This section describes the step-by-step procedure for calling a C MDER decode function. This method must be used if C code generation was done. This method can also be used as an alternative to using the control class interface if C++ code generation was done.

Before any decode function can be called; the user must first initialize a context variable. This is a variable of type OSCTXT. This variable holds all of the working data used during the decoding of a message. The context variable is declared as a normal automatic variable within the top-level calling function. It must be initialized before use. This can be accomplished as follows:

   OSCTXT ctxt;
   int stat;

   stat = mderInitContext (&ctxt);
   if (stat != 0) {
      rtxErrPrint (&ctxt);
      rtFreeContext (&ctxt);
      return stat;
   }

The next step is to create a stream reader that will read from the given source. In our example, we read from a file, but it is also possible to read data from a socket or other source as well.

A decode function can then be called to decode the message. If the return status indicates success, the C variable that was passed as an argument will contain the decoded message contents. Note that the decoder may have allocated dynamic memory and stored pointers to objects in the C structure. After processing on the C structure is complete, the run-time library function rtxMemFree should be called to free the allocated memory.

A program fragment that could be used to decode a simple PDU type follows:

   #include ISO-11073-20601-ASN1.h          /* include file generated by ASN1C */

   int main (void)
   {
      ApduType data;
      OSCTXT      ctxt;
      const char* filename = "message.dat";
      int         stat;

      /* Step 1: Initialize a context variable for decoding */

      stat = mderInitContext (&ctxt);
      if (stat != 0) {
         /* initialization failed, could be a license problem */
         rtxErrPrint (&ctxt);
         rtFreeContext (&ctxt);
         return stat;
      }

      /* Step 2: Initialize a stream reader */

      stat = rtxStreamFileCreateReader (&ctxt, filename);
      if (0 != stat) {
         rtxErrPrint (&ctxt);
         rtFreeContext (&ctxt);
         return stat;
      }
      
      /* Step 3: decode */
      stat = MDERDec_ApduType (&ctxt, &data);
      if (stat != 0) {
         printf ("decode of data failed\n");
         rtxErrPrint (&ctxt);
         rtFreeContext (&ctxt);
         return -1;
      }            
   }