Ada Programming/Pragmas/Pack

Pragmas Pack __TOC__ pragma Pack (local_name); Description Pack is a representation pragma that directs the compiler to use type representations that favor conservation of storage space, rather than ease of access. It can be used with composite types, like arrays and records. Using pragma Pack will usually result in smaller data structures at the cost of additional operations to retrieve each element (bit operations may be required in order to retrieve packed data). This may or may not result in a loss of execution time, since the increase of instructions executed to access each element can be cheaper than the cost to read an unpacked data structure from memory. Sub-optimal code for packed boolean arrays — bug or inherent limitation? Robert A. Duff 2007-07-09 comp.lang.ada wccodim33u0.fsf@shell01.TheWorld.com http://groups.google.es/group/comp.lang.ada/msg/685e3a9bc6dcf58d 2008-05-27 Example A boolean is typically stored in a byte by the compiler, not a bit An array of eight booleans will therefore occupy 64 bits, not 8! type Packed_Bool_Array is array (1 .. 8) of Boolean; With pragma pack, the compiler will try to compress the entire array, in this case probably into a single byte (but this is not guaranteed). Accessing one of these booleans, however, will require bit operations. Pack (Packed_Bool_Array); Incorrect usage Portability The pragma Pack is standard in the language since Ada 83. However, implementations are allowed to produce different results or to ignore it completely, and therefore it is not portable between compilers. It can produce different results depending on the target architecture too. Thus the pragma Pack should not be used when an exact representation is required for a type. Interfacing There is no equivalent standard feature in other programming languages as far as the authors know, so this pragma should not be used when interfacing. GCC provides an extension to the C programming language that can be used to pack structs, unions (equivalent to Ada's records) or enumeration types. Free Software Foundation Using the GNU Compiler Collection (GCC) 5.35 Specifying Attributes of Types — packed http://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html 2008-11-23 packed: This attribute, attached to struct or union type definition, specifies that each member (other than zero-width bitfields) of the structure or union is placed to minimize the memory required. When attached to an enum definition, it indicates that the smallest integral type should be used. Note that the packed attribute is not a standard feature of the C language, and cannot be used with arrays like in Ada. See also Wikibook Ada Programming Ada Programming/Pragmas Ada Programming/Pragmas/Optimize Ada Programming/Pragmas/Implicit_Packing (implementation defined) Ada Programming/Attributes/'Size Ada Programming/Attributes/'Component_Size Ada Programming/Types/mod Ada Reference Manual 13 2 Pragma Pack Ada 95 Quality and Style Guide 10 6 3 Bit Operations on Modular Types Ada 83 Rationale 15 4 1 Representation Pragmas 15 04 References Pack Pack