►Inside:   Racket C API
▼II Inside Racket BC (3m and CGC)
►11 Values and Types (BC)
On this page:
11.1 Standard Types
11.2 Global Constants
11.3 Strings
11.4 Value Functions
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11 Values and Types (BC)🔗ℹ

A Racket value is represented by a pointer-sized value. The low bit is a mark bit: a 1 in the low bit indicates an immediate integer, a 0 indicates a (word-aligned) pointer.

A pointer Racket value references a structure that begins with a Scheme_Object sub-structure, which in turn starts with a tag that has the C type Scheme_Type. The rest of the structure, following the Scheme_Object header, is type-dependent. Racket’s C interface gives Racket values the type Scheme_Object*. (The “object” here does not refer to objects in the sense of the racket/class library.)

Examples of Scheme_Type values include scheme_pair_type and scheme_symbol_type. Some of these are implemented as instances of Scheme_Simple_Object, which is defined in "scheme.h", but extension or embedding code should never access this structure directly. Instead, the code should use macros, such as SCHEME_CAR, that provide access to the data of common Racket types.

For most Racket types, a constructor is provided for creating values of the type. For example, scheme_make_pair takes two Scheme_Object* values and returns the cons of the values.

The macro SCHEME_TYPE takes a Scheme_Object * and returns the type of the object. This macro performs the tag-bit check, and returns scheme_integer_type when the value is an immediate integer; otherwise, SCHEME_TYPE follows the pointer to get the type tag. Macros are provided to test for common Racket types; for example, SCHEME_PAIRP returns 1 if the value is a cons cell, 0 otherwise.

In addition to providing constructors, Racket defines six global constant Racket values: scheme_true, scheme_false, scheme_null, scheme_eof, scheme_void, and scheme_undefined. Each of these has a type tag, but each is normally recognized via its constant address.

An extension or embedding application can create new a primitive data type by calling scheme_make_type, which returns a fresh Scheme_Type value. To create a collectable instance of this type, allocate memory for the instance with scheme_malloc_atomic. From Racket’s perspective, the main constraint on the data format of such an instance is that the first sizeof(Scheme_Object) bytes must correspond to a Scheme_Object record; furthermore, the first sizeof(Scheme_Type) bytes must contain the value returned by scheme_make_type. Extensions with modest needs can use scheme_make_cptr, instead of creating an entirely new type.

Racket values should never be allocated on the stack, and they should never contain pointers to values on the stack. Besides the problem of restricting the value’s lifetime to that of the stack frame, allocating values on the stack creates problems for continuations and threads, both of which copy into and out of the stack.

11.1 Standard Types🔗ℹ

The following are the Scheme_Type values for the standard types:

The following are the procedure types:

The predicate SCHEME_PROCP returns 1 for all procedure types and 0 for anything else.

The following are additional number predicates:

11.2 Global Constants🔗ℹ

There are six global constants:

In some embedding contexts, the function forms scheme_make_null, etc., must be used, instead.

11.3 Strings🔗ℹ

As noted in Racket BC, Unicode, Characters, and Strings, a Racket character is a Unicode code point represented by a mzchar value, and character strings are mzchar arrays. Racket also supplies byte strings, which are char arrays.

For a character string s, SCHEME_CHAR_STR_VAL(s) produces a pointer to mzchars, not chars. Convert a character string to its UTF-8 encoding as byte string with scheme_char_string_to_byte_string. For a byte string bs, SCHEME_BYTE_STR_VAL(bs) produces a pointer to chars. The function scheme_byte_string_to_char_string decodes a byte string as UTF-8 and produces a character string. The functions scheme_char_string_to_byte_string_locale and scheme_byte_string_to_char_string_locale are similar, but they use the current locale’s encoding instead of UTF-8.

For more fine-grained control over UTF-8 encoding, use the scheme_utf8_decode and scheme_utf8_encode functions, which are described in String Encodings (BC).

11.4 Value Functions🔗ℹ

Scheme_Object*

 

scheme_make_null

(

)

Returns scheme_null.

Scheme_Object*

 

scheme_make_eof

(

)

Returns scheme_eof.

Scheme_Object*

 

scheme_make_true

(

)

Returns scheme_true.

Scheme_Object*

 

scheme_make_false

(

)

Returns scheme_false.

Scheme_Object*

 

scheme_make_void

(

)

Returns scheme_void.

Scheme_Object*

 

scheme_make_char

(

mzchar ch)

Returns the character value. The ch value must be a legal Unicode code point (and not a surrogate, for example). The first 256 characters are represented by constant Racket values, and others are allocated.

Scheme_Object*

 

scheme_make_char_or_null

(

mzchar ch)

Like scheme_make_char, but the result is NULL if ch is not a legal Unicode code point.

Scheme_Object*

 

scheme_make_character

(

mzchar ch)

Returns the character value. This is a macro that directly accesses the array of constant characters when ch is less than 256.

Scheme_Object*

 

scheme_make_ascii_character

(

mzchar ch)

Returns the character value, assuming that ch is less than 256. (This is a macro.)

Scheme_Object*

 

scheme_make_integer

(

intptr_t i)

Returns the integer value; i must fit in a fixnum. (This is a macro.)

Scheme_Object*

 

scheme_make_integer_value

(

intptr_t i)

Returns the integer value. If i does not fit in a fixnum, a bignum is returned.

Scheme_Object*

scheme_make_integer_value_from_unsigned

(

uintptr_t i)

Like scheme_make_integer_value, but for unsigned integers.

Scheme_Object*

scheme_make_integer_value_from_long_long

(

mzlonglong i)

Like scheme_make_integer_value, but for mzlonglong values (see Racket BC Integers).

Scheme_Object*

scheme_make_integer_value_from_unsigned_long_long

(

umzlonglong i)

Like scheme_make_integer_value_from_long_long, but for unsigned integers.

Scheme_Object*

scheme_make_integer_value_from_long_halves

(

uintptr_t hi,

 

 

uintptr_t lo)

Creates an integer given the high and low intptr_ts of a signed integer. Note that on 64-bit platforms where long long is the same as intptr_t, the resulting integer has 128 bits. (See also Racket BC Integers.)

Scheme_Object*

scheme_make_integer_value_from_unsigned_long_halves

(

uintptr_t hi,

 

 

uintptr_t lo)

Creates an integer given the high and low intptr_ts of an unsigned integer. Note that on 64-bit platforms where long long is the same as intptr_t, the resulting integer has 128 bits.

int

 

scheme_get_int_val

(

Scheme_Object* o,

 

 

 

 

intptr_t* i)

Extracts the integer value. Unlike the SCHEME_INT_VAL macro, this procedure will extract an integer that fits in a intptr_t from a Racket bignum. If o fits in a intptr_t, the extracted integer is placed in *i and 1 is returned; otherwise, 0 is returned and *i is unmodified.

int

 

scheme_get_unsigned_int_val

(

Scheme_Object* o,

 

 

 

 

uintptr_t* i)

Like scheme_get_int_val, but for unsigned integers.

int

 

scheme_get_long_long_val

(

Scheme_Object* o,

 

 

 

 

mzlonglong* i)

Like scheme_get_int_val, but for mzlonglong values (see Racket BC Integers).

int

 

scheme_get_unsigned_long_long_val

(

Scheme_Object* o,

 

 

 

 

umzlonglong* i)

Like scheme_get_int_val, but for unsigned mzlonglong values (see Racket BC Integers).

Scheme_Object*

 

scheme_make_double

(

double d)

Creates a new floating-point value.

Scheme_Object*

 

scheme_make_float

(

float d)

Creates a new single-precision floating-point value. The procedure is available only when Racket is compiled with single-precision numbers enabled.

double

 

scheme_real_to_double

(

Scheme_Object* o)

Converts a Racket real number to a double-precision floating-point value.

Scheme_Object*

 

scheme_make_pair

(

Scheme_Object* carv,

 

 

 

 

Scheme_Object* cdrv)

Makes a cons pair.

Scheme_Object*

 

scheme_make_byte_string

(

char* bytes)

Makes a Racket byte string from a nul-terminated C string. The bytes string is copied.

Scheme_Object*

scheme_make_byte_string_without_copying

(

char* bytes)

Like scheme_make_byte_string, but the string is not copied.

Scheme_Object*

 

scheme_make_sized_byte_string

(

char* bytes,

 

 

 

 

intptr_t len,

 

 

 

 

int copy)

Makes a byte string value with size len. A copy of bytes is made if copy is not 0. The string bytes should contain len bytes; bytes can contain the nul byte at any position, and need not be nul-terminated if copy is non-zero. However, if len is negative, then the nul-terminated length of bytes is used for the length, and if copy is zero, then bytes must be nul-terminated.

Scheme_Object*

 

scheme_make_sized_offset_byte_string

(

char* bytes,

 

 

 

 

intptr_t d,

 

 

 

 

intptr_t len,

 

 

 

 

int copy)

Like scheme_make_sized_byte_string, except the len characters start from position d in bytes. If d is non-zero, then copy must be non-zero.

Scheme_Object*

 

scheme_alloc_byte_string

(

intptr_t size,

 

 

 

 

char fill)

Allocates a new Racket byte string.

Scheme_Object*

 

scheme_append_byte_string

(

Scheme_Object* a,

 

 

 

 

Scheme_Object* b)

Creates a new byte string by appending the two given byte strings.

Scheme_Object*

 

scheme_make_locale_string

(

char* bytes)

Makes a Racket string from a nul-terminated byte string that is a locale-specific encoding of a character string; a new string is allocated during decoding. The “locale in the name of this function thus refers to bytes, and not the resulting string (which is internally stored as UCS-4).

Scheme_Object*

 

scheme_make_utf8_string

(

char* bytes)

Makes a Racket string from a nul-terminated byte string that is a UTF-8 encoding. A new string is allocated during decoding. The “utf8” in the name of this function thus refers to bytes, and not the resulting string (which is internally stored as UCS-4).

Scheme_Object*

 

scheme_make_sized_utf8_string

(

char* bytes,

 

 

 

 

intptr_t len)

Makes a string value, based on len UTF-8-encoding bytes (so the resulting string is len characters or less). The string bytes should contain at least len bytes; bytes can contain the nul byte at any position, and need not be null-terminated. However, if len is negative, then the nul-terminated length of bytes is used for the length.

Scheme_Object*

 

scheme_make_sized_offset_utf8_string

(

char* bytes,

 

 

 

 

intptr_t d,

 

 

 

 

intptr_t len)

Like scheme_make_sized_char_string, except the len characters start from position d in bytes.

Scheme_Object*

 

scheme_make_char_string

(

mzchar* chars)

Makes a Racket string from a nul-terminated UCS-4 string. The chars string is copied.

Scheme_Object*

scheme_make_char_string_without_copying

(

mzchar* chars)

Like scheme_make_char_string, but the string is not copied.

Scheme_Object*

 

scheme_make_sized_char_string

(

mzchar* chars,

 

 

 

 

intptr_t len,

 

 

 

 

int copy)

Makes a string value with size len. A copy of chars is made if copy is not 0. The string chars should contain len characters; chars can contain the nul character at any position, and need not be nul-terminated if copy is non-zero. However, if len is negative, then the nul-terminated length of chars is used for the length, and if copy is zero, then the chars must be nul-terminated.

Scheme_Object*

scheme_make_sized_offset_char_string

(

mzchar* chars,

 

 

intptr_t d,

 

 

intptr_t len,

 

 

int copy)

Like scheme_make_sized_char_string, except the len characters start from position d in chars. If d is non-zero, then copy must be non-zero.

Scheme_Object*

 

scheme_alloc_char_string

(

intptr_t size,

 

 

 

 

mzchar fill)

Allocates a new Racket string.

Scheme_Object*

 

scheme_append_char_string

(

Scheme_Object* a,

 

 

 

 

Scheme_Object* b)

Creates a new string by appending the two given strings.

Scheme_Object*

scheme_char_string_to_byte_string

(

Scheme_Object* s)

Converts a Racket character string into a Racket byte string via UTF-8.

Scheme_Object*

scheme_byte_string_to_char_string

(

Scheme_Object* s)

Converts a Racket byte string into a Racket character string via UTF-8.

Scheme_Object*

scheme_char_string_to_byte_string_locale

(

Scheme_Object* s)

Converts a Racket character string into a Racket byte string via the locale’s encoding.

Scheme_Object*

scheme_byte_string_to_char_string_locale

(

Scheme_Object* s)

Converts a Racket byte string into a Racket character string via the locale’s encoding.

Scheme_Object*

 

scheme_intern_symbol

(

char* name)

Finds (or creates) the symbol matching the given nul-terminated, ASCII string (not UTF-8). The case of name is (non-destructively) normalized before interning if scheme_case_sensitive is 0.

Scheme_Object*

 

scheme_intern_exact_symbol

(

char* name,

 

 

 

 

int len)

Creates or finds a symbol given the symbol’s length in UTF-8-encoding bytes. The case of name is not normalized.

Scheme_Object*

 

scheme_intern_exact_char_symbol

(

mzchar* name,

 

 

 

 

int len)

Like scheme_intern_exact_symbol, but given a character array instead of a UTF-8-encoding byte array.

Scheme_Object*

 

scheme_make_symbol

(

char* name)

Creates an uninterned symbol from a nul-terminated, UTF-8-encoding string. The case is not normalized.

Scheme_Object*

 

scheme_make_exact_symbol

(

char* name,

 

 

 

 

int len)

Creates an uninterned symbol given the symbol’s length in UTF-8-encoded bytes.

Scheme_Object*

 

scheme_intern_exact_keyword

(

char* name,

 

 

 

 

int len)

Creates or finds a keyword given the keywords length in UTF-8-encoding bytes. The case of name is not normalized, and it should not include the leading hash and colon of the keyword’s printed form.

Scheme_Object*

 

scheme_intern_exact_char_keyword

(

mzchar* name,

 

 

 

 

int len)

Like scheme_intern_exact_keyword, but given a character array instead of a UTF-8-encoding byte array.

Scheme_Object*

 

scheme_make_vector

(

intptr_t size,

 

 

 

 

Scheme_Object* fill)

Allocates a new vector.

Scheme_Double_Vector*

 

scheme_alloc_flvector

(

intptr_t size)

Allocates an uninitialized flvector. The result type is effectively an alias for Scheme_Object*.

Scheme_Vector*

 

scheme_alloc_fxvector

(

intptr_t size)

Allocates an uninitialized fxvector. The result type is effectively an alias for Scheme_Object*.

Scheme_Object*

 

scheme_box

(

Scheme_Object* v)

Creates a new box containing the value v.

Scheme_Object*

 

scheme_make_weak_box

(

Scheme_Object* v)

Creates a new weak box containing the value v.

Scheme_Type

 

scheme_make_type

(

char* name)

Creates a new type (not a Racket value). The type tag is valid across all places.

Scheme_Object*

 

scheme_make_cptr

(

void* ptr,

 

 

 

 

const Scheme_Object* typetag)

Creates a C-pointer object that encapsulates ptr and uses typetag to identify the type of the pointer. The SCHEME_CPTRP macro recognizes objects created by scheme_make_cptr. The SCHEME_CPTR_VAL macro extracts the original ptr from the Racket object, and SCHEME_CPTR_TYPE extracts the type tag. The SCHEME_CPTR_OFFSETVAL macro returns 0 for the result Racket object.

The ptr can refer to either memory managed by the garbage collector or by some other memory manager. Beware, however, of retaining a ptr that refers to memory released by another memory manager, since the enclosing memory range might later become managed by the garbage collector (in which case ptr might become an invalid pointer that can crash the garbage collector).

Scheme_Object*

scheme_make_external_cptr

(

void* ptr,

 

 

const Scheme_Object* typetag)

Like scheme_make_cptr, but ptr is never treated as referencing memory managed by the garbage collector.

Scheme_Object*

scheme_make_offset_cptr

(

void* ptr,

 

 

intptr_t offset,

 

 

const Scheme_Object* typetag)

Creates a C-pointer object that encapsulates both ptr and offset. The SCHEME_CPTR_OFFSETVAL macro returns offset for the result Racket object (and the macro be used to change the offset, since it also works on objects with no offset).

The ptr can refer to either memory managed by the garbage collector or by some other memory manager; see also scheme_make_cptr.

Scheme_Object*

scheme_make_offset_external_cptr

(

void* ptr,

 

 

intptr_t offset,

 

 

const Scheme_Object* typetag)

Like scheme_make_offset_cptr, but ptr is never treated as referencing memory managed by the garbage collector.

void

 

scheme_set_type_printer

(

Scheme_Type type,

 

 

 

 

Scheme_Type_Printer printer)

Installs a printer to be used for printing (or writing or displaying) values that have the type tag type.

The type of printer is defined as follows:

  typedef void (*Scheme_Type_Printer)(Scheme_Object *v, int dis,

                                      Scheme_Print_Params *pp);

Such a printer must print a representation of the value using scheme_print_bytes and scheme_print_string. The first argument to the printer, v, is the value to be printed. The second argument indicates whether v is printed via write or display. The last argument is to be passed on to scheme_print_bytes or scheme_print_string to identify the printing context.

void

 

scheme_print_bytes

(

Scheme_Print_Params* pp,

 

 

 

 

const char* str,

 

 

 

 

int offset,

 

 

 

 

int len)

Writes the content of str — starting from offset and running len bytes — into a printing context determined by pp. This function is for use by a printer that is installed with scheme_set_type_printer.

void

 

scheme_print_string

(

Scheme_Print_Params* pp,

 

 

 

 

const mzchar* str,

 

 

 

 

int offset,

 

 

 

 

int len)

Writes the content of str — starting from offset and running len characters — into a printing context determined by pp. This function is for use by a printer that is installed with scheme_set_type_printer.

void

 

scheme_set_type_equality

(

Scheme_Type type,

 

 

 

 

Scheme_Equal_Proc equalp,

 

 

 

 

Scheme_Primary_Hash_Proc hash1,

 

 

 

 

Scheme_Secondary_Hash_Proc hash2)

Installs an equality predicate and associated hash functions for values that have the type tag type. The equalp predicate is only applied to values that both have tag type.

The type of equalp, hash1, and hash2 are defined as follows:

  typedef int (*Scheme_Equal_Proc)(Scheme_Object* obj1,

                                   Scheme_Object* obj2,

                                   void* cycle_data);

  typedef intptr_t (*Scheme_Primary_Hash_Proc)(Scheme_Object* obj,

                                           intptr_t base,

                                           void* cycle_data);

  typedef intptr_t (*Scheme_Secondary_Hash_Proc)(Scheme_Object* obj,

                                            void* cycle_data);

The two hash functions are used to generate primary and secondary keys for double hashing in an equal?-based hash table. The result of the primary-key function should depend on both obj and base.

The cycle_data argument in each case allows checking and hashing on cyclic values. It is intended for use in recursive checking or hashing via scheme_recur_equal, scheme_recur_equal_hash_key, and scheme_recur_equal_hash_key. That is, do not call plain scheme_equal, scheme_equal_hash_key, or scheme_equal_hash_key for recursive checking or hashing on sub-elements of the given value(s).

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