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(gcc-300.info)Type encoding

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Type encoding
=============

   The Objective-C compiler generates type encodings for all the types.
These type encodings are used at runtime to find out information about
selectors and methods and about objects and classes.

   The types are encoded in the following way:

`char'             `c'
`unsigned char'    `C'
`short'            `s'
`unsigned short'   `S'
`int'              `i'
`unsigned int'     `I'
`long'             `l'
`unsigned long'    `L'
`long long'        `q'
`unsigned long     `Q'
long'              
`float'            `f'
`double'           `d'
`void'             `v'
`id'               `@'
`Class'            `#'
`SEL'              `:'
`char*'            `*'
unknown type       `?'
bit-fields         `b' followed by the starting position of the
                   bit-field, the type of the bit-field and the size of
                   the bit-field (the bit-fields encoding was changed
                   from the NeXT's compiler encoding, see below)

   The encoding of bit-fields has changed to allow bit-fields to be
properly handled by the runtime functions that compute sizes and
alignments of types that contain bit-fields.  The previous encoding
contained only the size of the bit-field.  Using only this information
it is not possible to reliably compute the size occupied by the
bit-field.  This is very important in the presence of the Boehm's
garbage collector because the objects are allocated using the typed
memory facility available in this collector.  The typed memory
allocation requires information about where the pointers are located
inside the object.

   The position in the bit-field is the position, counting in bits, of
the bit closest to the beginning of the structure.

   The non-atomic types are encoded as follows:

pointers       `^' followed by the pointed type.
arrays         `[' followed by the number of elements in the array
               followed by the type of the elements followed by `]'
structures     `{' followed by the name of the structure (or `?' if the
               structure is unnamed), the `=' sign, the type of the
               members and by `}'
unions         `(' followed by the name of the structure (or `?' if the
               union is unnamed), the `=' sign, the type of the members
               followed by `)'

   Here are some types and their encodings, as they are generated by the
compiler on an i386 machine:


Objective-C type   Compiler encoding
     int a[10];    `[10i]'
     struct {      `{?=i[3f]b128i3b131i2c}'
       int i;      
       float f[3]; 
       int a:3;    
       int b:2;    
       char c;     
     }             


   In addition to the types the compiler also encodes the type
specifiers.  The table below describes the encoding of the current
Objective-C type specifiers:


Specifier          Encoding
`const'            `r'
`in'               `n'
`inout'            `N'
`out'              `o'
`bycopy'           `O'
`oneway'           `V'


   The type specifiers are encoded just before the type.  Unlike types
however, the type specifiers are only encoded when they appear in method
argument types.