Re: Theory question about "void pBuffer" - C / C++ / MFC Discussion Boards

Nelek9-Oct-07 4:38

9-Oct-07 4:38

Hi,

When using a buffer to intermediary help to make use of some functions…

If I want to have in the buffer the content of an Array, then I make:

pBuffer = &Array[0];  // or the same: pBuffer = Array;
file->Write (pBuffer , sizeof (Array));

And there is no problem because the array allocates contiguous memory to hold the members and the pointer says where the Array starts, and with the “size of (Array)” I say how many elements have to be written.

But doing the opposite…

::RegQueryValueEx (hWkKey, "SubKey", 0, &dwType, pBuffer, &lnLength); // Read the contents
if (dwType == REG_SZ)	// The registry value is a string, it could be something else though so we better make sure
	szValue = pBuffer;

else if (dwType == REG_BINARY)
	for (int i = 0; i < lnLength; i++)
		Array[i] = *(pBuffer + i);

delete [] pBuffer;
pBuffer = NULL;

The usual assignation works with the string because internally is like a char* and then both uses the start direction to allocate the data in their place till a '\0' is founded, but it should be made using a for when writing the data into the array because there is no way to say how long the array is (like '\0' by the char*) (isn't it?)

Array = *pBuffer;	//error C2440: impossible to convert from 'unsigned char' in 'unsigned char [1252]' 
;	//the fixed size from array makes problems
//or
&Array = pBuffer;	//error C2440: impossible to convert from 'unsigned char *' in 'unsigned char (*)[1252]' 
//or
Array = pBuffer;	//error C2240: impossible to convert from 'unsigned char *' in 'unsigned char [1252]'
//or
&Array[0] = pBuffer;	//error C2109: Left operator must be a "L-Value"
;	//MSDN says "The subscript was used on a variable that was not an array", does it means that a whole array is trying to be placed in one element??

The meaning of the errors (but C2109) and the reason to have them is clear, but...

Is there any other possibility to do it, or the “for” must be used?

Greetings.

--------
M.D.V.

If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about?

Help me to understand what I'm saying, and I'll explain it better to you

Wink | ;)

Re: Theory question about "void pBuffer"

led mike9-Oct-07 5:00

led mike

9-Oct-07 5:00

I am not sure what your question is but have a look at memcpy(...)

Re: Theory question about "void pBuffer"

Nelek10-Oct-07 0:27

Nelek

10-Oct-07 0:27

I have taken look to the documentation and it seems to be what I asked for. But...

Quote:
"The memcpy function copies count bytes of src to dest. If the source and destination overlap, this function does not ensure that the original source bytes in the overlapping region are copied before being overwritten. Use memmove to handle overlapping regions."

but in the example both makes exactly the same,

//Output of example code 1
Function:   memmove with overlap
Source:      quick brown fox jumps over the lazy dog
Destination:   brown fox jumps over the lazy dog
Result:      The quick quick brown fox jumps over the lazy dog
Length:      49 characters

//output of example code 2
Function:   memcpy with overlap
Source:      quick brown dog jumps over the lazy fox
Destination:   brown dog jumps over the lazy fox
Result:      The quick quick brown dog jumps over the lazy fox
Length:      49 characters

then... why is it said?

Can you tell me an example where the quote of MSDN is true?

Re: Theory question about "void pBuffer"

led mike10-Oct-07 4:46

led mike

10-Oct-07 4:46

Nelek wrote:
then... why is it said?

what is "what" said? The part about overlapping source and destination? ????

Re: Theory question about void pBuffer

Nelek10-Oct-07 20:45

Nelek

10-Oct-07 20:45

Yes, what I write in quote.

It is supposed to give problems while overlapping, but the code that they put in the example (I paste some parts)

char string1[60] = "The quick brown dog jumps over the lazy fox";
char string2[60] = "The quick brown fox jumps over the lazy dog";
/*                           1         2         3         4         5
 *                  12345678901234567890123456789012345678901234567890
 */

//Other function

   printf( "Function:\tmemmove with overlap\n" );
   printf( "Source:\t\t%s\n", string2 + 4 );
   printf( "Destination:\t%s\n", string2 + 10 );
   memmove( string2 + 10, string2 + 4, 40 );
   printf( "Result:\t\t%s\n", string2 );
   printf( "Length:\t\t%d characters\n\n", strlen( string2 ) );

   printf( "Function:\tmemcpy with overlap\n" );
   printf( "Source:\t\t%s\n", string1 + 4 );
   printf( "Destination:\t%s\n", string1 + 10 );
   memcpy( string1 + 10, string1 + 4, 40 );
   printf( "Result:\t\t%s\n", string1 );
   printf( "Length:\t\t%d characters\n\n", strlen( string1 ) );

gives the results that I worte in the message above, and both are making exactly the same and working good. So I don't understand where the problem is. If I could see an example where the quote is true, then I would understand it better.

Re: Theory question about void pBuffer

led mike11-Oct-07 4:59

led mike

11-Oct-07 4:59

Nelek wrote:
If I could see an example where the quote is true, then I would understand it better.

If the source and destination overlap, this function does not ensure that the original source bytes in the overlapping region are copied before being overwritten.

memmov "does" ensure that the original source bytes are copied before being overwritten.

There is no certain way to give an example where it memcpy fails because the results are not guaranteed. You would have to try experimenting to find a condition where it might fail repeatedly.

Also your documentation appears dated check this out[^]

Re: Theory question about void pBuffer

Nelek12-Oct-07 1:59

Nelek

12-Oct-07 1:59

Yes,

the only problem is that I am usually using VC++ 6.0 and it would be not the first time I check the MSDN page to look for something, try to use it, and lots of errors because it is for newer versions.

Because of that I use my program's help and my quotes sometimes are old.

But thank you for your time and explications. You have helped me to understand it better

Re: Theory question about "void pBuffer"

Bram van Kampen9-Oct-07 15:31

Bram van Kampen

9-Oct-07 15:31

Don't quite understand the question, but there seems to be confusion about Arrays, Strings and CStrings. An Array is a continguous section of memory of which the compiler is aware of how it is organised.(Element Size, and Number of Elements). It is closely related to the Pointer to an Element Concept. However, whereas you can change a pointer point to a different location in memory, you cannot do this with a variable declared as an array name.[Error C2240] (The Compiler would soon loose track if you could do these things). A Pointer is more versatile than an array in this respect. However, the compiler still keeps track of the type(and hence size) of the object it points to. A pointer to void should be interpreted as a pointer to 'Nothing in Particular' It has only an address value, all other interresting titbits are ommitted. That means that the compiler cannot dereference a void pointer, it has insufficient information to do so. We use a Pointer to void as a generic pointer, which can be passed on by value, but which we must render usable as pointer by adding information, (i.e. Casting)
A String of characters is closely related to an array of characters, there is actually only one difference, when initialised with a literal text constant, the compiler adds automatically a terminating zero to a string. It would for instance not do that if you initialise an array of integers.
This brings us to a CString. Now, this is an entirely different kettle of fish. The items discussed above are essentially C-constructs, which can ofcourse be used in CPP. A CString is firmly a CPP Class, it is definitely NOT the same as a character array. The properties of a CString are not determined by the Compiler (as in Strings and Arrays) but are implemented as part of the Microsoft Standard Libraries. What makes it confusing for a novice is that aparently CStrings and Strings can often be used interchangably, and that the CString can be initialised to the value of a String. This is all due (and purposely designed to work this way) to the gymnastics carried out in the various methods of the CString class. As an example, A CString contains indeed a buffer containing the Null terminated string representing the value. The Cast Operator (LPCSTR) is written in such a way that it returns a pointer to this buffer. I Suggest that you experiment with the CString Class to get a feel for what I am about here. As for your own Array's, why not write your own dedicated class which keeps track of buffers and sizes, and which 'act' like arrays by using the various 'operator' overrides

LateNightsInNewry

Re: Theory question about "void pBuffer"

Nelek9-Oct-07 22:14

Nelek

9-Oct-07 22:14

Thanks for such a good explanation.

So szValue = pBuffer works fine because the adress of pBuffer is actually being asigned to that intern buffer of the CString, and the rest of the information is find out internally (i.e. the length of the string is determined with the "\0"), but...

&Array[0] = pBuffer can not be made because, although the type of data is the same and both will point to the same place, the Array has a fixed length due to the allocation or the creation with [M], and the pointer can not really determine how many place is needed...

But pBuffer = &Array [0] works fine because when the pBuffer is used in other place, the most of the functions that use a buffer need a parameter "sizeof (...)" to determine the needed length of the data field. I.e. the &nLength by using RegQueryEx (..), or file->Write ( pBuffer, (n * sizeof (BYTE)) )
or sizeof (Object);

Is that correct?

MCI -> set video position at a specific time

sdancer759-Oct-07 4:36

sdancer75

9-Oct-07 4:36

Hi,

How can i set the video position using MCI to a specific time using the format hh:mm:ss ?

Regards,

sdancer75

Re: MCI -> set video position at a specific time

Hamid_RT9-Oct-07 9:46

Hamid_RT

9-Oct-07 9:46

Does MCI_WINDOW helpful?

Re: MCI -> set video position at a specific time

sdancer7510-Oct-07 0:39

sdancer75

10-Oct-07 0:39

hi,

i dont use the mci_window. I can use the Seek(frame_number) to position my video but it does not work when i want to use time format. Any suggestions?

Regards,

sdancer75

Re: MCI -> set video position at a specific time

Hamid_RT10-Oct-07 7:09

Hamid_RT

10-Oct-07 7:09

I wrote code for set postion of window for mci I think two or three years ago but I forgot it I will be see my projects and give you an answer.

Re: MCI -> set video position at a specific time

sdancer7510-Oct-07 20:00

sdancer75

10-Oct-07 20:00

Hey thanks,

I think I found a solution. All I had to do is to multiply the hour with 3600 ( sec) * 1000 to convert it to milliseconds the minutes with 60 * 1000 and seconds with 1000.
Finally I add the hour+minutes+secords and the final value is the position to mseconds.

mseconds = (hour * 3600 * 1000) + (minutes * 60 * 1000) + (seconds * 1000)

It worked.

Thank you.

sdancer75

Re: MCI -> set video position at a specific time

Hamid_RT10-Oct-07 21:32

Hamid_RT

10-Oct-07 21:32

I gald that your problem solved. Big Grin | :-D

Multiline textout

vSteve9-Oct-07 3:44

vSteve

9-Oct-07 3:44

I have an ActiveX control which is similar to a button, I write the caption on to the control with the following:

pdc->SetTextAlign(TA_CENTER | VTA_CENTER);
pdc->SetBkMode(TRANSPARENT);
pdc->ExtTextOutW(buttonCntr.x, buttonCntr.y, ETO_CLIPPED, rcBounds, InternalGetText(), NULL);

Which all works OK, until the caption of the control is long, can I get the text to wrap on to multiple lines by some means, as is the case with CButton?

Thanks

Kalvin

Re: Multiline textout

C^hris L^osinger9-Oct-07 3:51

C^hris L^osinger

9-Oct-07 3:51

try DrawText/DrawTextEx instead of ExtTextOut

-c

image processing toolkits | batch image processing

Re: Multiline textout

vSteve9-Oct-07 4:20

vSteve

9-Oct-07 4:20

Thanks, that's done the job, partly! I am now using:

pdc->DrawTextEx(InternalGetText(), (LPRECT)myRect, DT_CENTER | DT_WORDBREAK, NULL );

Which does multiline and centres the text horizontaly in the rect., but I can't get it to centre vertically.

DT_VCENTER would appear to do this but it is only effective when combined with DT_SINGLELINE, and single line is def. not what I want. Any further ideas?

Kalvin

Re: Multiline textout

led mike9-Oct-07 5:02

led mike

9-Oct-07 5:02

KalvinComer wrote:
but I can't get it to centre vertically.

KalvinComer wrote:
Any further ideas?

Center it yourself?

Buffer Overrun

LCI9-Oct-07 2:33

LCI

9-Oct-07 2:33

I have a buffer overrun problem that i am not 100% sure of the best way to fix it. If someone can give me some direction i would appreciate it.

I have the following :
Two structs with fields of different values that i am attempting to StrCpy();

typedef struct _myInfo
{
TCHAR szOwnerLastName[64];
TCHAR szOwnerFirstName[64];
} MyInfo;

typedef struct _otherInfo
{
TCHAR szOwnerLastName[32];
TCHAR szOwnerFirstName[32];
} OtherInfo;

MyInfo* pMyInfo;
OtherInfo* pOtherInfo;

After populating these fields, here is what i find:
If i do a :

StrCpy(pOtherInfo->szOwnerLastName, pMyInfo->szOwnerLastName);

Since MyInfo is bigger than OtherInfo, the above statement basically writes into the OtherInfo's Firstname field.

Is there a nice way to get around this? I cannot change the field sizes for either structure.
Thanks,
cw

Re: Buffer Overrun

David Crow9-Oct-07 2:51

David Crow

9-Oct-07 2:51

LCI wrote:
Is there a nice way to get around this?

Yes, use strncpy() instead. Or use std::string and not worry about it.

"A good athlete is the result of a good and worthy opponent." - David Crow

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

Re: Buffer Overrun

Stephen Hewitt9-Oct-07 12:29

Stephen Hewitt

9-Oct-07 12:29

Do you have to use raw strings? Why not use std::string or some other string class? What should happen to the bit of the string that can't fit in the smaller buffer?

Steve

DirectX - Generall Question

GermanGeorge9-Oct-07 2:05

GermanGeorge

9-Oct-07 2:05

Sorry for my poor englisch
i hope i can make myself understood
i need a generall help, how to solve this problem

i work with directx sdk 9.0 and visual studio 2005

i have to create a lens (in spectacles).
i got the modell of the lens with all the curves etc.

but:
the lens is made out of colored glass (let us say brown)
so what happens is, that in the thinner part of the lens the brown is not so intense. in the thicker part of the lens the lens is darker, as there is more material.

please help. how to i start to solve this problem

george

Re: DirectX - Generall Question

El Corazon9-Oct-07 15:17

El Corazon

9-Oct-07 15:17

GermanGeorge wrote:
the lens is made out of colored glass (let us say brown)
so what happens is, that in the thinner part of the lens the brown is not so intense. in the thicker part of the lens the lens is darker, as there is more material.

Are you only trying to change the color of the lens, or are you trying to model the refraction (light bending) of the lens? In the case of the color change set the color brown to levels from faint to heavy based on the thickness of the lens. Basically make a texture map to represent the color at each point in the material. Alternately, you can have a fixed color, brown, and vary only the alpha (transmitance) value based on the thickness of the lens.

If the idea is to model both color change and light bending of the glass lens, google ray-tracing, refraction, BRTDF caustics and/or global illumination. http://www.gpgpu.org/cgi-bin/blosxom.cgi/Advanced%20Rendering/Global%20Illumination/index.html[^]

_________________________
Asu no koto o ieba, tenjo de nezumi ga warau.
Talk about things of tomorrow and the mice in the ceiling laugh. (Japanese Proverb)

Re: DirectX - Generall Question

GermanGeorge9-Oct-07 20:37

GermanGeorge

9-Oct-07 20:37

Hi,

thank you for your answer.

no, I only need the colour.

three questions:

1)
can you give me some code for your ideas? i would be glad to pay you for it.

2)
as the thickness and colour can change with every spectacle: how do i create a texture map in my programm, i cannon hold a file for each possibility.

3)
which alpha do you mean? the one of the material, the ampient alpha?

sorry
george

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