{ $description "Outputs a freshly allocated byte-array whose elements are C type values from the given sequence." }
{ $notes "The appropriate specialized array vocabulary must be loaded; otherwise, an error will be thrown. See the " { $vocab-link "specialized-arrays" } " vocabulary for details on the underlying sequence type constructed." }
{ $notes "The appropriate specialized array vocabulary must be loaded; otherwise, an error will be thrown. See the " { $vocab-link "specialized-arrays" } " vocabulary for details on the underlying sequence type constructed." }
{ $description "Literal syntax, consists of a C-type followed by a series of values terminated by " { $snippet "}" } }
{ $notes "The appropriate specialized array vocabulary must be loaded; otherwise, an error will be thrown. 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 "Converts a byte array into a specialized array by interpreting the bytes in as machine-specific values. Code which uses this word is unportable." }
{ $notes "The appropriate specialized array vocabulary must be loaded; otherwise, an error will be thrown. 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. 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." } ;
{ $values { "c-types""a list of scoped allocation specifiers" } { "quot" quotation } }
{ $description "Allocates values on the call stack, calls the quotation, then deallocates the values as soon as the quotation returns."
$nl
"A scoped allocation specifier is either:"
{ $list
"a C type name,"
{ "or a triple with shape " { $snippet "{ c-type initial: initial }" } ", where " { $snippet "c-type" } " is a C type name and " { $snippet "initial" } " is a literal value." }
}
"If no initial value is specified, the contents of the allocated memory are undefined." }
{ $warning "Reading or writing a scoped allocation buffer outside of the given quotation will cause memory corruption." }
{ $description "Allocates values on the call stack, calls the quotation, then copies all stack allocated values to the data heap after the quotation returns."
$nl
"A scoped allocation specifier is either:"
{ $list
"a C type name,"
{ "or a triple with shape " { $snippet "{ c-type initial: initial }" } ", where " { $snippet "c-type" } " is a C type name and " { $snippet "initial" } " is a literal value." }
}
"If no initial value is specified, the contents of the allocated memory are undefined." }
{ $warning "Reading or writing a scoped allocation buffer outside of the given quotation will cause memory corruption." } ;
"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" } "." } ;
"Sometimes it is useful to create a byte array storing a single C value, like a struct with a single field. A pair of utility words exist to make this more convenient:"
{ $subsections <ref> deref }
"These words can be used to in conjuction with, or instead of, " { $link with-out-parameters } " to handle \"out-parameters\". For example, if a function is declared in the following way:"
{ $code
"FUNCTION: int do_foo ( int* a )"
}
"and writes to the pointer 'a', then it can be called like this:"
{ $code
"1234 int <ref> [ do_foo ] keep int deref"
}
"The stack will then contain the two integers emitted by the 'do_foo' function.";
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-STRUCT: } ". 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"
{ $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. 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 encoded to the requested encoding and a raw pointer is passed to the function. "
"Passing an already encoded " { $link byte-array } " also works and performs no conversion."
}
{ "Returning a C string from a C function allocates a Factor string in the Factor heap; the memory pointed to by the returned pointer is then decoded with the requested encoding into the Factor string." }
"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.";
"A frequently-occurring idiom in C code is the \"out parameter\". If a C function returns more than one value, the caller passes pointers of the correct type, and the C function writes its return values to those locations."
"The idiom is commonly used for passing back an error message if the function calls fails. For example, if a function is declared in the following way:"
{ $code
"FUNCTION: int do_frob ( int arg1, char** errptr )"
}
"Then it could return 1 on error and 0 otherwise. A correct way to call it would be:"
