C, Win32 API: l need help with ReadFile function[Solved]

User 12105981

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.

Jochen Arndt

After writing to the file, the file pointer is at the actual end of the file when the file has been created. If you then try to read from the file, there is nothing to read at that position (the end of the file). If you want to read the data that has just been written, you must set the file pointer using SetFilePointer function (Windows)[^]:

// Rewind to begin of file
SetFilePointer(hFile, 0, 0, FILE_BEGIN);

A return value of zero (FALSE) indicates that the function failed (see ReadFile function (Windows)[^]). Error code 998 is ERROR_NOACCESS / " Invalid access to memory location."

David Crow

Use double slashes in your file name. ReadFile() will read from the file pointer's current position. Perhaps you need to call SetFilePointer(FILE_BEGIN) in between write and read.

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User 12105981

Thanks. But l have made necessary corrections but the ReadFile is still not working and GetLasterror still returns 998. Also, direct check of ReadFile also returns 0. Help please.

Quote:

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
        SetFilePointer(hfile,0,0,FILE\_BEGIN);
        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);

Lost User

You should check the return value from all system calls to see whether the function succeeded or failed. Without that information you are just guessing.

Jochen Arndt

A probable error source is szBuf being NULL.