►The Racket Foreign Interface
▼3 C Types
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3.10 Enumerations and Masks

Although the constructors below are describes as procedures, they are implemented as syntax, so that error messages can report a type name where the syntactic context implies one.

procedure

(_enum symbols [basetype #:unknown unknown]) → ctype?

  symbols : list?
  basetype : ctype? = _ufixint
  unknown : any/c = (lambda (x) (error ....))
Takes a list of symbols and generates an enumeration type. The enumeration maps between a symbol in the given symbols list and corresponding integers, counting from 0.

The list symbols can also set the values of symbols by putting '= and an exact integer after the symbol. For example, the list '(x y = 10 z) maps 'x to 0, 'y to 10, and 'z to 11.

The basetype argument specifies the base type to use.

The unknown argument specifies the result of converting an unknown integer from the foreign side: it can be a one-argument function to be applied on the integer, or a value to return instead. The default is to throw an exception.

Examples:
; example from snappy-c.h
> (define _snappy_status
    (_enum '(ok = 0
             invalid_input
             buffer_too_small)))

procedure

(_bitmask symbols [basetype]) → ctype?

  symbols : (or symbol? list?)
  basetype : ctype? = _uint
Similar to _enum, but the resulting mapping translates a list of symbols to a number and back, using bitwise-ior on the values of individual symbols, where A single symbol is equivalent to a list containing just the symbol.

When a symbol does not have a given value (via '= after the symbol in symbols), its value is the next power of 2 greater than the previous symbol’s assignment (or 1 for the first symbol).

The default basetype is _uint, since high bits are often used for flags.

Examples:
; example from curl.h
> (define _curl_global_flag
    (_bitmask `(CURL_GLOBAL_SSL = 1
                CURL_GLOBAL_WIN32 = 2
                CURL_GLOBAL_ALL = 3
                CURL_GLOBAL_NOTHING = 0
                CURL_GLOBAL_DEFAULT = 3
                CURL_GLOBAL_ACK_EINTR = 4)))
; example from XOrg
> (define _Modifiers
    (_bitmask '(ShiftMask = 1
                LockMask = 2
                ControlMask = 4
                Mod1Mask = 8
                Mod2Mask = 16
                Mod3Mask = 32
                Mod4Mask = 64
                Mod5Mask = 128
                Button1Mask = 256
                Button2Mask = 512
                Button3Mask = 1024
                Button4Mask = 2048
                Button5Mask = 4096
                Any = 32768)))

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