C, Win32 API: l need help with ReadFile function

[evaboy]

Hello,
Happy New Month to you all.

However, l am playing with Createfile, WriteFile and ReadFile functions, Createfile and Writefile works well but my problem is ReadFile.

l want to read the data written to a buffer "szBuffer", by using "ReadFile" to read the data in the szBuffer into another buffer szBuf. But it doesn't work.

When l check the return value like error=ReadFile(hfile,szBuffer,10,&in,NULL), it returns 0 but when l use GetLastError() to check for the return value, it gives me 998. l learnt that the second parameter to ReadFile() is the address to store the data read.
Code snippet below:

hfile=CreateFile("C:\file.txt",GENERIC_READ|GENERIC_WRITE,FILE_SHARE_READ|FILE_SHARE_WRITE,NULL,CREATE_NEW,FILE_ATTRIBUTE_NORMAL,NULL);// this works well
				
				WriteFile(hfile,szBuffer,256,&in,NULL);//this works well
				
				
				ReadFile(hfile,szBuf,256,&in,NULL);//this does not read data
				CloseHandle(hfile);
					
					hdc=GetDC(hwnd);
					TextOut(hdc,50,300,szerror,wsprintf(szerror,"%i",GetLastError()));//check return value
					ReleaseDC(hwnd,hdc);

Can someone help me to overcome this?

Thanks.

[kbw]

You can lookup what the error means here.
https://msdn.microsoft.com/en-us/library/cc231199.aspx

What does the declaration of those variables look like? We can't give an answer without seeing them.

Your file should be "C:\\file.txt", you have to escape the \.

Where is the file position before the read? It's at the end of the file, right?

[freddie1]

As kbw intimated, the file pointer is likely at the end of the file due to the write operation. Possibly could use SetFilePointer() to set to beginning of file.

[evaboy]

Ok. l have done necessary corrections but the problem still persists.

static TCHAR szBuffer[100]="Read file testing"; static TCHAR  *szBuf; DWORD in; static TCHAR szerror[100];



hfile=CreateFile("C:\\file.txt",GENERIC_READ|GENERIC_WRITE,FILE_SHARE_READ|FILE_SHARE_WRITE,NULL,CREATE_NEW,FILE_ATTRIBUTE_NORMAL,NULL);// this works well
				
				WriteFile(hfile,szBuffer,10,&in,NULL);//this works well
				
				SetFilePointer(hfile,0,0,FILE_BEGIN);
				ReadFile(hfile,szBuf,10,&in,NULL);//this does not read data
				CloseHandle(hfile);
					
					hdc=GetDC(hwnd);
TextOut(hdc,0,100,szBuf,10)// to see print out, but nothing prints.
					TextOut(hdc,50,300,szerror,wsprintf(szerror,"%i",GetLastError()));//check return value
					ReleaseDC(hwnd,hdc);

When l check for ReadFile return value directly, it gives me 0 but when l check with GetLastError, it gives me 998.

l need directions please.

[freddie1]

Is all the code shown above within a WM_PAINT handler?

[evaboy]

@Freddie1: No. But l have gotten the answer being that the szbuf was not initialized.
So l set it static TCHAR szBuf[100]={0};

and now it's working well.

Thanks alot for your concern. God bless you all.