{ $description "Creates a byte array large enough to hold " { $snippet "n" } " values of a C type." }
{ $notes "The appropriate specialized array vocabulary must be loaded; otherwise, an error will be thrown. The vocabulary can be loaded with the " { $link require-c-array } " word. See the " { $vocab-link "specialized-arrays" } " vocabulary for details on the underlying sequence type constructed." }
{ $errors "Throws an error if the type does not exist, the necessary specialized array vocabulary is not loaded, or the requested size is negative." } ;
{ $description "Allocates an unmanaged memory block large enough to hold " { $snippet "n" } " values of a C type, then wraps the memory in a sequence object using " { $link <c-direct-array> } "." }
{ $notes "The appropriate specialized array vocabulary must be loaded; otherwise, an error will be thrown. The vocabulary can be loaded with the " { $link require-c-array } " word. See the " { $vocab-link "specialized-arrays" } " vocabulary for details on the underlying sequence type constructed." }
{ $warning "Don't forget to deallocate the memory with a call to " { $link free } "." }
{ $errors "Throws an error if the type does not exist, if the requested size is negative, if a direct specialized array class appropriate to the type is not loaded, or if memory allocation fails." } ;
"Sometimes data passed to C functions must be allocated at a fixed address. See " { $link "byte-arrays-gc" } " for an explanation of when this is the case."
"The above words record memory allocations, to help catch double frees and track down memory leaks with " { $link "tools.destructors" } ". To free memory allocated by a C library, another word can be used:"
{ "Any data type which defines a method on " { $link >c-ptr } ". This includes " { $link "classes.struct" } " and " { $link "specialized-arrays" } "." }
"The Factor garbage collector can move byte arrays around, and code passing byte arrays, or objects backed by byte arrays, must obey important guidelines. See " { $link "byte-arrays-gc" } "." } ;
ARTICLE: "c-data""Passing data between Factor and C"
"Two defining characteristics of Factor are dynamic typing and automatic memory management, which are somewhat incompatible with the machine-level data model exposed by C. Factor's C library interface defines its own set of C data types, distinct from Factor language types, together with automatic conversion between Factor values and C types. For example, C integer types must be declared and are fixed-width, whereas Factor supports arbitrary-precision integers."
$nl
"Furthermore, Factor's garbage collector can move objects in memory; for a discussion of the consequences, see " { $link "byte-arrays-gc" } "."
"C struct and union types can be defined with " { $link POSTPONE:STRUCT: } " and " { $link POSTPONE:UNION: } ". See " { $link "classes.struct" } " for details. For passing arrays to and from C, use the " { $link "specialized-arrays" } " vocabulary.";
{ $description "Encodes a string together with a trailing null code point using the given encoding, and stores the resulting bytes in a freshly-allocated unmanaged memory block." }
{ $warning "Don't forget to deallocate the memory with a call to " { $link free } "." }
{ $errors "Throws an error if one of the following conditions occurs:"
{ $list
"the string contains null code points"
"the string contains characters not representable using the encoding specified"
"memory allocation fails"
}
} ;
HELP:require-c-array
{ $values { "c-type""a C type" } }
{ $description "Generates a specialized array of " { $snippet "c-type" } " using the " { $link <c-array> } " or " { $link <c-direct-array> } " vocabularies." }
{ $notes "This word must be called inside a compilation unit. See the " { $vocab-link "specialized-arrays" } " vocabulary for details on the underlying sequence types loaded." } ;
{ $description "Constructs a new specialized array of length " { $snippet "len" } " and element type " { $snippet "c-type" } " over the range of memory referenced by " { $snippet "alien" } "." }
{ $notes "The appropriate specialized array vocabulary must be loaded; otherwise, an error will be thrown. The vocabulary can be loaded with the " { $link require-c-array } " word. See the " { $vocab-link "specialized-arrays" } " vocabulary for details on the underlying sequence type constructed." } ;
"C string types are arrays with shape " { $snippet "{ c-string encoding }" } ", where " { $snippet "encoding" } " is an encoding descriptor. The type " { $link c-string } " is an alias for " { $snippet "{ c-string utf8 }" } ". See " { $link "encodings-descriptors" } " for information about encoding descriptors. In " { $link POSTPONE:TYPEDEF: } ", " { $link POSTPONE:FUNCTION: } ", " { $link POSTPONE:CALLBACK: } ", and " { $link POSTPONE:STRUCT: } " definitions, the shorthand syntax " { $snippet "c-string[encoding]" } " can be used to specify the string encoding."
"Passing a Factor string to a C function expecting a " { $link c-string } " allocates a " { $link byte-array } " in the Factor heap; the string is then converted to the requested format and a raw pointer is passed to the function."
"If the conversion fails, for example if the string contains null bytes or characters with values higher than 255, a " { $link c-string-error. } " is thrown."
"Care must be taken if the C function expects a pointer to a string with its length represented by another parameter rather than a null terminator. Passing the result of calling " { $link length } " on the string object will not suffice. This is because a Factor string of " { $emphasis "n" } " characters will not necessarily encode to " { $emphasis "n" } " bytes. The correct idiom for C functions which take a string with a length is to first encode the string using " { $link encode } ", and then pass the resulting byte array together with the length of this byte array."
"Sometimes a C function has a parameter type of " { $link void* } ", and various data types, among them strings, can be passed in. In this case, strings are not automatically converted to aliens, and instead you must call one of these words:"
"The first allocates " { $link byte-array } "s, and the latter allocates manually-managed memory which is not moved by the garbage collector and has to be explicitly freed by calling " { $link free } ". See " { $link "byte-arrays-gc" } " for a discussion of the two approaches."
"The C type " { $snippet "char*" } " represents a generic pointer to " { $snippet "char" } "; arguments with this type will expect and return " { $link alien } "s, and won't perform any implicit string conversion."
"For example, if a C function returns a " { $link c-string } " but stipulates that the caller must deallocate the memory afterward, you must define the function as returning " { $snippet "char*" } " and call " { $link (free) } " yourself.";
