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Allows a protected-mode OS/2 session to communicate with a virtual device driver (VDD).


DosRequestVDD(hvdd, sgid, cmd, cbInput, pInput, cbOutput, pOutput)


hvdd (HVDD) - input 
The handle of a VDD returned by a previous call to DosOpenVDD.
sgid (SGID) - input 
The identifier of a specific DOS session, or null.
cmd (ULONG) - input 
A function code that is specific to a virtual device.
cbInput (ULONG) - input 
The length, in bytes, of the application data in pInput.
pInput (PVOID) - input 
The address of the command-specific information.
The system sends this data to the virtual device driver to process the specified command.
cbOutput (ULONG) - input 
The length, in bytes, of pOutput.
pOutput (PVOID) - output 
The address of the buffer where the VDD returns the information for the specified command.
This information is specific to the command and the virtual device driver.

Return Code

ulrc (APIRET) - returns
DosRequestVDD returns one of the following values:
  • 0 NO_ERROR


The system calls every DosRequestVDD procedure registered by VDHRegisterVDD under the VDD name associated with the specified handle. This calling continues until a virtual device driver gives a return code other than VDDREQ_PASS. There is no predefined order to the calling sequence.

Example Code

The following is NOT a complete C program. It is simply intended to provide an idea of how a protected-mode OS/2 process can communicate with a virtual device driver (VDD).

This example shows a protected-mode process calling a hypothetical VDD with a request to read a string of bytes from the VDD. Assume that the session identifier of the specified DOS session has been placed into SessionID already and that the sample virtual device driver has registered the name "VDD007" with the operating system.

 #define INCL_DOSMVDM     /* Multiple DOS sessions values */
 #define INCL_DOSERRORS   /* DOS Error values */
 #include <os2.h>
 #include <stdio.h>

 UCHAR   VDDName[10] = "VDD007";    /* Name of VDD */
 HVDD    VDDHandle   = NULLHANDLE;  /* Handle of VDD */
 SGID    SessionID   = 0;           /* Session identifier (should be initialized */
 ULONG   Command     = 3;           /* VDD function code (hypothetical) */
 APIRET  rc          = NO_ERROR;    /* Return code */
 UCHAR   InputBuffer[30]  = "Your command here";  /* Command information    */
 UCHAR   OutputBuffer[30] = "";                   /* Output data (returned) */

    rc = DosOpenVDD(VDDName, &VDDHandle);
    if (rc != NO_ERROR) {
       printf("VDD %s was not found.\n", rc);
       return 1;

    rc = DosRequestVDD(VDDHandle,              /* Handle of VDD */
                       SessionID,              /* Session ID */
                       Command,                /* Command to send to VDD */
                       sizeof(InputBuffer),    /* Length of input */
                       InputBuffer,            /* Input buffer */
                       sizeof(OutputBuffer),   /* Length of output area */
                       OutputBuffer);          /* Output from command */
    if (rc != NO_ERROR) {
        printf("DosRequestVDD error: return code = %u\n", rc);
        return 1;

    rc = DosCloseVDD(VDDHandle);               /* Close the VDD */
    if (rc != NO_ERROR) {
        printf("DosCloseVDD error: return code = %u\n", rc);
        return 1;

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