fseek() causing an overlap in the data

Im reading a specified chunk of a file with fseek and fread functions and then writing it to another file. For some reason in the destination file I get about 20 bytes overlap between every chunk written in it.

Could anyone, please, help me identifying the source of this garb开发者_StackOverflow社区age? It is definitely caused by the fseek function, but I cant figure out why.

FILE *pSrcFile; 
FILE *pDstFile; 

int main()
{
int buff[512], i;
long bytesRead;

pSrcFile = fopen ( "test.txt" , "r" );
pDstFile = fopen ( "result1.txt", "a+");

for(i = 0; i < 5; i++)
{
    bytesRead = _readFile ( &i, buff, 512);
    _writeFile( &i, buff, bytesRead);
}

fclose (pSrcFile);
fclose (pDstFile);
}

int _readFile (void* chunkNumber, void* Dstc, long len) 
{
int bytesRead;
long offset = (512) * (*(int*)chunkNumber);

fseek( pSrcFile, offset, SEEK_SET);

bytesRead = fread (Dstc , 1, len, pSrcFile);

return bytesRead;
}

int _writeFile (void* chunkNumber, void const * Src, long len) 
{
int bytesWritten;
long offset = (512) * (*(int*)chunkNumber);

bytesWritten = fwrite( Src , 1 , len , pDstFile );

return bytesWritten;
}

I'm guessing you're on Windows and suffering from the evils of Windows' text mode. Add "b" to the flags you pass to fopen, i.e.

pSrcFile = fopen ( "test.txt" , "rb" );
pDstFile = fopen ( "result1.txt", "a+b");

It seems you are reading from Dest file

bytesRead = fread (Dstc , 1, len, pSrcFile);

and writing to source

bytesWritten = fwrite( Src , 1 , len , pDstFile );

Probably, you have to change Dest to Src.