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Problem reading binary file

Started by Austrian Coder Jul 16, 2003 at 5:13 AM 7 replies 500+ views

Original Post

Jul 16, 2003 at 5:13 AM

Here is my source, to read an binary file completly in an buffer.

		FILE*			pfile;
		unsigned int	Size;
		char*			buffer;

		if ((pfile = fopen(File, "rb")) != 0)
		{
			 // obtain file size.

			fseek (pfile , 0 , SEEK_END);
			Size = ftell (pfile);
			rewind (pfile);
		
			// allocate a buffer, which is one byte

			// biger, so that at the end of the

			// buffer is the needed 0.		

			buffer = (char*) malloc (Size + 1);
			memset (buffer, 0, Size + 1); 

			fread (buffer, 1, Size, pfile);

			// check if reading was ok

			if (!buffer)
			{
				cout << "Error while reading file!" << endl;
				return false;
			}

			// close file

			fclose(pfile);
		}
		else
		{
			// we could not open the file

			cout << "Could not open file" << endl;
			return false;
		}

If i want to load now a bmp completly into a buffer, the buffer is only 4 elements big. Whats wrong? Thanks, Christian

Fruny

203

Fruny

Jul 16, 2003 at 5:55 AM

quote:

// allocate a buffer, which is one byte
// biger, so that at the end of the
// buffer is the needed 0.

What needed zero are you talking about ? This is not a C string.
Additionally, there are few cases where malloc is useful in C++.
This is not one of them.


#include <fstream>    // for std::ifstream

#include <vector>     // for std::vector

#include <iterator>   // for std::istreambuf_iterator

#include <algorithm>  // for std::copy


using namespace std;  // So that you can omit the std:: prefix


// open the file

ifstream ifs( File, ios::binary );

// declare the buffer

vector<chat> buffer;

// read the file

buffer.assign( istreambuf_iterator<char>( ifs ),
               istreambuf_iterator<char>() );

// close the file (automatic when ifs gets destroyed)

ifs.close()

// If *really* necessary, copy it into a char* buffer

char* cbuf = new char[buffer.size()];
copy( buffer.begin(), buffer.end(), cbuf );

// sometime later, don't forget to do

delete[] cbuf;

T2k

Jul 16, 2003 at 5:57 AM

quote:
if (!buffer)

this will only fail if ur malloc (mem allocation) fails, not if u cant read the file, place it directly after malloc to prevent writing into an invalid buffer

quote:
Size + 1

why +1 ??? do u check 4 nulltermination? if so dont do it!

... ur code looks ok, reads in the file into the buffer, u dont need to set memory before reading (memset) and ur buffer has the size of ''Size + 1''

but u maybe need to understand how a bmp is build, see BMP Fileformat

T2k

Jul 16, 2003 at 5:58 AM

What makes you say it''s only 4 elements long?

Try:

Checking the value of size
Checking the return value of fread

Fruny

203

Fruny

Jul 16, 2003 at 5:59 AM

quote:
Original post by JY
What makes you say it''s only 4 elements long?

I bet he''s taking the size of the pointer for the size of the array.

Austrian Coder

Author

Austrian Coder

Jul 16, 2003 at 6:15 AM

Ok... i have made some changes:


		FILE*			pfile;
		unsigned int	Size;
		char*			buffer;			

		// no we try to open the file

		// try to open 2 source files

		if ((pfile = fopen(File, "rb")) != 0)
		{
			 // obtain file size.

			fseek (pfile , 0 , SEEK_END);
			Size = ftell (pfile);
			rewind (pfile);
		
			// allocate a buffer, which is one byte

			buffer = new char[Size];

			fread (buffer, 1, Size, pfile);

			// check if reading was ok

			if (!buffer)
			{
				cout << "Error while reading file!" << endl;
				return false;
			}

			// close file

			fclose(pfile);
		}
		else
		{
			// we could not open the file

			cout << "Could not open file" << endl;
			return false;
		}
		...
		delete[] buffer;

If i now make a debug breakpoint at if (!buffer) then in msvc 6.0 it shows me this: BM` and two black quads. Thats all... I need the whole file in a buffer, to compress it and save it again.

[edited by - Austrian Coder on July 16, 2003 7:20:18 AM]

[edited by - Austrian Coder on July 16, 2003 7:21:00 AM]

Jul 16, 2003 at 6:31 AM

quote:
I bet he''s taking the size of the pointer for the size of the array.

Yip, that would be my guess. If you are using Visual Studio, it looks at char * buffers as text strings so stops displaying them once it gets to a \0 character. A bitmap is a binary file which can quite possibly contain \0 characters.

Try casting buffer to a BITMAPFILEHEADER structure pointer and check the members. You should then see at least the size of the file by looking at the bdSize member.

Yanroy

Jul 16, 2003 at 8:46 AM

I''m not sure if it will make a difference or not, but I would declare buffer as type BYTE. Because, after all, it''s not a character array. Although BYTE = unsigned char, so maybe that''s not any better

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Austrian Coder

Author

Austrian Coder

Jul 16, 2003 at 9:10 AM

Now it seems to run... thanks all for their help!

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