An extended standard interface between an application and a file system driver is provided by DosFSCtl. This function is similar to DosDevIOCtl, which provides a standard interface between an application and a device driver. An application sends a request to the file system driver by specifying a particular function code. Data is exchanged through data areas and parameter lists.

DosFSCtl can be used to establish open connections to file system drivers that are not attached to a name in the operating system's name space. (A name space is a set of names that are known to the file system. For example, CON and PRN are always in the OS/2 file system's name space.)

The following code fragment demonstrates how a process can communicate with a file system driver (FSD). Assume that the calling process has placed an appropriate file handle into FileHandle. Assume that the specified file system recognizes a function code of hex 8100, and that the function code accepts an ASCII string as input, requires no specific command parameter list, and returns a string of ASCII characters to the caller.

    #define INCL_DOSFILEMGR   /* File System values */
    #include <os2.h>
    #include <stdio.h>
    #include <string.h>

    UCHAR   ucDataArea[100];    /* Data area                         */
    ULONG   ulDataLengthMax;    /* Max. length of Data area          */
    ULONG   ulDataLengthInOut;  /* Data area length, in and out      */
    PVOID   pParmList;          /* Parameter list                    */
    ULONG   ulParmLengthMax;    /* Max. length of Parameter list     */
    ULONG   ulParmLengthInOut;  /* Parameter list length, in and out */
    ULONG   ulFunctionCode;     /* Function code                     */
    PSZ     pszRouteName;       /* Path or FSD name                  */
    HFILE   hfFileHandle;       /* File handle                       */
    ULONG   ulRouteMethod;      /* Method for routing                */
    APIRET  ulrc;               /* Return code                       */

    ulFunctionCode = 0x8100;    /* Indicate the function to request  */
                                /* of the file system                */

    strcpy(ucDataArea,
           "PARM1: 98"); /* ASCII string to pass to file system */

    ulDataLengthMax = 100;      /* Tell the file system the maximum  */
                                /* amount of data it can return      */

    ulDataLengthInOut = strlen(ucDataArea);
                              /* On input, this is the number of   */
                              /* bytes sent to the file system     */

    pParmList = 0;              /* In this example, assume that no   */
    ulParmLengthMax = 0;        /* specific command parameter list   */
    ulParmLengthInOut = 0;      /* is required by the file system    */
                              /* for this function code            */

    ulRouteMethod = 1;          /* Indicate that the file handle     */
    pszRouteName = 0;           /* directs routing (this implies     */
                                /* that the RouteName variable is    */
                                /* unused in this example)           */

    ulrc = DosFSCtl(ucDataArea,
                    ulDataLengthMax,
                    &ulDataLengthInOut,
                    pParmList,
                    ulParmLengthMax,
                    &ulParmLengthInOut,
                    ulFunctionCode,
                    pszRouteName,
                    hfFileHandle,
                    ulRouteMethod);

    if (ulrc != 0) {
        printf("DosFSCtl error: return code = %ld", ulrc);
        return;
    }

In this example, the the DataArea buffer is used to store the ASCII string sent by the file system in response to the function request, and the DataLengthInOut variable is used to store the number of bytes placed in the buffer by the file system.