C / C++ / MFC

Re: reading a string from an unknown position in a file

5-Mar-06 1:30

Hi,

I have an MFC application in which I have to read a string from a file.
For Example, somewhere in the file I have something like: <>12345<>, and I have to find the <> and then read the 12345 string.
The string can be somewhere in the file - in the beginning, end or somewhere in the middle.
I understand I can read the file using the CFile::Read(), but the problem is that I don't know how many bytes I want to read, and read requires this info as an argument to the function.

how can I find the string in a file, if I don't know how many bytes I have to read?

Gary R. Wheeler5-Mar-06 2:13

5-Mar-06 2:13

You've got a couple options here.

1. If the file is relatively small, read the entire file into a string and then use one of the string search functions (like strstr) to find the opening marker <<A>>. Extract the data from where you find it.

2. If the file is large, read it a chunk at a time. Look for the first character in the marker in the chunk. If you find it, read enough more out of the file, appending to the chunk, that you're sure you have all of the data you're interested in. Extract the data.

3. Given your example, it almost sounds like you're trying to read XML. If that's the case, there's plenty of code around for helping you do that.

Software Zen: delete this;

Re: reading a string from an unknown position in a file

SWDevil5-Mar-06 2:27

Re: reading a string from an unknown position in a file

5-Mar-06 2:27

1. no - I'm not trying to read an XML file. I'm trying to read a picture file that has a header from which I'm trying to extract the string... when I open the picture file with a text editor I see the string I have to read... and yes, the file is big... Smile | :)

2. How can I read chunk by chunk? with the read function?
right now, I'm reading like this:

int MaxBufferSize = 1000;
char cTemp[MaxBufferSize + 1];
cf.Read(cTemp, MaxBufferSize);
//then look for the string in the cTemp array.

is this the correct way to do this?
how can I search for the string in the cTemp array? with a for loop?

Gary R. Wheeler5-Mar-06 2:53

5-Mar-06 2:53

Try looking up the strstr function, which searches for one string within another.

You can also do the search yourself by looping through the data, something like this:

for (int offset = 0; offset < MaxBufferSize; offset++) {
  if (CTemp[offset] == '<') {
    if (strcmp(&CTemp[offset],"<<A>>") == 0) {
      // data is at CTemp[offset + 6]
    }
  }
}

Software Zen: delete this;

Re: reading a string from an unknown position in a file

Stephen Hewitt5-Mar-06 17:13

Re: reading a string from an unknown position in a file

5-Mar-06 17:13

This loop will not work. The strcmp function matches two NULL terminated strings - As such it will never match the sequence unless it's at the end of the buffer (the only place where it could be NULL terminated).

Steve

SWDevil5-Mar-06 20:06

Re: reading a string from an unknown position in a file

5-Mar-06 20:06

I didn't use the loop. I used the strstr function in a different loop, but I think I have an infinite loop. This is my code:

CFile cf(sCurrImgPath, CFile::modeRead);
const int MaxBufferSize = 100000;
char cTemp[MaxBufferSize + 1];

while (pStr == NULL)
{
iBytesRead = cf.Read(cTemp, MaxBufferSize);
pStr = strstr(cTemp, "<>");
}

My file is about 2200KB, so if I'm reading 100,000 bytes each iteration , then there should be about 22 iterations.
The problem is that it keeps going much further than 22 iterations, and doesn't read anymore data. it keeps going because pStr is still NULL because the string isn't found in CTemp...

any ideas?

Stephen Hewitt5-Mar-06 20:24

Re: reading a string from an unknown position in a file

5-Mar-06 20:24

Try this:

CFile cf(sCurrImgPath, CFile::modeRead);
const int MaxBufferSize = 100000;
char cTemp[MaxBufferSize + 1];

const char *pStringToFind = "<<A>>";
size_t LengthOfStringToFind = strlen(pStringToFind);

while (pStr == NULL)
{
     iBytesRead = cf.Read(cTemp, MaxBufferSize);
     pStr = strstr(cTemp, pStringToFind);
     if ( pStr != NULL )
     {
         // Process the found item here...
         pStr += LengthOfStringToFind ; // Skip over the one we found.
     }
}

Steve

SWDevil5-Mar-06 20:52

Re: reading a string from an unknown position in a file

5-Mar-06 20:52

Ok - it still doesn't work....

I think the problem is that I am trying to read text from a file which is actually an image file.

I'll explain my problem in a different manner...
what I have is a *.png file (image file), which has a row of text somewhere in the file. This row starts with: >tEXtComment, and continues with the data I need to read...
how can I extract this row that starts with ›tEXtComment from the image file?

Stephen Hewitt5-Mar-06 20:56

Re: reading a string from an unknown position in a file

5-Mar-06 20:56

Before you worry about that test your code on a plain text file you create.

Steve

SWDevil5-Mar-06 21:24

Re: reading a string from an unknown position in a file

5-Mar-06 21:24

Ok - so I copied the string to a text file and renamed it as a *.png file, and it finds the string, and works fine...

Stephen Hewitt5-Mar-06 21:32

Re: reading a string from an unknown position in a file

5-Mar-06 21:32

Ok. Now open the .png file with Notepad and see it actually contains the string.

Steve

SWDevil5-Mar-06 21:37

Re: reading a string from an unknown position in a file

5-Mar-06 21:37

what .png file? I didn't understand what u meant..

I already opened the .png files with notepad, and they all contain the string. That includes the .png file that is the actual image, and the .png file I just created...

Stephen Hewitt5-Mar-06 21:47

Re: reading a string from an unknown position in a file

5-Mar-06 21:47

I can't think of what could be going wrong then. If it finds it the text file it should work for the image file - assuming the string is present. The only think I can suggest is to check that the cases are consistent. i.e. Search string is in upper cast but string in file is in lower case.

Steve

SWDevil5-Mar-06 21:53

Re: reading a string from an unknown position in a file

5-Mar-06 21:53

I'll tell u something - I copied all of the data from the .png file that is an image file(the original file), to the .png file I constructed from Notepad, and the search for the string works...
So I think the problem is in the fact that the original .png file was originally constructed as an image file...

Gary R. Wheeler6-Mar-06 12:41

6-Mar-06 12:41

Oops. That should have been strncmp D'Oh! | :doh:

Software Zen: delete this;

Re: reading a string from an unknown position in a file

David Crow6-Mar-06 3:18

David Crow

6-Mar-06 3:18

Gary R. Wheeler wrote:
2. If the file is large, read it a chunk at a time.

But what if the 'chunk' split the <> in two?

"Let us be thankful for the fools. But for them the rest of us could not succeed." - Mark Twain

"There is no death, only a change of worlds." - Native American Proverb

Re: reading a string from an unknown position in a file

Gary R. Wheeler6-Mar-06 12:44

6-Mar-06 12:44

If you read the rest of #2, you'll see I tell him to read more data out of the file, appending to the original chunk. That covers the case where you haven't read enough of the data to include an entire marker.

Software Zen: delete this;