
Say my array is array[10] and I want to compare it with the constant
0x0001020304050 6070809 where array[0] == 0x00, array[1] == 0x01, etc.
How do I do that. Obviously I can't use memcmp() with the
0x0001020304050 6070809 since the compiler will use that constant as an
address and give me a segfault. Other than doing an if (array[i] =
0x0i) for all 10 values of i, how can I do such a compare? I thought of
converting the constant to a string and doing a memcmp/strcmp using
that string but not all the bytes are printable ASCII characters.
>

Just convert your constant to an array of unsigned char and use memcmp.

How do you represent a ten byte constant?

--
Ian Collins.

[galapogos]

Re: Comparing memory with a constant


Ian Collins wrote:

galapogos wrote:

Hi,
I'm trying to compare an array of unsigned chars(basically just data
without any context) with a constant, and I'm not sure how to do that.
Say my array is array[10] and I want to compare it with the constant
0x0001020304050 6070809 where array[0] == 0x00, array[1] == 0x01, etc.
How do I do that. Obviously I can't use memcmp() with the
0x0001020304050 6070809 since the compiler will use that constant as an
address and give me a segfault. Other than doing an if (array[i] =
0x0i) for all 10 values of i, how can I do such a compare? I thought of
converting the constant to a string and doing a memcmp/strcmp using
that string but not all the bytes are printable ASCII characters.

Just convert your constant to an array of unsigned char and use memcmp.
>
How do you represent a ten byte constant?
>
--
Ian Collins.

That was what I was hoping to avoid. If I have to convert the constant
to an array, I might as well save some space and compare each byte of
the array with each byte of the constant. I guess there's no easier way?

[Ian Collins]

Re: Comparing memory with a constant

galapogos wrote:

Ian Collins wrote:
>

>>galapogos wrote:
>>

>>>Hi,
>>>I'm trying to compare an array of unsigned chars(basically just data
>>>without any context) with a constant, and I'm not sure how to do that.
>>>Say my array is array[10] and I want to compare it with the constant
>>>0x0001020304 0506070809 where array[0] == 0x00, array[1] == 0x01, etc.
>>>How do I do that. Obviously I can't use memcmp() with the
>>>0x0001020304 0506070809 since the compiler will use that constant as an
>>>address and give me a segfault. Other than doing an if (array[i] =
>>>0x0i) for all 10 values of i, how can I do such a compare? I thought of
>>>converting the constant to a string and doing a memcmp/strcmp using
>>>that string but not all the bytes are printable ASCII characters.
>>>

>>
>>Just convert your constant to an array of unsigned char and use memcmp.
>>
>>How do you represent a ten byte constant?
>>

That was what I was hoping to avoid. If I have to convert the constant
to an array, I might as well save some space and compare each byte of
the array with each byte of the constant. I guess there's no easier way?
>

How is the constant represented?

--
Ian Collins.

[galapogos]

Re: Comparing memory with a constant


Ian Collins wrote:

galapogos wrote:

Ian Collins wrote:

>galapogos wrote:
>
>>Hi,
>>I'm trying to compare an array of unsigned chars(basically just data
>>without any context) with a constant, and I'm not sure how to do that.
>>Say my array is array[10] and I want to compare it with the constant
>>0x00010203040 506070809 where array[0] == 0x00, array[1] == 0x01, etc.
>>How do I do that. Obviously I can't use memcmp() with the
>>0x00010203040 506070809 since the compiler will use that constant as an
>>address and give me a segfault. Other than doing an if (array[i] =
>>0x0i) for all 10 values of i, how can I do such a compare? I thought of
>>converting the constant to a string and doing a memcmp/strcmp using
>>that string but not all the bytes are printable ASCII characters.
>>
>
>Just convert your constant to an array of unsigned char and use memcmp.
>
>How do you represent a ten byte constant?
>

That was what I was hoping to avoid. If I have to convert the constant
to an array, I might as well save some space and compare each byte of
the array with each byte of the constant. I guess there's no easier way?

How is the constant represented?
>
--
Ian Collins.

That's a problem isn't it? A 10 byte constant can't be represented in
any data format, so I guess I have to split it up?

[Ian Collins]

Re: Comparing memory with a constant

galapogos wrote:

Ian Collins wrote:
>

>>galapogos wrote:
>>

>>>Ian Collins wrote:
>>>
>>>
>>>>galapogos wrote:
>>>>
>>>>
>>>>>Hi,
>>>>>I'm trying to compare an array of unsigned chars(basically just data
>>>>>without any context) with a constant, and I'm not sure how to do that.
>>>>>Say my array is array[10] and I want to compare it with the constant
>>>>>0x00010203 040506070809 where array[0] == 0x00, array[1] == 0x01, etc.
>>>>>How do I do that. Obviously I can't use memcmp() with the
>>>>>0x00010203 040506070809 since the compiler will use that constant as an
>>>>>address and give me a segfault. Other than doing an if (array[i] =
>>>>>0x0i) for all 10 values of i, how can I do such a compare? I thought of
>>>>>converti ng the constant to a string and doing a memcmp/strcmp using
>>>>>that string but not all the bytes are printable ASCII characters.
>>>>>
>>>>
>>>>Just convert your constant to an array of unsigned char and use memcmp.
>>>>
>>>>How do you represent a ten byte constant?
>>>>
>>>
>>>That was what I was hoping to avoid. If I have to convert the constant
>>>to an array, I might as well save some space and compare each byte of
>>>the array with each byte of the constant. I guess there's no easier way?
>>>

>>
>>How is the constant represented?
>>

>
That's a problem isn't it? A 10 byte constant can't be represented in
any data format, so I guess I have to split it up?
>

Please trim signatures in replies.

const unsigned char ref[] =
{0x00,0x01,0x02 ,0x03,0x04,0x05 ,0x06,0x07,0x08 ,0x09};


--
Ian Collins.

[galapogos]

Re: Comparing memory with a constant


Ian Collins wrote:

galapogos wrote:

Ian Collins wrote:

>galapogos wrote:
>
>>Ian Collins wrote:
>>
>>
>>>galapogos wrote:
>>>
>>>
>>>>Hi,
>>>>I'm trying to compare an array of unsigned chars(basically just data
>>>>without any context) with a constant, and I'm not sure how to do that.
>>>>Say my array is array[10] and I want to compare it with the constant
>>>>0x000102030 40506070809 where array[0] == 0x00, array[1] == 0x01, etc.
>>>>How do I do that. Obviously I can't use memcmp() with the
>>>>0x000102030 40506070809 since the compiler will use that constant as an
>>>>address and give me a segfault. Other than doing an if (array[i] =
>>>>0x0i) for all 10 values of i, how can I do such a compare? I thought of
>>>>convertin g the constant to a string and doing a memcmp/strcmp using
>>>>that string but not all the bytes are printable ASCII characters.
>>>>
>>>
>>>Just convert your constant to an array of unsigned char and use memcmp.
>>>
>>>How do you represent a ten byte constant?
>>>
>>
>>That was what I was hoping to avoid. If I have to convert the constant
>>to an array, I might as well save some space and compare each byte of
>>the array with each byte of the constant. I guess there's no easier way?
>>
>
>How is the constant represented?
>

That's a problem isn't it? A 10 byte constant can't be represented in
any data format, so I guess I have to split it up?

Please trim signatures in replies.
>
const unsigned char ref[] =
{0x00,0x01,0x02 ,0x03,0x04,0x05 ,0x06,0x07,0x08 ,0x09};

Thanks. I thought of doing that, but I'm not really comparing to just 1
constant, so having multiple constants would be kinda unwieldy. What
I'm doing is something like that

switch (array) {
case bitpattern1: ...
case bitpattern2: ...
etc...
}

where bitpattern is my 0x0001020304050 6070809 or whatever else. Now
obviously the above code won't work but I think you get my idea?

[Ian Collins]

Re: Comparing memory with a constant

galapogos wrote:

Ian Collins wrote:
>

>>galapogos wrote:
>>

>>>Ian Collins wrote:
>>>
>>>
>>>>galapogos wrote:
>>>>
>>>>>That was what I was hoping to avoid. If I have to convert the constant
>>>>>to an array, I might as well save some space and compare each byte of
>>>>>the array with each byte of the constant. I guess there's no easier way?
>>>>>
>>>>
>>>>How is the constant represented?
>>>>
>>>
>>>That's a problem isn't it? A 10 byte constant can't be represented in
>>>any data format, so I guess I have to split it up?
>>>

>>
>>Please trim signatures in replies.
>>
> const unsigned char ref[] =
>>{0x00,0x01,0x 02,0x03,0x04,0x 05,0x06,0x07,0x 08,0x09};

>
>
Thanks. I thought of doing that, but I'm not really comparing to just 1
constant, so having multiple constants would be kinda unwieldy. What
I'm doing is something like that
>
switch (array) {
case bitpattern1: ...
case bitpattern2: ...
etc...
}
>
where bitpattern is my 0x0001020304050 6070809 or whatever else. Now
obviously the above code won't work but I think you get my idea?
>

But you still have to represent the constants, don't you?

You'll have to fall back on a set of if/else tests with memcmp and const
unsigned char arrays.

--
Ian Collins.

[galapogos]

Re: Comparing memory with a constant


Ian Collins wrote:

galapogos wrote:

Ian Collins wrote:

>galapogos wrote:
>
>>Ian Collins wrote:
>>
>>
>>>galapogos wrote:
>>>
>>>>That was what I was hoping to avoid. If I have to convert the constant
>>>>to an array, I might as well save some space and compare each byte of
>>>>the array with each byte of the constant. I guess there's no easier way?
>>>>
>>>
>>>How is the constant represented?
>>>
>>
>>That's a problem isn't it? A 10 byte constant can't be represented in
>>any data format, so I guess I have to split it up?
>>
>
>Please trim signatures in replies.
>
const unsigned char ref[] =
>{0x00,0x01,0x0 2,0x03,0x04,0x0 5,0x06,0x07,0x0 8,0x09};

Thanks. I thought of doing that, but I'm not really comparing to just 1
constant, so having multiple constants would be kinda unwieldy. What
I'm doing is something like that

switch (array) {
case bitpattern1: ...
case bitpattern2: ...
etc...
}

where bitpattern is my 0x0001020304050 6070809 or whatever else. Now
obviously the above code won't work but I think you get my idea?

But you still have to represent the constants, don't you?
>
You'll have to fall back on a set of if/else tests with memcmp and const
unsigned char arrays.
>

Yes I know that's the worst case scenario that would work. I was just
hoping there was an easier way that I can implement the switch/case
statements rather than using a bunch of if-else statements that I'm
currently doing.

[Chris Torek]

Re: Comparing memory with a constant

>Ian Collins wrote:

> const unsigned char ref[] =
>{0x00,0x01,0x0 2,0x03,0x04,0x0 5,0x06,0x07,0x0 8,0x09};

In article <1164163513.385 700.320670@h48g 2000cwc.googleg roups.com>
galapogos <goister@gmail. comwrote:

>Thanks. I thought of doing that, but I'm not really comparing to just 1
>constant, so having multiple constants would be kinda unwieldy. What
>I'm doing is something like that
>
>switch (array) {
case bitpattern1: ...
case bitpattern2: ...
etc...
>}
>
>where bitpattern is my 0x0001020304050 6070809 or whatever else. Now
>obviously the above code won't work but I think you get my idea?

Indeed, you cannot do this at all.

If you dislike both:

if (memcmp(array, ref, sizeof ref) == 0)
... handle the case where the pattern is 0x00 0x01 0x02 ...

and:

if (memcmp(array, "\x00\x01\x02\x 03\x04\x05\x06\ x07\x08\x09", 10) == 0)
... handle the case where the pattern is 0x00 0x01 0x02 ...

you can always write a program "P" to produce a C program (or program
fragment) to do the comparisons. You can then decide how fancy Program
P should be; it can even do things like:

/* program P has observed that these three bytes
combine to a value in [0..9180] that is unique for
all the "interestin g" values, so that we only have to
verify that all ten values are correct */
switch (array[2] + (3 * array[3]) + (array[7] << 5)) {
static const unsigned char patterns[N][10] = {
{ 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09 },
... others ...
};
case DIGESTED_CONST_ 0:
if (memcmp(array, patterns[0], 10) != 0) goto Default;
... matched pattern #0 ...
break;
case DIGESTED_CONST_ 1:
if (memcmp(array, patterns[1], 10) != 0) goto Default;
... matched pattern #1 ...
break;
... other cases here ...
default:
Default:
... did not match any pattern ...
}

Look up "perfect hash" to find strategies for producing a formula
for computing a number to switch on, and case-labels.
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[james of tucson]

Re: Comparing memory with a constant

Chris Torek wrote:

>where bitpattern is my 0x0001020304050 6070809 or whatever else. Now
>obviously the above code won't work but I think you get my idea?

>
Indeed, you cannot do this at all.

I still don't understand how the basic idea can work, given byteorder
and data alignment considerations.

There is no guarantee that the data in an array are aligned anything
like the arbitrary structure of bits that it's being compared to, (is
there?)

[Ian Collins]

Re: Comparing memory with a constant

james of tucson wrote:

Chris Torek wrote:
>
>

>>>where bitpattern is my 0x0001020304050 6070809 or whatever else. Now
>>>obviously the above code won't work but I think you get my idea?

>>
>>Indeed, you cannot do this at all.

>
>
I still don't understand how the basic idea can work, given byteorder
and data alignment considerations.
>
There is no guarantee that the data in an array are aligned anything
like the arbitrary structure of bits that it's being compared to, (is
there?)
>

The data and constants are represented as arrays of unsigned char.

--
Ian Collins.

[galapogos]

Re: Comparing memory with a constant


Chris Torek wrote:

Ian Collins wrote:

const unsigned char ref[] =
{0x00,0x01,0x02 ,0x03,0x04,0x05 ,0x06,0x07,0x08 ,0x09};

>
In article <1164163513.385 700.320670@h48g 2000cwc.googleg roups.com>
galapogos <goister@gmail. comwrote:

Thanks. I thought of doing that, but I'm not really comparing to just 1
constant, so having multiple constants would be kinda unwieldy. What
I'm doing is something like that

switch (array) {
case bitpattern1: ...
case bitpattern2: ...
etc...
}

where bitpattern is my 0x0001020304050 6070809 or whatever else. Now
obviously the above code won't work but I think you get my idea?

>
Indeed, you cannot do this at all.
>
If you dislike both:
>
if (memcmp(array, ref, sizeof ref) == 0)
... handle the case where the pattern is 0x00 0x01 0x02 ...
>
and:
>
if (memcmp(array, "\x00\x01\x02\x 03\x04\x05\x06\ x07\x08\x09", 10) == 0)
... handle the case where the pattern is 0x00 0x01 0x02 ...
>
you can always write a program "P" to produce a C program (or program
fragment) to do the comparisons. You can then decide how fancy Program
P should be; it can even do things like:
>
/* program P has observed that these three bytes
combine to a value in [0..9180] that is unique for
all the "interestin g" values, so that we only have to
verify that all ten values are correct */
switch (array[2] + (3 * array[3]) + (array[7] << 5)) {
static const unsigned char patterns[N][10] = {
{ 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09 },
... others ...
};
case DIGESTED_CONST_ 0:
if (memcmp(array, patterns[0], 10) != 0) goto Default;
... matched pattern #0 ...
break;
case DIGESTED_CONST_ 1:
if (memcmp(array, patterns[1], 10) != 0) goto Default;
... matched pattern #1 ...
break;
... other cases here ...
default:
Default:
... did not match any pattern ...
}
>
Look up "perfect hash" to find strategies for producing a formula
for computing a number to switch on, and case-labels.
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

Thanks! I don't think I'll do the perfect hash way though it is pretty
interested if a tad convoluted. The 2nd idea is great though. I've been
looking for a way to represent the pattern with a constant, even a
string, but for some reason I didn't think of that method. I just tried
it and it works. That'll save me the memory for declaring the patterns
as constant arrays. I can just put a couple of #defines with this :)

[J. J. Farrell]

Re: Comparing memory with a constant


galapogos wrote:

Chris Torek wrote:

If you dislike both:

if (memcmp(array, ref, sizeof ref) == 0)
... handle the case where the pattern is 0x00 0x01 0x02 ...

and:

if (memcmp(array, "\x00\x01\x02\x 03\x04\x05\x06\ x07\x08\x09", 10) == 0)
... handle the case where the pattern is 0x00 0x01 0x02 ...
...

>
Thanks! I don't think I'll do the perfect hash way though it is pretty
interested if a tad convoluted. The 2nd idea is great though. I've been
looking for a way to represent the pattern with a constant, even a
string, but for some reason I didn't think of that method. I just tried
it and it works. That'll save me the memory for declaring the patterns
as constant arrays. I can just put a couple of #defines with this :)

How will it save you memory? memcmp() compares two chunks of memory.
Making one of those chunks anonymous doesn't take it out of memory yet
leave it accessible to memcmp().