forked from LuaJIT/LuaJIT
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathext_ffi_semantics.html
1245 lines (1130 loc) · 51.8 KB
/
ext_ffi_semantics.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>FFI Semantics</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta name="Author" content="Mike Pall">
<meta name="Copyright" content="Copyright (C) 2005-2017, Mike Pall">
<meta name="Language" content="en">
<link rel="stylesheet" type="text/css" href="bluequad.css" media="screen">
<link rel="stylesheet" type="text/css" href="bluequad-print.css" media="print">
<style type="text/css">
table.convtable { line-height: 1.2; }
tr.convhead td { font-weight: bold; }
td.convop { font-style: italic; width: 40%; }
</style>
</head>
<body>
<div id="site">
<a href="http://luajit.org"><span>Lua<span id="logo">JIT</span></span></a>
</div>
<div id="head">
<h1>FFI Semantics</h1>
</div>
<div id="nav">
<ul><li>
<a href="luajit.html">LuaJIT</a>
<ul><li>
<a href="http://luajit.org/download.html">Download <span class="ext">»</span></a>
</li><li>
<a href="install.html">Installation</a>
</li><li>
<a href="running.html">Running</a>
</li></ul>
</li><li>
<a href="extensions.html">Extensions</a>
<ul><li>
<a href="ext_ffi.html">FFI Library</a>
<ul><li>
<a href="ext_ffi_tutorial.html">FFI Tutorial</a>
</li><li>
<a href="ext_ffi_api.html">ffi.* API</a>
</li><li>
<a class="current" href="ext_ffi_semantics.html">FFI Semantics</a>
</li></ul>
</li><li>
<a href="ext_jit.html">jit.* Library</a>
</li><li>
<a href="ext_c_api.html">Lua/C API</a>
</li></ul>
</li><li>
<a href="status.html">Status</a>
<ul><li>
<a href="changes.html">Changes</a>
</li></ul>
</li><li>
<a href="faq.html">FAQ</a>
</li><li>
<a href="http://luajit.org/performance.html">Performance <span class="ext">»</span></a>
</li><li>
<a href="http://wiki.luajit.org/">Wiki <span class="ext">»</span></a>
</li><li>
<a href="http://luajit.org/list.html">Mailing List <span class="ext">»</span></a>
</li></ul>
</div>
<div id="main">
<p>
This page describes the detailed semantics underlying the FFI library
and its interaction with both Lua and C code.
</p>
<p>
Given that the FFI library is designed to interface with C code
and that declarations can be written in plain C syntax, <b>it
closely follows the C language semantics</b>, wherever possible.
Some minor concessions are needed for smoother interoperation with Lua
language semantics.
</p>
<p>
Please don't be overwhelmed by the contents of this page — this
is a reference and you may need to consult it, if in doubt. It doesn't
hurt to skim this page, but most of the semantics "just work" as you'd
expect them to work. It should be straightforward to write
applications using the LuaJIT FFI for developers with a C or C++
background.
</p>
<h2 id="clang">C Language Support</h2>
<p>
The FFI library has a built-in C parser with a minimal memory
footprint. It's used by the <a href="ext_ffi_api.html">ffi.* library
functions</a> to declare C types or external symbols.
</p>
<p>
It's only purpose is to parse C declarations, as found e.g. in
C header files. Although it does evaluate constant expressions,
it's <em>not</em> a C compiler. The body of <tt>inline</tt>
C function definitions is simply ignored.
</p>
<p>
Also, this is <em>not</em> a validating C parser. It expects and
accepts correctly formed C declarations, but it may choose to
ignore bad declarations or show rather generic error messages. If in
doubt, please check the input against your favorite C compiler.
</p>
<p>
The C parser complies to the <b>C99 language standard</b> plus
the following extensions:
</p>
<ul>
<li>The <tt>'\e'</tt> escape in character and string literals.</li>
<li>The C99/C++ boolean type, declared with the keywords <tt>bool</tt>
or <tt>_Bool</tt>.</li>
<li>Complex numbers, declared with the keywords <tt>complex</tt> or
<tt>_Complex</tt>.</li>
<li>Two complex number types: <tt>complex</tt> (aka
<tt>complex double</tt>) and <tt>complex float</tt>.</li>
<li>Vector types, declared with the GCC <tt>mode</tt> or
<tt>vector_size</tt> attribute.</li>
<li>Unnamed ('transparent') <tt>struct</tt>/<tt>union</tt> fields
inside a <tt>struct</tt>/<tt>union</tt>.</li>
<li>Incomplete <tt>enum</tt> declarations, handled like incomplete
<tt>struct</tt> declarations.</li>
<li>Unnamed <tt>enum</tt> fields inside a
<tt>struct</tt>/<tt>union</tt>. This is similar to a scoped C++
<tt>enum</tt>, except that declared constants are visible in the
global namespace, too.</li>
<li>Scoped <tt>static const</tt> declarations inside a
<tt>struct</tt>/<tt>union</tt> (from C++).</li>
<li>Zero-length arrays (<tt>[0]</tt>), empty
<tt>struct</tt>/<tt>union</tt>, variable-length arrays (VLA,
<tt>[?]</tt>) and variable-length structs (VLS, with a trailing
VLA).</li>
<li>C++ reference types (<tt>int &x</tt>).</li>
<li>Alternate GCC keywords with '<tt>__</tt>', e.g.
<tt>__const__</tt>.</li>
<li>GCC <tt>__attribute__</tt> with the following attributes:
<tt>aligned</tt>, <tt>packed</tt>, <tt>mode</tt>,
<tt>vector_size</tt>, <tt>cdecl</tt>, <tt>fastcall</tt>,
<tt>stdcall</tt>, <tt>thiscall</tt>.</li>
<li>The GCC <tt>__extension__</tt> keyword and the GCC
<tt>__alignof__</tt> operator.</li>
<li>GCC <tt>__asm__("symname")</tt> symbol name redirection for
function declarations.</li>
<li>MSVC keywords for fixed-length types: <tt>__int8</tt>,
<tt>__int16</tt>, <tt>__int32</tt> and <tt>__int64</tt>.</li>
<li>MSVC <tt>__cdecl</tt>, <tt>__fastcall</tt>, <tt>__stdcall</tt>,
<tt>__thiscall</tt>, <tt>__ptr32</tt>, <tt>__ptr64</tt>,
<tt>__declspec(align(n))</tt> and <tt>#pragma pack</tt>.</li>
<li>All other GCC/MSVC-specific attributes are ignored.</li>
</ul>
<p>
The following C types are pre-defined by the C parser (like
a <tt>typedef</tt>, except re-declarations will be ignored):
</p>
<ul>
<li>Vararg handling: <tt>va_list</tt>, <tt>__builtin_va_list</tt>,
<tt>__gnuc_va_list</tt>.</li>
<li>From <tt><stddef.h></tt>: <tt>ptrdiff_t</tt>,
<tt>size_t</tt>, <tt>wchar_t</tt>.</li>
<li>From <tt><stdint.h></tt>: <tt>int8_t</tt>, <tt>int16_t</tt>,
<tt>int32_t</tt>, <tt>int64_t</tt>, <tt>uint8_t</tt>,
<tt>uint16_t</tt>, <tt>uint32_t</tt>, <tt>uint64_t</tt>,
<tt>intptr_t</tt>, <tt>uintptr_t</tt>.</li>
</ul>
<p>
You're encouraged to use these types in preference to
compiler-specific extensions or target-dependent standard types.
E.g. <tt>char</tt> differs in signedness and <tt>long</tt> differs in
size, depending on the target architecture and platform ABI.
</p>
<p>
The following C features are <b>not</b> supported:
</p>
<ul>
<li>A declaration must always have a type specifier; it doesn't
default to an <tt>int</tt> type.</li>
<li>Old-style empty function declarations (K&R) are not allowed.
All C functions must have a proper prototype declaration. A
function declared without parameters (<tt>int foo();</tt>) is
treated as a function taking zero arguments, like in C++.</li>
<li>The <tt>long double</tt> C type is parsed correctly, but
there's no support for the related conversions, accesses or arithmetic
operations.</li>
<li>Wide character strings and character literals are not
supported.</li>
<li><a href="#status">See below</a> for features that are currently
not implemented.</li>
</ul>
<h2 id="convert">C Type Conversion Rules</h2>
<h3 id="convert_tolua">Conversions from C types to Lua objects</h3>
<p>
These conversion rules apply for <em>read accesses</em> to
C types: indexing pointers, arrays or
<tt>struct</tt>/<tt>union</tt> types; reading external variables or
constant values; retrieving return values from C calls:
</p>
<table class="convtable">
<tr class="convhead">
<td class="convin">Input</td>
<td class="convop">Conversion</td>
<td class="convout">Output</td>
</tr>
<tr class="odd separate">
<td class="convin"><tt>int8_t</tt>, <tt>int16_t</tt></td><td class="convop">→<sup>sign-ext</sup> <tt>int32_t</tt> → <tt>double</tt></td><td class="convout">number</td></tr>
<tr class="even">
<td class="convin"><tt>uint8_t</tt>, <tt>uint16_t</tt></td><td class="convop">→<sup>zero-ext</sup> <tt>int32_t</tt> → <tt>double</tt></td><td class="convout">number</td></tr>
<tr class="odd">
<td class="convin"><tt>int32_t</tt>, <tt>uint32_t</tt></td><td class="convop">→ <tt>double</tt></td><td class="convout">number</td></tr>
<tr class="even">
<td class="convin"><tt>int64_t</tt>, <tt>uint64_t</tt></td><td class="convop">boxed value</td><td class="convout">64 bit int cdata</td></tr>
<tr class="odd separate">
<td class="convin"><tt>double</tt>, <tt>float</tt></td><td class="convop">→ <tt>double</tt></td><td class="convout">number</td></tr>
<tr class="even separate">
<td class="convin"><tt>bool</tt></td><td class="convop">0 → <tt>false</tt>, otherwise <tt>true</tt></td><td class="convout">boolean</td></tr>
<tr class="odd separate">
<td class="convin"><tt>enum</tt></td><td class="convop">boxed value</td><td class="convout">enum cdata</td></tr>
<tr class="even">
<td class="convin">Complex number</td><td class="convop">boxed value</td><td class="convout">complex cdata</td></tr>
<tr class="odd">
<td class="convin">Vector</td><td class="convop">boxed value</td><td class="convout">vector cdata</td></tr>
<tr class="even">
<td class="convin">Pointer</td><td class="convop">boxed value</td><td class="convout">pointer cdata</td></tr>
<tr class="odd separate">
<td class="convin">Array</td><td class="convop">boxed reference</td><td class="convout">reference cdata</td></tr>
<tr class="even">
<td class="convin"><tt>struct</tt>/<tt>union</tt></td><td class="convop">boxed reference</td><td class="convout">reference cdata</td></tr>
</table>
<p>
Bitfields are treated like their underlying type.
</p>
<p>
Reference types are dereferenced <em>before</em> a conversion can take
place — the conversion is applied to the C type pointed to
by the reference.
</p>
<h3 id="convert_fromlua">Conversions from Lua objects to C types</h3>
<p>
These conversion rules apply for <em>write accesses</em> to
C types: indexing pointers, arrays or
<tt>struct</tt>/<tt>union</tt> types; initializing cdata objects;
casts to C types; writing to external variables; passing
arguments to C calls:
</p>
<table class="convtable">
<tr class="convhead">
<td class="convin">Input</td>
<td class="convop">Conversion</td>
<td class="convout">Output</td>
</tr>
<tr class="odd separate">
<td class="convin">number</td><td class="convop">→</td><td class="convout"><tt>double</tt></td></tr>
<tr class="even">
<td class="convin">boolean</td><td class="convop"><tt>false</tt> → 0, <tt>true</tt> → 1</td><td class="convout"><tt>bool</tt></td></tr>
<tr class="odd separate">
<td class="convin">nil</td><td class="convop"><tt>NULL</tt> →</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="even">
<td class="convin">lightuserdata</td><td class="convop">lightuserdata address →</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="odd">
<td class="convin">userdata</td><td class="convop">userdata payload →</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="even">
<td class="convin">io.* file</td><td class="convop">get FILE * handle →</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="odd separate">
<td class="convin">string</td><td class="convop">match against <tt>enum</tt> constant</td><td class="convout"><tt>enum</tt></td></tr>
<tr class="even">
<td class="convin">string</td><td class="convop">copy string data + zero-byte</td><td class="convout"><tt>int8_t[]</tt>, <tt>uint8_t[]</tt></td></tr>
<tr class="odd">
<td class="convin">string</td><td class="convop">string data →</td><td class="convout"><tt>const char[]</tt></td></tr>
<tr class="even separate">
<td class="convin">function</td><td class="convop"><a href="#callback">create callback</a> →</td><td class="convout">C function type</td></tr>
<tr class="odd separate">
<td class="convin">table</td><td class="convop"><a href="#init_table">table initializer</a></td><td class="convout">Array</td></tr>
<tr class="even">
<td class="convin">table</td><td class="convop"><a href="#init_table">table initializer</a></td><td class="convout"><tt>struct</tt>/<tt>union</tt></td></tr>
<tr class="odd separate">
<td class="convin">cdata</td><td class="convop">cdata payload →</td><td class="convout">C type</td></tr>
</table>
<p>
If the result type of this conversion doesn't match the
C type of the destination, the
<a href="#convert_between">conversion rules between C types</a>
are applied.
</p>
<p>
Reference types are immutable after initialization ("no re-seating of
references"). For initialization purposes or when passing values to
reference parameters, they are treated like pointers. Note that unlike
in C++, there's no way to implement automatic reference generation of
variables under the Lua language semantics. If you want to call a
function with a reference parameter, you need to explicitly pass a
one-element array.
</p>
<h3 id="convert_between">Conversions between C types</h3>
<p>
These conversion rules are more or less the same as the standard
C conversion rules. Some rules only apply to casts, or require
pointer or type compatibility:
</p>
<table class="convtable">
<tr class="convhead">
<td class="convin">Input</td>
<td class="convop">Conversion</td>
<td class="convout">Output</td>
</tr>
<tr class="odd separate">
<td class="convin">Signed integer</td><td class="convop">→<sup>narrow or sign-extend</sup></td><td class="convout">Integer</td></tr>
<tr class="even">
<td class="convin">Unsigned integer</td><td class="convop">→<sup>narrow or zero-extend</sup></td><td class="convout">Integer</td></tr>
<tr class="odd">
<td class="convin">Integer</td><td class="convop">→<sup>round</sup></td><td class="convout"><tt>double</tt>, <tt>float</tt></td></tr>
<tr class="even">
<td class="convin"><tt>double</tt>, <tt>float</tt></td><td class="convop">→<sup>trunc</sup> <tt>int32_t</tt> →<sup>narrow</sup></td><td class="convout"><tt>(u)int8_t</tt>, <tt>(u)int16_t</tt></td></tr>
<tr class="odd">
<td class="convin"><tt>double</tt>, <tt>float</tt></td><td class="convop">→<sup>trunc</sup></td><td class="convout"><tt>(u)int32_t</tt>, <tt>(u)int64_t</tt></td></tr>
<tr class="even">
<td class="convin"><tt>double</tt>, <tt>float</tt></td><td class="convop">→<sup>round</sup></td><td class="convout"><tt>float</tt>, <tt>double</tt></td></tr>
<tr class="odd separate">
<td class="convin">Number</td><td class="convop">n == 0 → 0, otherwise 1</td><td class="convout"><tt>bool</tt></td></tr>
<tr class="even">
<td class="convin"><tt>bool</tt></td><td class="convop"><tt>false</tt> → 0, <tt>true</tt> → 1</td><td class="convout">Number</td></tr>
<tr class="odd separate">
<td class="convin">Complex number</td><td class="convop">convert real part</td><td class="convout">Number</td></tr>
<tr class="even">
<td class="convin">Number</td><td class="convop">convert real part, imag = 0</td><td class="convout">Complex number</td></tr>
<tr class="odd">
<td class="convin">Complex number</td><td class="convop">convert real and imag part</td><td class="convout">Complex number</td></tr>
<tr class="even separate">
<td class="convin">Number</td><td class="convop">convert scalar and replicate</td><td class="convout">Vector</td></tr>
<tr class="odd">
<td class="convin">Vector</td><td class="convop">copy (same size)</td><td class="convout">Vector</td></tr>
<tr class="even separate">
<td class="convin"><tt>struct</tt>/<tt>union</tt></td><td class="convop">take base address (compat)</td><td class="convout">Pointer</td></tr>
<tr class="odd">
<td class="convin">Array</td><td class="convop">take base address (compat)</td><td class="convout">Pointer</td></tr>
<tr class="even">
<td class="convin">Function</td><td class="convop">take function address</td><td class="convout">Function pointer</td></tr>
<tr class="odd separate">
<td class="convin">Number</td><td class="convop">convert via <tt>uintptr_t</tt> (cast)</td><td class="convout">Pointer</td></tr>
<tr class="even">
<td class="convin">Pointer</td><td class="convop">convert address (compat/cast)</td><td class="convout">Pointer</td></tr>
<tr class="odd">
<td class="convin">Pointer</td><td class="convop">convert address (cast)</td><td class="convout">Integer</td></tr>
<tr class="even">
<td class="convin">Array</td><td class="convop">convert base address (cast)</td><td class="convout">Integer</td></tr>
<tr class="odd separate">
<td class="convin">Array</td><td class="convop">copy (compat)</td><td class="convout">Array</td></tr>
<tr class="even">
<td class="convin"><tt>struct</tt>/<tt>union</tt></td><td class="convop">copy (identical type)</td><td class="convout"><tt>struct</tt>/<tt>union</tt></td></tr>
</table>
<p>
Bitfields or <tt>enum</tt> types are treated like their underlying
type.
</p>
<p>
Conversions not listed above will raise an error. E.g. it's not
possible to convert a pointer to a complex number or vice versa.
</p>
<h3 id="convert_vararg">Conversions for vararg C function arguments</h3>
<p>
The following default conversion rules apply when passing Lua objects
to the variable argument part of vararg C functions:
</p>
<table class="convtable">
<tr class="convhead">
<td class="convin">Input</td>
<td class="convop">Conversion</td>
<td class="convout">Output</td>
</tr>
<tr class="odd separate">
<td class="convin">number</td><td class="convop">→</td><td class="convout"><tt>double</tt></td></tr>
<tr class="even">
<td class="convin">boolean</td><td class="convop"><tt>false</tt> → 0, <tt>true</tt> → 1</td><td class="convout"><tt>bool</tt></td></tr>
<tr class="odd separate">
<td class="convin">nil</td><td class="convop"><tt>NULL</tt> →</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="even">
<td class="convin">userdata</td><td class="convop">userdata payload →</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="odd">
<td class="convin">lightuserdata</td><td class="convop">lightuserdata address →</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="even separate">
<td class="convin">string</td><td class="convop">string data →</td><td class="convout"><tt>const char *</tt></td></tr>
<tr class="odd separate">
<td class="convin"><tt>float</tt> cdata</td><td class="convop">→</td><td class="convout"><tt>double</tt></td></tr>
<tr class="even">
<td class="convin">Array cdata</td><td class="convop">take base address</td><td class="convout">Element pointer</td></tr>
<tr class="odd">
<td class="convin"><tt>struct</tt>/<tt>union</tt> cdata</td><td class="convop">take base address</td><td class="convout"><tt>struct</tt>/<tt>union</tt> pointer</td></tr>
<tr class="even">
<td class="convin">Function cdata</td><td class="convop">take function address</td><td class="convout">Function pointer</td></tr>
<tr class="odd">
<td class="convin">Any other cdata</td><td class="convop">no conversion</td><td class="convout">C type</td></tr>
</table>
<p>
To pass a Lua object, other than a cdata object, as a specific type,
you need to override the conversion rules: create a temporary cdata
object with a constructor or a cast and initialize it with the value
to pass:
</p>
<p>
Assuming <tt>x</tt> is a Lua number, here's how to pass it as an
integer to a vararg function:
</p>
<pre class="code">
ffi.cdef[[
int printf(const char *fmt, ...);
]]
ffi.C.printf("integer value: %d\n", ffi.new("int", x))
</pre>
<p>
If you don't do this, the default Lua number → <tt>double</tt>
conversion rule applies. A vararg C function expecting an integer
will see a garbled or uninitialized value.
</p>
<h2 id="init">Initializers</h2>
<p>
Creating a cdata object with
<a href="ext_ffi_api.html#ffi_new"><tt>ffi.new()</tt></a> or the
equivalent constructor syntax always initializes its contents, too.
Different rules apply, depending on the number of optional
initializers and the C types involved:
</p>
<ul>
<li>If no initializers are given, the object is filled with zero bytes.</li>
<li>Scalar types (numbers and pointers) accept a single initializer.
The Lua object is <a href="#convert_fromlua">converted to the scalar
C type</a>.</li>
<li>Valarrays (complex numbers and vectors) are treated like scalars
when a single initializer is given. Otherwise they are treated like
regular arrays.</li>
<li>Aggregate types (arrays and structs) accept either a single cdata
initializer of the same type (copy constructor), a single
<a href="#init_table">table initializer</a>, or a flat list of
initializers.</li>
<li>The elements of an array are initialized, starting at index zero.
If a single initializer is given for an array, it's repeated for all
remaining elements. This doesn't happen if two or more initializers
are given: all remaining uninitialized elements are filled with zero
bytes.</li>
<li>Byte arrays may also be initialized with a Lua string. This copies
the whole string plus a terminating zero-byte. The copy stops early only
if the array has a known, fixed size.</li>
<li>The fields of a <tt>struct</tt> are initialized in the order of
their declaration. Uninitialized fields are filled with zero
bytes.</li>
<li>Only the first field of a <tt>union</tt> can be initialized with a
flat initializer.</li>
<li>Elements or fields which are aggregates themselves are initialized
with a <em>single</em> initializer, but this may be a table
initializer or a compatible aggregate.</li>
<li>Excess initializers cause an error.</li>
</ul>
<h2 id="init_table">Table Initializers</h2>
<p>
The following rules apply if a Lua table is used to initialize an
Array or a <tt>struct</tt>/<tt>union</tt>:
</p>
<ul>
<li>If the table index <tt>[0]</tt> is non-<tt>nil</tt>, then the
table is assumed to be zero-based. Otherwise it's assumed to be
one-based.</li>
<li>Array elements, starting at index zero, are initialized one-by-one
with the consecutive table elements, starting at either index
<tt>[0]</tt> or <tt>[1]</tt>. This process stops at the first
<tt>nil</tt> table element.</li>
<li>If exactly one array element was initialized, it's repeated for
all the remaining elements. Otherwise all remaining uninitialized
elements are filled with zero bytes.</li>
<li>The above logic only applies to arrays with a known fixed size.
A VLA is only initialized with the element(s) given in the table.
Depending on the use case, you may need to explicitly add a
<tt>NULL</tt> or <tt>0</tt> terminator to a VLA.</li>
<li>A <tt>struct</tt>/<tt>union</tt> can be initialized in the
order of the declaration of its fields. Each field is initialized with
consecutive table elements, starting at either index <tt>[0]</tt>
or <tt>[1]</tt>. This process stops at the first <tt>nil</tt> table
element.</li>
<li>Otherwise, if neither index <tt>[0]</tt> nor <tt>[1]</tt> is present,
a <tt>struct</tt>/<tt>union</tt> is initialized by looking up each field
name (as a string key) in the table. Each non-<tt>nil</tt> value is
used to initialize the corresponding field.</li>
<li>Uninitialized fields of a <tt>struct</tt> are filled with zero
bytes, except for the trailing VLA of a VLS.</li>
<li>Initialization of a <tt>union</tt> stops after one field has been
initialized. If no field has been initialized, the <tt>union</tt> is
filled with zero bytes.</li>
<li>Elements or fields which are aggregates themselves are initialized
with a <em>single</em> initializer, but this may be a nested table
initializer (or a compatible aggregate).</li>
<li>Excess initializers for an array cause an error. Excess
initializers for a <tt>struct</tt>/<tt>union</tt> are ignored.
Unrelated table entries are ignored, too.</li>
</ul>
<p>
Example:
</p>
<pre class="code">
local ffi = require("ffi")
ffi.cdef[[
struct foo { int a, b; };
union bar { int i; double d; };
struct nested { int x; struct foo y; };
]]
ffi.new("int[3]", {}) --> 0, 0, 0
ffi.new("int[3]", {1}) --> 1, 1, 1
ffi.new("int[3]", {1,2}) --> 1, 2, 0
ffi.new("int[3]", {1,2,3}) --> 1, 2, 3
ffi.new("int[3]", {[0]=1}) --> 1, 1, 1
ffi.new("int[3]", {[0]=1,2}) --> 1, 2, 0
ffi.new("int[3]", {[0]=1,2,3}) --> 1, 2, 3
ffi.new("int[3]", {[0]=1,2,3,4}) --> error: too many initializers
ffi.new("struct foo", {}) --> a = 0, b = 0
ffi.new("struct foo", {1}) --> a = 1, b = 0
ffi.new("struct foo", {1,2}) --> a = 1, b = 2
ffi.new("struct foo", {[0]=1,2}) --> a = 1, b = 2
ffi.new("struct foo", {b=2}) --> a = 0, b = 2
ffi.new("struct foo", {a=1,b=2,c=3}) --> a = 1, b = 2 'c' is ignored
ffi.new("union bar", {}) --> i = 0, d = 0.0
ffi.new("union bar", {1}) --> i = 1, d = ?
ffi.new("union bar", {[0]=1,2}) --> i = 1, d = ? '2' is ignored
ffi.new("union bar", {d=2}) --> i = ?, d = 2.0
ffi.new("struct nested", {1,{2,3}}) --> x = 1, y.a = 2, y.b = 3
ffi.new("struct nested", {x=1,y={2,3}}) --> x = 1, y.a = 2, y.b = 3
</pre>
<h2 id="cdata_ops">Operations on cdata Objects</h2>
<p>
All of the standard Lua operators can be applied to cdata objects or a
mix of a cdata object and another Lua object. The following list shows
the pre-defined operations.
</p>
<p>
Reference types are dereferenced <em>before</em> performing each of
the operations below — the operation is applied to the
C type pointed to by the reference.
</p>
<p>
The pre-defined operations are always tried first before deferring to a
metamethod or index table (if any) for the corresponding ctype (except
for <tt>__new</tt>). An error is raised if the metamethod lookup or
index table lookup fails.
</p>
<h3 id="cdata_array">Indexing a cdata object</h3>
<ul>
<li><b>Indexing a pointer/array</b>: a cdata pointer/array can be
indexed by a cdata number or a Lua number. The element address is
computed as the base address plus the number value multiplied by the
element size in bytes. A read access loads the element value and
<a href="#convert_tolua">converts it to a Lua object</a>. A write
access <a href="#convert_fromlua">converts a Lua object to the element
type</a> and stores the converted value to the element. An error is
raised if the element size is undefined or a write access to a
constant element is attempted.</li>
<li><b>Dereferencing a <tt>struct</tt>/<tt>union</tt> field</b>: a
cdata <tt>struct</tt>/<tt>union</tt> or a pointer to a
<tt>struct</tt>/<tt>union</tt> can be dereferenced by a string key,
giving the field name. The field address is computed as the base
address plus the relative offset of the field. A read access loads the
field value and <a href="#convert_tolua">converts it to a Lua
object</a>. A write access <a href="#convert_fromlua">converts a Lua
object to the field type</a> and stores the converted value to the
field. An error is raised if a write access to a constant
<tt>struct</tt>/<tt>union</tt> or a constant field is attempted.
Scoped enum constants or static constants are treated like a constant
field.</li>
<li><b>Indexing a complex number</b>: a complex number can be indexed
either by a cdata number or a Lua number with the values 0 or 1, or by
the strings <tt>"re"</tt> or <tt>"im"</tt>. A read access loads the
real part (<tt>[0]</tt>, <tt>.re</tt>) or the imaginary part
(<tt>[1]</tt>, <tt>.im</tt>) part of a complex number and
<a href="#convert_tolua">converts it to a Lua number</a>. The
sub-parts of a complex number are immutable — assigning to an
index of a complex number raises an error. Accessing out-of-bound
indexes returns unspecified results, but is guaranteed not to trigger
memory access violations.</li>
<li><b>Indexing a vector</b>: a vector is treated like an array for
indexing purposes, except the vector elements are immutable —
assigning to an index of a vector raises an error.</li>
</ul>
<p>
A ctype object can be indexed with a string key, too. The only
pre-defined operation is reading scoped constants of
<tt>struct</tt>/<tt>union</tt> types. All other accesses defer
to the corresponding metamethods or index tables (if any).
</p>
<p>
Note: since there's (deliberately) no address-of operator, a cdata
object holding a value type is effectively immutable after
initialization. The JIT compiler benefits from this fact when applying
certain optimizations.
</p>
<p>
As a consequence, the <em>elements</em> of complex numbers and
vectors are immutable. But the elements of an aggregate holding these
types <em>may</em> be modified of course. I.e. you cannot assign to
<tt>foo.c.im</tt>, but you can assign a (newly created) complex number
to <tt>foo.c</tt>.
</p>
<p>
The JIT compiler implements strict aliasing rules: accesses to different
types do <b>not</b> alias, except for differences in signedness (this
applies even to <tt>char</tt> pointers, unlike C99). Type punning
through unions is explicitly detected and allowed.
</p>
<h3 id="cdata_call">Calling a cdata object</h3>
<ul>
<li><b>Constructor</b>: a ctype object can be called and used as a
<a href="ext_ffi_api.html#ffi_new">constructor</a>. This is equivalent
to <tt>ffi.new(ct, ...)</tt>, unless a <tt>__new</tt> metamethod is
defined. The <tt>__new</tt> metamethod is called with the ctype object
plus any other arguments passed to the contructor. Note that you have to
use <tt>ffi.new</tt> inside of it, since calling <tt>ct(...)</tt> would
cause infinite recursion.</li>
<li><b>C function call</b>: a cdata function or cdata function
pointer can be called. The passed arguments are
<a href="#convert_fromlua">converted to the C types</a> of the
parameters given by the function declaration. Arguments passed to the
variable argument part of vararg C function use
<a href="#convert_vararg">special conversion rules</a>. This
C function is called and the return value (if any) is
<a href="#convert_tolua">converted to a Lua object</a>.<br>
On Windows/x86 systems, <tt>__stdcall</tt> functions are automatically
detected and a function declared as <tt>__cdecl</tt> (the default) is
silently fixed up after the first call.</li>
</ul>
<h3 id="cdata_arith">Arithmetic on cdata objects</h3>
<ul>
<li><b>Pointer arithmetic</b>: a cdata pointer/array and a cdata
number or a Lua number can be added or subtracted. The number must be
on the right hand side for a subtraction. The result is a pointer of
the same type with an address plus or minus the number value
multiplied by the element size in bytes. An error is raised if the
element size is undefined.</li>
<li><b>Pointer difference</b>: two compatible cdata pointers/arrays
can be subtracted. The result is the difference between their
addresses, divided by the element size in bytes. An error is raised if
the element size is undefined or zero.</li>
<li><b>64 bit integer arithmetic</b>: the standard arithmetic
operators (<tt>+ - * / % ^</tt> and unary
minus) can be applied to two cdata numbers, or a cdata number and a
Lua number. If one of them is an <tt>uint64_t</tt>, the other side is
converted to an <tt>uint64_t</tt> and an unsigned arithmetic operation
is performed. Otherwise both sides are converted to an
<tt>int64_t</tt> and a signed arithmetic operation is performed. The
result is a boxed 64 bit cdata object.<br>
If one of the operands is an <tt>enum</tt> and the other operand is a
string, the string is converted to the value of a matching <tt>enum</tt>
constant before the above conversion.<br>
These rules ensure that 64 bit integers are "sticky". Any
expression involving at least one 64 bit integer operand results
in another one. The undefined cases for the division, modulo and power
operators return <tt>2LL ^ 63</tt> or
<tt>2ULL ^ 63</tt>.<br>
You'll have to explicitly convert a 64 bit integer to a Lua
number (e.g. for regular floating-point calculations) with
<tt>tonumber()</tt>. But note this may incur a precision loss.</li>
</ul>
<h3 id="cdata_comp">Comparisons of cdata objects</h3>
<ul>
<li><b>Pointer comparison</b>: two compatible cdata pointers/arrays
can be compared. The result is the same as an unsigned comparison of
their addresses. <tt>nil</tt> is treated like a <tt>NULL</tt> pointer,
which is compatible with any other pointer type.</li>
<li><b>64 bit integer comparison</b>: two cdata numbers, or a
cdata number and a Lua number can be compared with each other. If one
of them is an <tt>uint64_t</tt>, the other side is converted to an
<tt>uint64_t</tt> and an unsigned comparison is performed. Otherwise
both sides are converted to an <tt>int64_t</tt> and a signed
comparison is performed.<br>
If one of the operands is an <tt>enum</tt> and the other operand is a
string, the string is converted to the value of a matching <tt>enum</tt>
constant before the above conversion.<br>
<li><b>Comparisons for equality/inequality</b> never raise an error.
Even incompatible pointers can be compared for equality by address. Any
other incompatible comparison (also with non-cdata objects) treats the
two sides as unequal.</li>
</ul>
<h3 id="cdata_key">cdata objects as table keys</h3>
<p>
Lua tables may be indexed by cdata objects, but this doesn't provide
any useful semantics — <b>cdata objects are unsuitable as table
keys!</b>
</p>
<p>
A cdata object is treated like any other garbage-collected object and
is hashed and compared by its address for table indexing. Since
there's no interning for cdata value types, the same value may be
boxed in different cdata objects with different addresses. Thus
<tt>t[1LL+1LL]</tt> and <tt>t[2LL]</tt> usually <b>do not</b> point to
the same hash slot and they certainly <b>do not</b> point to the same
hash slot as <tt>t[2]</tt>.
</p>
<p>
It would seriously drive up implementation complexity and slow down
the common case, if one were to add extra handling for by-value
hashing and comparisons to Lua tables. Given the ubiquity of their use
inside the VM, this is not acceptable.
</p>
<p>
There are three viable alternatives, if you really need to use cdata
objects as keys:
</p>
<ul>
<li>If you can get by with the precision of Lua numbers
(52 bits), then use <tt>tonumber()</tt> on a cdata number or
combine multiple fields of a cdata aggregate to a Lua number. Then use
the resulting Lua number as a key when indexing tables.<br>
One obvious benefit: <tt>t[tonumber(2LL)]</tt> <b>does</b> point to
the same slot as <tt>t[2]</tt>.</li>
<li>Otherwise use either <tt>tostring()</tt> on 64 bit integers
or complex numbers or combine multiple fields of a cdata aggregate to
a Lua string (e.g. with
<a href="ext_ffi_api.html#ffi_string"><tt>ffi.string()</tt></a>). Then
use the resulting Lua string as a key when indexing tables.</li>
<li>Create your own specialized hash table implementation using the
C types provided by the FFI library, just like you would in
C code. Ultimately this may give much better performance than the
other alternatives or what a generic by-value hash table could
possibly provide.</li>
</ul>
<h2 id="param">Parameterized Types</h2>
<p>
To facilitate some abstractions, the two functions
<a href="ext_ffi_api.html#ffi_typeof"><tt>ffi.typeof</tt></a> and
<a href="ext_ffi_api.html#ffi_cdef"><tt>ffi.cdef</tt></a> support
parameterized types in C declarations. Note: none of the other API
functions taking a cdecl allow this.
</p>
<p>
Any place you can write a <b><tt>typedef</tt> name</b>, an
<b>identifier</b> or a <b>number</b> in a declaration, you can write
<tt>$</tt> (the dollar sign) instead. These placeholders are replaced in
order of appearance with the arguments following the cdecl string:
</p>
<pre class="code">
-- Declare a struct with a parameterized field type and name:
ffi.cdef([[
typedef struct { $ $; } foo_t;
]], type1, name1)
-- Anonymous struct with dynamic names:
local bar_t = ffi.typeof("struct { int $, $; }", name1, name2)
-- Derived pointer type:
local bar_ptr_t = ffi.typeof("$ *", bar_t)
-- Parameterized dimensions work even where a VLA won't work:
local matrix_t = ffi.typeof("uint8_t[$][$]", width, height)
</pre>
<p>
Caveat: this is <em>not</em> simple text substitution! A passed ctype or
cdata object is treated like the underlying type, a passed string is
considered an identifier and a number is considered a number. You must
not mix this up: e.g. passing <tt>"int"</tt> as a string doesn't work in
place of a type, you'd need to use <tt>ffi.typeof("int")</tt> instead.
</p>
<p>
The main use for parameterized types are libraries implementing abstract
data types
(<a href="https://www.freelists.org/post/luajit/ffi-type-of-pointer-to,8">example</a>),
similar to what can be achieved with C++ template metaprogramming.
Another use case are derived types of anonymous structs, which avoids
pollution of the global struct namespace.
</p>
<p>
Please note that parameterized types are a nice tool and indispensable
for certain use cases. But you'll want to use them sparingly in regular
code, e.g. when all types are actually fixed.
</p>
<h2 id="gc">Garbage Collection of cdata Objects</h2>
<p>
All explicitly (<tt>ffi.new()</tt>, <tt>ffi.cast()</tt> etc.) or
implicitly (accessors) created cdata objects are garbage collected.
You need to ensure to retain valid references to cdata objects
somewhere on a Lua stack, an upvalue or in a Lua table while they are
still in use. Once the last reference to a cdata object is gone, the
garbage collector will automatically free the memory used by it (at
the end of the next GC cycle).
</p>
<p>
Please note that pointers themselves are cdata objects, however they
are <b>not</b> followed by the garbage collector. So e.g. if you
assign a cdata array to a pointer, you must keep the cdata object
holding the array alive as long as the pointer is still in use:
</p>
<pre class="code">
ffi.cdef[[
typedef struct { int *a; } foo_t;
]]
local s = ffi.new("foo_t", ffi.new("int[10]")) -- <span style="color:#c00000;">WRONG!</span>
local a = ffi.new("int[10]") -- <span style="color:#00a000;">OK</span>
local s = ffi.new("foo_t", a)
-- Now do something with 's', but keep 'a' alive until you're done.
</pre>
<p>
Similar rules apply for Lua strings which are implicitly converted to
<tt>"const char *"</tt>: the string object itself must be
referenced somewhere or it'll be garbage collected eventually. The
pointer will then point to stale data, which may have already been
overwritten. Note that <em>string literals</em> are automatically kept
alive as long as the function containing it (actually its prototype)
is not garbage collected.
</p>
<p>
Objects which are passed as an argument to an external C function
are kept alive until the call returns. So it's generally safe to
create temporary cdata objects in argument lists. This is a common
idiom for <a href="#convert_vararg">passing specific C types to
vararg functions</a>.
</p>
<p>
Memory areas returned by C functions (e.g. from <tt>malloc()</tt>)
must be manually managed, of course (or use
<a href="ext_ffi_api.html#ffi_gc"><tt>ffi.gc()</tt></a>). Pointers to
cdata objects are indistinguishable from pointers returned by C
functions (which is one of the reasons why the GC cannot follow them).
</p>
<h2 id="callback">Callbacks</h2>
<p>
The LuaJIT FFI automatically generates special callback functions
whenever a Lua function is converted to a C function pointer. This
associates the generated callback function pointer with the C type
of the function pointer and the Lua function object (closure).
</p>
<p>
This can happen implicitly due to the usual conversions, e.g. when
passing a Lua function to a function pointer argument. Or you can use
<tt>ffi.cast()</tt> to explicitly cast a Lua function to a
C function pointer.
</p>
<p>
Currently only certain C function types can be used as callback
functions. Neither C vararg functions nor functions with
pass-by-value aggregate argument or result types are supported. There
are no restrictions for the kind of Lua functions that can be called
from the callback — no checks for the proper number of arguments
are made. The return value of the Lua function will be converted to the
result type and an error will be thrown for invalid conversions.
</p>
<p>
It's allowed to throw errors across a callback invocation, but it's not
advisable in general. Do this only if you know the C function, that
called the callback, copes with the forced stack unwinding and doesn't
leak resources.
</p>
<p>
One thing that's not allowed, is to let an FFI call into a C function
get JIT-compiled, which in turn calls a callback, calling into Lua again.
Usually this attempt is caught by the interpreter first and the
C function is blacklisted for compilation.
</p>
<p>
However, this heuristic may fail under specific circumstances: e.g. a
message polling function might not run Lua callbacks right away and the call
gets JIT-compiled. If it later happens to call back into Lua (e.g. a rarely
invoked error callback), you'll get a VM PANIC with the message
<tt>"bad callback"</tt>. Then you'll need to manually turn off
JIT-compilation with
<a href="ext_jit.html#jit_onoff_func"><tt>jit.off()</tt></a> for the
surrounding Lua function that invokes such a message polling function (or
similar).
</p>
<h3 id="callback_resources">Callback resource handling</h3>
<p>
Callbacks take up resources — you can only have a limited number
of them at the same time (500 - 1000, depending on the
architecture). The associated Lua functions are anchored to prevent
garbage collection, too.
</p>
<p>
<b>Callbacks due to implicit conversions are permanent!</b> There is no
way to guess their lifetime, since the C side might store the
function pointer for later use (typical for GUI toolkits). The associated
resources cannot be reclaimed until termination:
</p>
<pre class="code">
ffi.cdef[[
typedef int (__stdcall *WNDENUMPROC)(void *hwnd, intptr_t l);
int EnumWindows(WNDENUMPROC func, intptr_t l);
]]
-- Implicit conversion to a callback via function pointer argument.
local count = 0
ffi.C.EnumWindows(function(hwnd, l)
count = count + 1
return true
end, 0)
-- The callback is permanent and its resources cannot be reclaimed!
-- Ok, so this may not be a problem, if you do this only once.
</pre>
<p>
Note: this example shows that you <em>must</em> properly declare
<tt>__stdcall</tt> callbacks on Windows/x86 systems. The calling
convention cannot be automatically detected, unlike for
<tt>__stdcall</tt> calls <em>to</em> Windows functions.
</p>
<p>
For some use cases it's necessary to free up the resources or to
dynamically redirect callbacks. Use an explicit cast to a
C function pointer and keep the resulting cdata object. Then use
the <a href="ext_ffi_api.html#callback_free"><tt>cb:free()</tt></a>
or <a href="ext_ffi_api.html#callback_set"><tt>cb:set()</tt></a> methods
on the cdata object:
</p>
<pre class="code">
-- Explicitly convert to a callback via cast.
local count = 0
local cb = ffi.cast("WNDENUMPROC", function(hwnd, l)