REDIRECTSEEK - Raster Imaging C API Help

REDIRECTSEEK

#include "l_bitmap.h"

L_SSIZE_T pEXT_CALLBACK YourSeek(nFd, nPos, nOrigin, pUserData)

L_HFILE nFd;

file handle

L_SSIZE_T nPos;

number of bytes to move

L_INT nOrigin;

position to move from

L_VOID* pUserData;

pointer to additional parameters

Replaces the normal LEADTOOLS function for repositioning a file pointer.

Parameter Description
nFd Identifies the file, using the file handle returned by the REDIRECTOPEN callback function.
nPos Specifies the number of bytes the pointer is to be moved.
nOrigin Specifies the starting position and direction of the pointer. This parameter must be one of the following values:
  Value Meaning
  0 Move the file pointer nPos bytes from the beginning of the file.
  1 Move the file pointer nPos bytes from its current position.
  2 Move the file pointer nPos bytes from the end of the file.
pUserData A void pointer that you can use to access a variable or structure containing data that your callback function needs. This gives you a way to receive data indirectly from the function that uses this callback function. (This is the same pointer that you pass in the pUserData parameter of the calling function.)
  Keep in mind that this is a void pointer, which must be cast to the appropriate data type within your callback function.

Returns

>0

The number of bytes that the function wrote to the file.

-1

The function failed.

Comments

For information about using this type of callback, refer to L_RedirectIO.

Required DLLs and Libraries

LTKRN

For a listing of the exact DLLs and Libraries needed, based on the toolkit version, refer to Files To Be Included With Your Application

Example

To see how the following function is defined and called, refer to L_RedirectIO. This procedure is a replacement to the built-in seek procedure. This emulates the whole seek operation on a memory block, and adjusts the internal data structures accordingly.

/************************** Global Declarations **********************************/ 
 
typedef struct tagUSERDATA 
{ 
   L_INT32 dwSize; /* Size of the buffer. */ 
   L_INT32 dwUsed; /* Number of bytes used. */ 
   L_CHAR *pData; /* Pointer to the buffer. */ 
   L_CHAR *pCurData; /* Current pointer location. */ 
} USERDATA, * LPUSERDATA; 
 
 
/***************************************************************************************/ 
L_INT32  WindowsSeek (L_INT FD, L_INT32 lnPos, L_INT nOrigin, LPUSERDATA pUserData) 
{ 
   UNREFERENCED_PARAMETER (FD); 
 
   switch (nOrigin) 
   { 
   case 0:                     /* SEEK_SET */ 
      pUserData->pCurData = pUserData->pData + lnPos; 
      pUserData->dwUsed = lnPos; 
      break; 
   case 1:                     /* SEEK_CUR */ 
      pUserData->pCurData += lnPos; 
      pUserData->dwUsed += lnPos; 
      break; 
   case 2:                     /* SEEK_END */ 
      if (0 <= lnPos)           /* Positive value, but at the end, so go to 
                                   the end. */ 
         lnPos = 0; 
      else 
         lnPos = -(lnPos);      /* Seek backwards from the end of the buffer. */ 
      pUserData->pCurData = pUserData->pData + pUserData->dwSize - lnPos; 
      pUserData->dwUsed = pUserData->dwSize - lnPos; 
      break; 
   } 
   return ((L_INT)(pUserData->pCurData - pUserData->pData)); 
} 

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