How do I work with OPTIONAL and DEFAULT components?

A structure which has OPTIONAL and DEFAULT components in it has a bit_mask field generated for it along with #define constants for each non-pointer optional field. For example,

Module DEFINITIONS ::= BEGIN
DataCard ::= SEQUENCE {
a [0] BOOLEAN OPTIONAL,
b [1] BOOLEAN OPTIONAL,
c [2] BOOLEAN
}
END 

After passing the above notation through the ASN.1 compiler, the following is generated in the header file:

typedef struct DataCard {
unsigned char bit_mask;
# define a_present 0x80
# define b_present 0x40
ossBoolean a; 
ossBoolean b; 
ossBoolean c;
} DataCard; 

To indicate the presence of one particular field, you simply set the bit_mask field equal to the corresponding #define constant. To indicate the presence of more than one optional field, you simply perform a logical OR with the #define constants corresponding to the desired optional fields and assign the result to the bit_mask field. Sample code to illustrate:

DataCard myData; 
myData.bit_mask &= a_present; 

In the case of pointered elements, a value of NULL indicates absence of the optional fields while a non-NULL value indicates presence.

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