► |  | The Racket Foreign Interface

1 | Overview
2 | Loading Foreign Libraries
3 | C Types
4 | Pointer Functions
5 | Derived Utilities
6 | Miscellaneous Support
7 | Unexported Primitive Functions
| Bibliography
| Index

▼ | 3 | C Types

3.1 | Type Constructors
3.2 | Numeric Types
3.3 | Other Atomic Types
3.4 | String Types
3.5 | Pointer Types
3.6 | Function Types
3.7 | C Struct Types
3.8 | C Array Types
3.9 | C Union Types
3.10 | Enumerations and Masks

► | 3.4 | String Types

3.4.1 | Primitive String Types
3.4.2 | Fixed Auto-Converting String Types
3.4.3 | Variable Auto-Converting String Type
3.4.4 | Other String Types

On this page:

3.4.1`` Primitive String Types
_ string/ ucs-4
_ string/ utf-16
_ path
_ symbol
3.4.2`` Fixed Auto-Converting String Types
_ string/ utf-8
_ string/ latin-1
_ string/ locale
_ string*/ utf-8
_ string*/ latin-1
_ string*/ locale
3.4.3`` Variable Auto-Converting String Type
_ string
default-_ string-type
3.4.4`` Other String Types
_ file
_ bytes/ eof
_ string/ eof

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#### 3.4`` String Types

##### 3.4.1`` Primitive String Types

The five primitive string types correspond to cases where a C representation matches Racket’s representation without encodings.

The form _bytes form can be used type for Racket byte strings, which corresponds to C’s char* type. In addition to translating byte strings, #f corresponds to the NULL pointer.

> > value

> > _string/ucs-4 : ctype?

A type for Racket’s native Unicode strings, which are in UCS-4 format. These correspond to the C mzchar* type used by Racket. As usual, the types treat #f as NULL and vice versa.

> > value

> > _string/utf-16 : ctype?

Unicode strings in UTF-16 format. As usual, the types treat #f as NULL and vice versa.

> > value

> > _path : ctype?

Simple char* strings, corresponding to Racket’s paths. As usual, the types treat #f as NULL and vice versa.

Beware that changing the current directory via current-directory does not change the OS-level current directory as seen by foreign library functions. Paths normally should be converted to absolute form using path->complete-path (which uses the current-directory parameter) before passing them to a foreign function.

> > value

> > _symbol : ctype?

Simple char* strings as Racket symbols (encoded in UTF-8). Return values using this type are interned as symbols.

##### 3.4.2`` Fixed Auto-Converting String Types

> > value

> > _string/utf-8 : ctype?

> _string/latin-1 : ctype?
> _string/locale : ctype?

Types that correspond to (character) strings on the Racket side and char* strings on the C side. The bridge between the two requires a transformation on the content of the string. As usual, the types treat #f as NULL and vice versa.

> > value

> > _string*/utf-8 : ctype?

> _string*/latin-1 : ctype?
> _string*/locale : ctype?

Similar to _string/utf-8, etc., but accepting a wider range of values: Racket byte strings are allowed and passed as is, and Racket paths are converted using path->bytes.

##### 3.4.3`` Variable Auto-Converting String Type

The _string/ucs-4 type is rarely useful when interacting with foreign code, while using _bytes is somewhat unnatural, since it forces Racket programmers to use byte strings. Using _string/utf-8, etc., meanwhile, may prematurely commit to a particular encoding of strings as bytes. The _string type supports conversion between Racket strings and char* strings using a parameter-determined conversion.

> > value

> > _string : ctype?

Expands to a use of the default-_string-type parameter. The parameter’s value is consulted when _string is evaluated, so the parameter should be set before any interface definition that uses _string.

> > parameter

> > (default-_string-type) → ctype?

> (default-_string-type type) → void?
> type : ctype?

A parameter that determines the current meaning of _string. It is initially set to _string*/utf-8. If you change it, do so before interfaces are defined.

##### 3.4.4`` Other String Types

> > value

> > _file : ctype?

Like _path, but when values go from Racket to C, cleanse-path is used on the given value. As an output value, it is identical to _path.

> > value

> > _bytes/eof : ctype?

Similar to the _bytes type, except that a foreign return value of NULL is translated to a Racket eof value.

> > value

> > _string/eof : ctype?

Similar to the _string type, except that a foreign return value of NULL is translated to a Racket eof value.
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