C-Compiler Requirements for MIDL

The MIDL compiler requires the C compiler to support a packing level of 1, 2, 4, or 8. The command-line option for Microsoft C compilers that controls packing is /Zplevel, where level is the packing level 1, 2, 4, or 8. The following rules govern the alignment of compound types:

• Base-type fields of size < packing level start on a (0 modulo size) address.
• Base-type fields of size >= packing level start on a (0 modulo packing level) address.
• The compound type itself (and any field of compound type) is aligned according to the strictest alignment requirement on any of its fields.
• Compound types are padded to the next (0 modulo level) address. This padding appears in the size returned by the C SIZEOF macro.

As an example, consider a compound type consisting of a 1-byte character, an integer 4 bytes long, and a 1-byte character:

struct mystructtype {    
    char c1;  /* requires 1 byte  */
    long l2;  /* requires 4 bytes */
    char c3;  /* requires 1 byte  */
 } mystruct;
 

For packing level 4, the structure mystruct is aligned on a (0 mod 4) boundary and sizeof(struct mystructtype) = 12.

For packing level 2, the structure mystruct is aligned on a (0 mod 2) boundary and sizeof(struct mystructtype) = 8.