The max_is Attribute

You can specify the valid bounds of the array using the max_is attribute.

/* IDL file */
[ uuid(20B309B1-015C-101A-B308-02608C4C9B53),
  version(5.0)
]
interface arraytest
{
void fArray5([in] short sMax,
             [in, out, max_is(sMax)]  char achArray[]);
}
 

Field attributes can be supplied in various combinations as long as the stub can use the information to determine the size of the array and the number of bytes to transmit to the server. The relationships between the attributes are defined using the following formulas:

size_is = max_is + 1;
length_is = last_is - first_is + 1;
 

The values associated with the attributes must obey several common-sense rules based on those formulas. These are:

• The first_is index value cannot be smaller than zero and last_is cannot be greater than max_is.
• Do not specify a negative size for an array. Define the first and last elements so they result in a length value of zero or greater. Define the max_is value so the size is zero or greater. If MIDL was invoked with the /error bounds_check option, then the stub raises an exception when the size is less than zero, or the transmitted length is less than zero.
• You cannnot use the length_is and last_is attributes at the same time, nor can you use the size_is and max_is attributes at the same time.

Because of the close relationship in C between arrays and pointers, MIDL also lets you declare arrays in parameter lists using pointer notation. MIDL treats a parameter that is a pointer to a type as an array of that type if the parameter has any of the attributes commonly associated with arrays.

/* IDL file */
[ uuid(20B309B1-015C-101A-B308-02608C4C9B53)
  version(6.0) 
]
interface arraytest
{
void fArray6([in, out, size_is(sSize)] char * p1, 
             [in] short sSize);
void fArray7([in, out, size_is(sSize)] char achArray[], 
             [in] short sSize);
}
 

In the preceding example, the array and pointer parameters in the functions fArray6 and fArray7 are equivalent.