Flexible Array Member: what changed

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Title: Flexible Array Member

New		Line
1	+	Is a rarely used feature of C language. It was introduced with C99 standard. The feature was never adopted by C++ compiler (and that might be the reason of the rate use). Some C-like APIs could benefit from the feature, especially low level ones.
2	+
3	+	## Introduction
4	+
5	+	Disclaimer: this section is pretty much a copy-paste from [Wikipedia article](https://en.wikipedia.org/wiki/Flexible_array_member)
6	+
7	+	Flexible Array Member is a member of a struct, which is an array without a given dimension. It must be the last member of such a struct and it must be accompanied by at least one other member, as in the following example:
8	+
9	+	``` c
10	+	typedef struct mystruct
11	+	{
12	+	DWORD a;
13	+	DWORD b;
14	+	DWORD c[]; // the flexible array member must be last
15	+	};
16	+	```
17	+
18	+	Typically, such structures serve as the header in a larger, variable memory allocation:
19	+
20	+	``` c
21	+	struct mystruct* st= malloc(...);
22	+	...
23	+	for (int i = 0; i < st->b; i++)
24	+	st->c[i] = ...;
25	+	```
26	+
27	+	The sizeof operator on such a struct gives the size of the structure as if the flexible array member was empty. This may include padding added to accommodate the flexible member; the compiler is also free to re-use such padding as part of the array itself.
28	+
29	+	It is common to allocate sizeof(struct) + array_len*sizeof(array element) bytes
30	+
31	+	## Pascal
32	+
33	+	Pascal doesn't have flexible array member, thus the structure should be declared without the additional member, in order for sizeof(mystruct) to return the same value as in C.
34	+
35	+	``` pascal
36	+	type
37	+	mystruct = record
38	+	a : DWORD;
39	+	b : DWORD;
40	+	end;
41	+	pmystruct = ^mystruct;
42	+	```
43	+
44	+	There are a few options on how the additional elements can be accessed:
45	+
46	+	### Direct Addressing
47	+
48	+	By explicitly getting the address that goes after the header struct
49	+
50	+	``` pascal
51	+	var
52	+	i : integer;
53	+	buf : Pmystruct;
54	+	c : PDword;
55	+	begin
56	+	GetMem(buf, sizeof(mystruct)+ extrasize);
57	+	...
58	+	c:=Pointer(buf)+sizeof(mystruct);
59	+
60	+	for i := 0 to buf.b-1 do
61	+	c[i] := ...;
62	+	```
63	+
64	+	### Using Additional Record
65	+
66	+	Depending on the use, the additional record could overlap with the original record using [[absolute]].
67	+
68	+	``` pascal
69	+	type
70	+	mystructEx = record
71	+	hdr : mystruct;
72	+	c : PDword;
73	+	end;
74	+	pmystructEx = ^mystructEx;
75	+
76	+	var
77	+	i : integer;
78	+	buf : PmystructEx;
79	+	begin
80	+	GetMem(buf, sizeof(mystruct)+ extrasize);
81	+
82	+	for i := 0 to buf.hdr.b-1 do
83	+	buf.c[i] := ...;
84	+	```
85	+
86	+	### Using Advanced Record
87	+
88	+	Advanced records allow to methods and properties for records. Thus the field could be emulated through a method/property
89	+
90	+	``` pascal
91	+	{$modeswitch advancedrecords}
92	+
93	+	type
94	+	mystruct = packed record
95	+	a,b: DWORD;
96	+	function c: PDWORD; inline;
97	+	end;
98	+
99	+	function mystruct.c: PDWORD;
100	+	begin
101	+	Result := PDWORD(pbyte(@self.b)+sizeOf(self.b));
102	+	end;
103	+	```
104	+
105	+	OR using an empty record. (An empty record has not struct, yet allows to get an address, which should save some time on address calculation)
106	+
107	+	``` pascal
108	+	{$modeswitch advancedrecords}
109	+
110	+	type
111	+	mystruct = packed record
112	+	a,b: DWORD;
113	+	function c: PDWORD; inline;
114	+	strict private
115	+	cStart: record end;
116	+	end;
117	+
118	+	function mystruct.c: PDWORD;
119	+	begin
120	+	Result := PDWORD(@cStart);
121	+	end;
122	+	```
123	+
124	+	## See Also
125	+
126	+	- <https://forum.lazarus.freepascal.org/index.php/topic,45998.0.html> - original thread that started the discussion
127	+	- <https://en.wikipedia.org/wiki/Flexible_array_member> - wikipedia on Flexible Array Member
128	+	- [[Common problems when converting C header files]]
129	+
130	+	---
131	+	From the Free Pascal wiki, page "Flexible Array Member", as it stood on 2022-04-30 after a change by E-ric, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).

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