diff options
Diffstat (limited to 'txr.1')
| -rw-r--r-- | txr.1 | 100 |
1 files changed, 97 insertions, 3 deletions
@@ -62011,6 +62011,22 @@ and .code bit compound type operators. +A structure member must not be an incomplete or zero sized array, +unless it is the last member. If the last member of FFI structure is +an incomplete array, then it is a flexible structure. + +A structure member must not be a flexible structure, unless it is the +last member; the containing structure is then itself a flexible structure. + +Flexible structures correspond to the C concept of a "flexible array member": +the idea that the last member of a structure may be an array of unknown size, +which allows for variable-length data at the end of a structure, provided +that the memory is suitably allocated. + +Flexible structures are subject to special restrictions and requirements. See +the section Flexible Structures below. In particular, flexible structures +may not be passed or returned by value. + See also: the .code make-zstruct function and the @@ -63025,6 +63041,73 @@ members of structs and elements of arrays. representation to the foreign representations exhibiting the specified endianness. +.SS* Incomplete Types + +In the \*(TL FFI type system, the following types are +.IR incomplete : +the type +.codn void , +arrays of unspecified size, and any +.code struct +whose last element is of incomplete type. + +An incomplete type cannot used as a function parameter type, or a return +value type. It may not be used as an array element or union member type. +A struct member type may be incomplete only if it is the last member. + +An incomplete structure whose last member is an array is a +.IR "flexible structure" . + +.SS* Flexible Structures + +If a FFI +.code struct +type is defined with an incomplete array (an array of unspecified size) as its +last member, then it specifies an incomplete type known as a +.IR "flexible structure" . +That array is the +.IR "terminating array" . +The terminating array corresponds to a slot in the Lisp structure; that +slot is the +.IR "last slot" . + +A structure which has a flexible structure as its last member is also, +effectively, a flexible structure. + +When a Lisp structure is being converted to the foreign representation +under the control of a flexible structure FFI type, the number of elements +in the terminating array is determined from the length of the object +stored in the last slot of the Lisp structure. The length includes the +terminating null element for +.code zarray +types. The conversion is consistent with the semantics of an incomplete +arrays that is not a structure member. + +In the reverse direction, when a foreign representation is being converted +to a Lisp structure under the control of a flexible structure FFI type, +the size of the array that is accessed and extracted is determined from +the length of the object stored in the last slot, or, if the array type +is a +.code zarray +from detecting null-termination of the foreign array. The conversion of +the array itself is consistent with the semantics of an incomplete +arrays that is not a structure member. +Before the conversion takes place, all of the members of the +structure prior to the the terminating array, are extracted and converted to +Lisp representations. The corresponding slots of the Lisp structure are +updated. Then if the Lisp structure type has a +.code length +method, that method is invoked. The return value of the method is used +to perform an adjustment on the object in the last slot. +If the existing object in the last slot is a vector, its length is adjusted to +the value returned by the method. If the existing +object isn't a vector, then it is replaced by a new +.codn nil -filled +vector, whose length is given by the return value of +.codn length . +The conversion of the terminating array to Lisp representation the proceeds +after this adjustment, using the adjusted last slot object. + .SS* Bitfield Allocation Rules The \*(TL FFI type system follows rules for bitfield allocation which were experimentally derived from the behavior of the GNU C compiler on several @@ -64112,12 +64195,11 @@ expression is evaluated to an object value. If .code type-syntax -denotes a variable length type, and the +denotes an incomplete array or structure type, and the .meta object-expr argument is present, then a .I "dynamic size" is computed: the actual number of bytes required to store -that object value as a foreign representation of the specified variable length -type. +that object value as a foreign representation. The .code sizeof @@ -64130,6 +64212,18 @@ is omitted, or if it is a constant expression according to the .code constantp function. +For the type +.codn void , +incomplete array types, and bitfield types, the one-argument form of +.code sizeof +reports zero. + +For incomplete structure types, the one-argument +.code sizeof +reports a size which is equivalent to the offset of the last member. +The size of an incomplete structure does not include padding +for the most strictly aligned member. + .coNP Macro @ alignof .synb .mets (alignof << type-syntax ) |