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
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
| | @c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1993, 1998-1999, 2001-2017 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node GNU Emacs Internals
@appendix GNU Emacs Internals
This chapter describes how the runnable Emacs executable is dumped with
the preloaded Lisp libraries in it, how storage is allocated, and some
internal aspects of GNU Emacs that may be of interest to C programmers.
@menu
* Building Emacs:: How the dumped Emacs is made.
* Pure Storage:: Kludge to make preloaded Lisp functions shareable.
* Garbage Collection:: Reclaiming space for Lisp objects no longer used.
* Stack-allocated Objects:: Temporary conses and strings on C stack.
* Memory Usage:: Info about total size of Lisp objects made so far.
* C Dialect:: What C variant Emacs is written in.
* Writing Emacs Primitives:: Writing C code for Emacs.
* Object Internals:: Data formats of buffers, windows, processes.
* C Integer Types:: How C integer types are used inside Emacs.
@end menu
@node Building Emacs
@section Building Emacs
@cindex building Emacs
@pindex temacs
This section explains the steps involved in building the Emacs
executable. You don't have to know this material to build and install
Emacs, since the makefiles do all these things automatically. This
information is pertinent to Emacs developers.
Building Emacs requires GNU Make version 3.81 or later.
Compilation of the C source files in the @file{src} directory
produces an executable file called @file{temacs}, also called a
@dfn{bare impure Emacs}. It contains the Emacs Lisp interpreter and
I/O routines, but not the editing commands.
@cindex @file{loadup.el}
The command @w{@command{temacs -l loadup}} would run @file{temacs}
and direct it to load @file{loadup.el}. The @code{loadup} library
loads additional Lisp libraries, which set up the normal Emacs editing
environment. After this step, the Emacs executable is no longer
@dfn{bare}.
@cindex dumping Emacs
Because it takes some time to load the standard Lisp files, the
@file{temacs} executable usually isn't run directly by users.
Instead, as one of the last steps of building Emacs, the command
@samp{temacs -batch -l loadup dump} is run. The special @samp{dump}
argument causes @command{temacs} to dump out an executable program,
called @file{emacs}, which has all the standard Lisp files preloaded.
(The @samp{-batch} argument prevents @file{temacs} from trying to
initialize any of its data on the terminal, so that the tables of
terminal information are empty in the dumped Emacs.)
@cindex preloaded Lisp files
@vindex preloaded-file-list
The dumped @file{emacs} executable (also called a @dfn{pure} Emacs)
is the one which is installed. The variable
@code{preloaded-file-list} stores a list of the Lisp files preloaded
into the dumped Emacs. If you port Emacs to a new operating system,
and are not able to implement dumping, then Emacs must load
@file{loadup.el} each time it starts.
@cindex build details
@cindex deterministic build
@cindex @option{--disable-build-details} option to @command{configure}
By default the dumped @file{emacs} executable records details such
as the build time and host name. Use the
@option{--disable-build-details} option of @command{configure} to
suppress these details, so that building and installing Emacs twice
from the same sources is more likely to result in identical copies of
Emacs.
@cindex @file{site-load.el}
You can specify additional files to preload by writing a library named
@file{site-load.el} that loads them. You may need to rebuild Emacs
with an added definition
@example
#define SITELOAD_PURESIZE_EXTRA @var{n}
@end example
@noindent
to make @var{n} added bytes of pure space to hold the additional files;
see @file{src/puresize.h}.
(Try adding increments of 20000 until it is big enough.) However, the
advantage of preloading additional files decreases as machines get
faster. On modern machines, it is usually not advisable.
After @file{loadup.el} reads @file{site-load.el}, it finds the
documentation strings for primitive and preloaded functions (and
variables) in the file @file{etc/DOC} where they are stored, by
calling @code{Snarf-documentation} (@pxref{Definition of
Snarf-documentation,, Accessing Documentation}).
@cindex @file{site-init.el}
@cindex preloading additional functions and variables
You can specify other Lisp expressions to execute just before dumping
by putting them in a library named @file{site-init.el}. This file is
executed after the documentation strings are found.
If you want to preload function or variable definitions, there are
three ways you can do this and make their documentation strings
accessible when you subsequently run Emacs:
@itemize @bullet
@item
Arrange to scan these files when producing the @file{etc/DOC} file,
and load them with @file{site-load.el}.
@item
Load the files with @file{site-init.el}, then copy the files into the
installation directory for Lisp files when you install Emacs.
@item
Specify a @code{nil} value for @code{byte-compile-dynamic-docstrings}
as a local variable in each of these files, and load them with either
@file{site-load.el} or @file{site-init.el}. (This method has the
drawback that the documentation strings take up space in Emacs all the
time.)
@end itemize
@cindex change @code{load-path} at configure time
@cindex @option{--enable-locallisppath} option to @command{configure}
It is not advisable to put anything in @file{site-load.el} or
@file{site-init.el} that would alter any of the features that users
expect in an ordinary unmodified Emacs. If you feel you must override
normal features for your site, do it with @file{default.el}, so that
users can override your changes if they wish. @xref{Startup Summary}.
Note that if either @file{site-load.el} or @file{site-init.el} changes
@code{load-path}, the changes will be lost after dumping.
@xref{Library Search}. To make a permanent change to
@code{load-path}, use the @option{--enable-locallisppath} option
of @command{configure}.
In a package that can be preloaded, it is sometimes necessary (or
useful) to delay certain evaluations until Emacs subsequently starts
up. The vast majority of such cases relate to the values of
customizable variables. For example, @code{tutorial-directory} is a
variable defined in @file{startup.el}, which is preloaded. The default
value is set based on @code{data-directory}. The variable needs to
access the value of @code{data-directory} when Emacs starts, not when
it is dumped, because the Emacs executable has probably been installed
in a different location since it was dumped.
@defun custom-initialize-delay symbol value
This function delays the initialization of @var{symbol} to the next
Emacs start. You normally use this function by specifying it as the
@code{:initialize} property of a customizable variable. (The argument
@var{value} is unused, and is provided only for compatibility with the
form Custom expects.)
@end defun
In the unlikely event that you need a more general functionality than
@code{custom-initialize-delay} provides, you can use
@code{before-init-hook} (@pxref{Startup Summary}).
@defun dump-emacs to-file from-file
@cindex unexec
This function dumps the current state of Emacs into an executable file
@var{to-file}. It takes symbols from @var{from-file} (this is normally
the executable file @file{temacs}).
If you want to use this function in an Emacs that was already dumped,
you must run Emacs with @samp{-batch}.
@end defun
@node Pure Storage
@section Pure Storage
@cindex pure storage
Emacs Lisp uses two kinds of storage for user-created Lisp objects:
@dfn{normal storage} and @dfn{pure storage}. Normal storage is where
all the new data created during an Emacs session are kept
(@pxref{Garbage Collection}). Pure storage is used for certain data
in the preloaded standard Lisp files---data that should never change
during actual use of Emacs.
Pure storage is allocated only while @command{temacs} is loading the
standard preloaded Lisp libraries. In the file @file{emacs}, it is
marked as read-only (on operating systems that permit this), so that
the memory space can be shared by all the Emacs jobs running on the
machine at once. Pure storage is not expandable; a fixed amount is
allocated when Emacs is compiled, and if that is not sufficient for
the preloaded libraries, @file{temacs} allocates dynamic memory for
the part that didn't fit. The resulting image will work, but garbage
collection (@pxref{Garbage Collection}) is disabled in this situation,
causing a memory leak. Such an overflow normally won't happen unless
you try to preload additional libraries or add features to the
standard ones. Emacs will display a warning about the overflow when
it starts. If this happens, you should increase the compilation
parameter @code{SYSTEM_PURESIZE_EXTRA} in the file
@file{src/puresize.h} and rebuild Emacs.
@defun purecopy object
This function makes a copy in pure storage of @var{object}, and returns
it. It copies a string by simply making a new string with the same
characters, but without text properties, in pure storage. It
recursively copies the contents of vectors and cons cells. It does
not make copies of other objects such as symbols, but just returns
them unchanged. It signals an error if asked to copy markers.
This function is a no-op except while Emacs is being built and dumped;
it is usually called only in preloaded Lisp files.
@end defun
@defvar pure-bytes-used
The value of this variable is the number of bytes of pure storage
allocated so far. Typically, in a dumped Emacs, this number is very
close to the total amount of pure storage available---if it were not,
we would preallocate less.
@end defvar
@defvar purify-flag
This variable determines whether @code{defun} should make a copy of the
function definition in pure storage. If it is non-@code{nil}, then the
function definition is copied into pure storage.
This flag is @code{t} while loading all of the basic functions for
building Emacs initially (allowing those functions to be shareable and
non-collectible). Dumping Emacs as an executable always writes
@code{nil} in this variable, regardless of the value it actually has
before and after dumping.
You should not change this flag in a running Emacs.
@end defvar
@node Garbage Collection
@section Garbage Collection
@cindex memory allocation
When a program creates a list or the user defines a new function
(such as by loading a library), that data is placed in normal storage.
If normal storage runs low, then Emacs asks the operating system to
allocate more memory. Different types of Lisp objects, such as
symbols, cons cells, small vectors, markers, etc., are segregated in
distinct blocks in memory. (Large vectors, long strings, buffers and
certain other editing types, which are fairly large, are allocated in
individual blocks, one per object; small strings are packed into blocks
of 8k bytes, and small vectors are packed into blocks of 4k bytes).
@cindex vector-like objects, storage
@cindex storage of vector-like Lisp objects
Beyond the basic vector, a lot of objects like window, buffer, and
frame are managed as if they were vectors. The corresponding C data
structures include the @code{struct vectorlike_header} field whose
@code{size} member contains the subtype enumerated by @code{enum pvec_type}
and an information about how many @code{Lisp_Object} fields this structure
contains and what the size of the rest data is. This information is
needed to calculate the memory footprint of an object, and used
by the vector allocation code while iterating over the vector blocks.
@cindex garbage collection
It is quite common to use some storage for a while, then release it
by (for example) killing a buffer or deleting the last pointer to an
object. Emacs provides a @dfn{garbage collector} to reclaim this
abandoned storage. The garbage collector operates by finding and
marking all Lisp objects that are still accessible to Lisp programs.
To begin with, it assumes all the symbols, their values and associated
function definitions, and any data presently on the stack, are
accessible. Any objects that can be reached indirectly through other
accessible objects are also accessible.
When marking is finished, all objects still unmarked are garbage. No
matter what the Lisp program or the user does, it is impossible to refer
to them, since there is no longer a way to reach them. Their space
might as well be reused, since no one will miss them. The second
(sweep) phase of the garbage collector arranges to reuse them.
@c ??? Maybe add something describing weak hash tables here?
@cindex free list
The sweep phase puts unused cons cells onto a @dfn{free list}
for future allocation; likewise for symbols and markers. It compacts
the accessible strings so they occupy fewer 8k blocks; then it frees the
other 8k blocks. Unreachable vectors from vector blocks are coalesced
to create largest possible free areas; if a free area spans a complete
4k block, that block is freed. Otherwise, the free area is recorded
in a free list array, where each entry corresponds to a free list
of areas of the same size. Large vectors, buffers, and other large
objects are allocated and freed individually.
@cindex CL note---allocate more storage
@quotation
@b{Common Lisp note:} Unlike other Lisps, GNU Emacs Lisp does not
call the garbage collector when the free list is empty. Instead, it
simply requests the operating system to allocate more storage, and
processing continues until @code{gc-cons-threshold} bytes have been
used.
This means that you can make sure that the garbage collector will not
run during a certain portion of a Lisp program by calling the garbage
collector explicitly just before it (provided that portion of the
program does not use so much space as to force a second garbage
collection).
@end quotation
@deffn Command garbage-collect
This command runs a garbage collection, and returns information on
the amount of space in use. (Garbage collection can also occur
spontaneously if you use more than @code{gc-cons-threshold} bytes of
Lisp data since the previous garbage collection.)
@code{garbage-collect} returns a list with information on amount of space in
use, where each entry has the form @samp{(@var{name} @var{size} @var{used})}
or @samp{(@var{name} @var{size} @var{used} @var{free})}. In the entry,
@var{name} is a symbol describing the kind of objects this entry represents,
@var{size} is the number of bytes used by each one, @var{used} is the number
of those objects that were found live in the heap, and optional @var{free} is
the number of those objects that are not live but that Emacs keeps around for
future allocations. So an overall result is:
@example
((@code{conses} @var{cons-size} @var{used-conses} @var{free-conses})
(@code{symbols} @var{symbol-size} @var{used-symbols} @var{free-symbols})
(@code{miscs} @var{misc-size} @var{used-miscs} @var{free-miscs})
(@code{strings} @var{string-size} @var{used-strings} @var{free-strings})
(@code{string-bytes} @var{byte-size} @var{used-bytes})
(@code{vectors} @var{vector-size} @var{used-vectors})
(@code{vector-slots} @var{slot-size} @var{used-slots} @var{free-slots})
(@code{floats} @var{float-size} @var{used-floats} @var{free-floats})
(@code{intervals} @var{interval-size} @var{used-intervals} @var{free-intervals})
(@code{buffers} @var{buffer-size} @var{used-buffers})
(@code{heap} @var{unit-size} @var{total-size} @var{free-size}))
@end example
Here is an example:
@example
(garbage-collect)
@result{} ((conses 16 49126 8058) (symbols 48 14607 0)
(miscs 40 34 56) (strings 32 2942 2607)
(string-bytes 1 78607) (vectors 16 7247)
(vector-slots 8 341609 29474) (floats 8 71 102)
(intervals 56 27 26) (buffers 944 8)
(heap 1024 11715 2678))
@end example
Below is a table explaining each element. Note that last @code{heap} entry
is optional and present only if an underlying @code{malloc} implementation
provides @code{mallinfo} function.
@table @var
@item cons-size
Internal size of a cons cell, i.e., @code{sizeof (struct Lisp_Cons)}.
@item used-conses
The number of cons cells in use.
@item free-conses
The number of cons cells for which space has been obtained from
the operating system, but that are not currently being used.
@item symbol-size
Internal size of a symbol, i.e., @code{sizeof (struct Lisp_Symbol)}.
@item used-symbols
The number of symbols in use.
@item free-symbols
The number of symbols for which space has been obtained from
the operating system, but that are not currently being used.
@item misc-size
Internal size of a miscellaneous entity, i.e.,
@code{sizeof (union Lisp_Misc)}, which is a size of the
largest type enumerated in @code{enum Lisp_Misc_Type}.
@item used-miscs
The number of miscellaneous objects in use. These include markers
and overlays, plus certain objects not visible to users.
@item free-miscs
The number of miscellaneous objects for which space has been obtained
from the operating system, but that are not currently being used.
@item string-size
Internal size of a string header, i.e., @code{sizeof (struct Lisp_String)}.
@item used-strings
The number of string headers in use.
@item free-strings
The number of string headers for which space has been obtained
from the operating system, but that are not currently being used.
@item byte-size
This is used for convenience and equals to @code{sizeof (char)}.
@item used-bytes
The total size of all string data in bytes.
@item vector-size
Internal size of a vector header, i.e., @code{sizeof (struct Lisp_Vector)}.
@item used-vectors
The number of vector headers allocated from the vector blocks.
@item slot-size
Internal size of a vector slot, always equal to @code{sizeof (Lisp_Object)}.
@item used-slots
The number of slots in all used vectors.
@item free-slots
The number of free slots in all vector blocks.
@item float-size
Internal size of a float object, i.e., @code{sizeof (struct Lisp_Float)}.
(Do not confuse it with the native platform @code{float} or @code{double}.)
@item used-floats
The number of floats in use.
@item free-floats
The number of floats for which space has been obtained from
the operating system, but that are not currently being used.
@item interval-size
Internal size of an interval object, i.e., @code{sizeof (struct interval)}.
@item used-intervals
The number of intervals in use.
@item free-intervals
The number of intervals for which space has been obtained from
the operating system, but that are not currently being used.
@item buffer-size
Internal size of a buffer, i.e., @code{sizeof (struct buffer)}.
(Do not confuse with the value returned by @code{buffer-size} function.)
@item used-buffers
The number of buffer objects in use. This includes killed buffers
invisible to users, i.e., all buffers in @code{all_buffers} list.
@item unit-size
The unit of heap space measurement, always equal to 1024 bytes.
@item total-size
Total heap size, in @var{unit-size} units.
@item free-size
Heap space which is not currently used, in @var{unit-size} units.
@end table
If there was overflow in pure space (@pxref{Pure Storage}),
@code{garbage-collect} returns @code{nil}, because a real garbage
collection cannot be done.
@end deffn
@defopt garbage-collection-messages
If this variable is non-@code{nil}, Emacs displays a message at the
beginning and end of garbage collection. The default value is
@code{nil}.
@end defopt
@defvar post-gc-hook
This is a normal hook that is run at the end of garbage collection.
Garbage collection is inhibited while the hook functions run, so be
careful writing them.
@end defvar
@defopt gc-cons-threshold
The value of this variable is the number of bytes of storage that must
be allocated for Lisp objects after one garbage collection in order to
trigger another garbage collection. You can use the result returned by
@code{garbage-collect} to get an information about size of the particular
object type; space allocated to the contents of buffers does not count.
Note that the subsequent garbage collection does not happen immediately
when the threshold is exhausted, but only the next time the Lisp interpreter
is called.
The initial threshold value is @code{GC_DEFAULT_THRESHOLD}, defined in
@file{alloc.c}. Since it's defined in @code{word_size} units, the value
is 400,000 for the default 32-bit configuration and 800,000 for the 64-bit
one. If you specify a larger value, garbage collection will happen less
often. This reduces the amount of time spent garbage collecting, but
increases total memory use. You may want to do this when running a program
that creates lots of Lisp data.
You can make collections more frequent by specifying a smaller value, down
to 1/10th of @code{GC_DEFAULT_THRESHOLD}. A value less than this minimum
will remain in effect only until the subsequent garbage collection, at which
time @code{garbage-collect} will set the threshold back to the minimum.
@end defopt
@defopt gc-cons-percentage
The value of this variable specifies the amount of consing before a
garbage collection occurs, as a fraction of the current heap size.
This criterion and @code{gc-cons-threshold} apply in parallel, and
garbage collection occurs only when both criteria are satisfied.
As the heap size increases, the time to perform a garbage collection
increases. Thus, it can be desirable to do them less frequently in
proportion.
@end defopt
The value returned by @code{garbage-collect} describes the amount of
memory used by Lisp data, broken down by data type. By contrast, the
function @code{memory-limit} provides information on the total amount of
memory Emacs is currently using.
@defun memory-limit
This function returns the address of the last byte Emacs has allocated,
divided by 1024. We divide the value by 1024 to make sure it fits in a
Lisp integer.
You can use this to get a general idea of how your actions affect the
memory usage.
@end defun
@defvar memory-full
This variable is @code{t} if Emacs is nearly out of memory for Lisp
objects, and @code{nil} otherwise.
@end defvar
@defun memory-use-counts
This returns a list of numbers that count the number of objects
created in this Emacs session. Each of these counters increments for
a certain kind of object. See the documentation string for details.
@end defun
@defun memory-info
This functions returns an amount of total system memory and how much
of it is free. On an unsupported system, the value may be @code{nil}.
@end defun
@defvar gcs-done
This variable contains the total number of garbage collections
done so far in this Emacs session.
@end defvar
@defvar gc-elapsed
This variable contains the total number of seconds of elapsed time
during garbage collection so far in this Emacs session, as a
floating-point number.
@end defvar
@node Stack-allocated Objects
@section Stack-allocated Objects
@cindex stack allocated Lisp objects
@cindex Lisp objects, stack-allocated
The garbage collector described above is used to manage data visible
from Lisp programs, as well as most of the data internally used by the
Lisp interpreter. Sometimes it may be useful to allocate temporary
internal objects using the C stack of the interpreter. This can help
performance, as stack allocation is typically faster than using heap
memory to allocate and the garbage collector to free. The downside is
that using such objects after they are freed results in undefined
behavior, so uses should be well thought out and carefully debugged by
using the @code{GC_CHECK_MARKED_OBJECTS} feature (see
@file{src/alloc.c}). In particular, stack-allocated objects should
never be made visible to user Lisp code.
Currently, cons cells and strings can be allocated this way. This
is implemented by C macros like @code{AUTO_CONS} and
@code{AUTO_STRING} that define a named @code{Lisp_Object} with block
lifetime. These objects are not freed by the garbage collector;
instead, they have automatic storage duration, i.e., they are
allocated like local variables and are automatically freed at the end
of execution of the C block that defined the object.
For performance reasons, stack-allocated strings are limited to
@acronym{ASCII} characters, and many of these strings are immutable,
i.e., calling @code{ASET} on them produces undefined behavior.
@node Memory Usage
@section Memory Usage
@cindex memory usage
These functions and variables give information about the total amount
of memory allocation that Emacs has done, broken down by data type.
Note the difference between these and the values returned by
@code{garbage-collect}; those count objects that currently exist, but
these count the number or size of all allocations, including those for
objects that have since been freed.
@defvar cons-cells-consed
The total number of cons cells that have been allocated so far
in this Emacs session.
@end defvar
@defvar floats-consed
The total number of floats that have been allocated so far
in this Emacs session.
@end defvar
@defvar vector-cells-consed
The total number of vector cells that have been allocated so far
in this Emacs session.
@end defvar
@defvar symbols-consed
The total number of symbols that have been allocated so far
in this Emacs session.
@end defvar
@defvar string-chars-consed
The total number of string characters that have been allocated so far
in this session.
@end defvar
@defvar misc-objects-consed
The total number of miscellaneous objects that have been allocated so
far in this session. These include markers and overlays, plus
certain objects not visible to users.
@end defvar
@defvar intervals-consed
The total number of intervals that have been allocated so far
in this Emacs session.
@end defvar
@defvar strings-consed
The total number of strings that have been allocated so far in this
Emacs session.
@end defvar
@node C Dialect
@section C Dialect
@cindex C programming language
The C part of Emacs is portable to C99 or later: C11-specific features such
as @samp{<stdalign.h>} and @samp{_Noreturn} are not used without a check,
typically at configuration time, and the Emacs build procedure
provides a substitute implementation if necessary. Some C11 features,
such as anonymous structures and unions, are too difficult to emulate,
so they are avoided entirely.
At some point in the future the base C dialect will no doubt change to C11.
@node Writing Emacs Primitives
@section Writing Emacs Primitives
@cindex primitive function internals
@cindex writing Emacs primitives
Lisp primitives are Lisp functions implemented in C@. The details of
interfacing the C function so that Lisp can call it are handled by a few
C macros. The only way to really understand how to write new C code is
to read the source, but we can explain some things here.
An example of a special form is the definition of @code{or}, from
@file{eval.c}. (An ordinary function would have the same general
appearance.)
@smallexample
@group
DEFUN ("or", For, Sor, 0, UNEVALLED, 0,
doc: /* Eval args until one of them yields non-nil, then return
that value.
The remaining args are not evalled at all.
If all args return nil, return nil.
@end group
@group
usage: (or CONDITIONS...) */)
(Lisp_Object args)
@{
Lisp_Object val = Qnil;
@end group
@group
while (CONSP (args))
@{
val = eval_sub (XCAR (args));
if (!NILP (val))
break;
args = XCDR (args);
maybe_quit ();
@}
@end group
@group
return val;
@}
@end group
@end smallexample
@cindex @code{DEFUN}, C macro to define Lisp primitives
Let's start with a precise explanation of the arguments to the
@code{DEFUN} macro. Here is a template for them:
@example
DEFUN (@var{lname}, @var{fname}, @var{sname}, @var{min}, @var{max}, @var{interactive}, @var{doc})
@end example
@table @var
@item lname
This is the name of the Lisp symbol to define as the function name; in
the example above, it is @code{or}.
@item fname
This is the C function name for this function. This is the name that
is used in C code for calling the function. The name is, by
convention, @samp{F} prepended to the Lisp name, with all dashes
(@samp{-}) in the Lisp name changed to underscores. Thus, to call
this function from C code, call @code{For}.
@item sname
This is a C variable name to use for a structure that holds the data for
the subr object that represents the function in Lisp. This structure
conveys the Lisp symbol name to the initialization routine that will
create the symbol and store the subr object as its definition. By
convention, this name is always @var{fname} with @samp{F} replaced with
@samp{S}.
@item min
This is the minimum number of arguments that the function requires. The
function @code{or} allows a minimum of zero arguments.
@item max
This is the maximum number of arguments that the function accepts, if
there is a fixed maximum. Alternatively, it can be @code{UNEVALLED},
indicating a special form that receives unevaluated arguments, or
@code{MANY}, indicating an unlimited number of evaluated arguments (the
equivalent of @code{&rest}). Both @code{UNEVALLED} and @code{MANY} are
macros. If @var{max} is a number, it must be more than @var{min} but
less than 8.
@cindex interactive specification in primitives
@item interactive
This is an interactive specification, a string such as might be used
as the argument of @code{interactive} in a Lisp function. In the case
of @code{or}, it is 0 (a null pointer), indicating that @code{or}
cannot be called interactively. A value of @code{""} indicates a
function that should receive no arguments when called interactively.
If the value begins with a @samp{"(}, the string is evaluated as a
Lisp form. For example:
@example
@group
DEFUN ("foo", Ffoo, Sfoo, 0, UNEVALLED,
"(list (read-char-by-name \"Insert character: \")\
(prefix-numeric-value current-prefix-arg)\
t))",
doc: /* @dots{} /*)
@end group
@end example
@item doc
This is the documentation string. It uses C comment syntax rather
than C string syntax because comment syntax requires nothing special
to include multiple lines. The @samp{doc:} identifies the comment
that follows as the documentation string. The @samp{/*} and @samp{*/}
delimiters that begin and end the comment are not part of the
documentation string.
If the last line of the documentation string begins with the keyword
@samp{usage:}, the rest of the line is treated as the argument list
for documentation purposes. This way, you can use different argument
names in the documentation string from the ones used in the C code.
@samp{usage:} is required if the function has an unlimited number of
arguments.
All the usual rules for documentation strings in Lisp code
(@pxref{Documentation Tips}) apply to C code documentation strings
too.
@end table
After the call to the @code{DEFUN} macro, you must write the
argument list for the C function, including the types for the
arguments. If the primitive accepts a fixed maximum number of Lisp
arguments, there must be one C argument for each Lisp argument, and
each argument must be of type @code{Lisp_Object}. (Various macros and
functions for creating values of type @code{Lisp_Object} are declared
in the file @file{lisp.h}.) If the primitive has no upper limit on
the number of Lisp arguments, it must have exactly two C arguments:
the first is the number of Lisp arguments, and the second is the
address of a block containing their values. These have types
@code{int} and @w{@code{Lisp_Object *}} respectively. Since
@code{Lisp_Object} can hold any Lisp object of any data type, you
can determine the actual data type only at run time; so if you want
a primitive to accept only a certain type of argument, you must check
the type explicitly using a suitable predicate (@pxref{Type Predicates}).
@cindex type checking internals
@cindex garbage collection protection
@cindex protect C variables from garbage collection
Within the function @code{For} itself, the local variable
@code{args} refers to objects controlled by Emacs's stack-marking
garbage collector. Although the garbage collector does not reclaim
objects reachable from C @code{Lisp_Object} stack variables, it may
move non-object components of an object, such as string contents; so
functions that access non-object components must take care to refetch
their addresses after performing Lisp evaluation. Lisp evaluation can
occur via calls to @code{eval_sub} or @code{Feval}, either directly or
indirectly.
@cindex @code{maybe_quit}, use in Lisp primitives
Note the call to @code{maybe_quit} inside the loop: this function
checks whether the user pressed @kbd{C-g}, and if so, aborts the
processing. You should do that in any loop that can potentially
require a large number of iterations; in this case, the list of
arguments could be very long. This increases Emacs responsiveness and
improves user experience.
You must not use C initializers for static or global variables unless
the variables are never written once Emacs is dumped. These variables
with initializers are allocated in an area of memory that becomes
read-only (on certain operating systems) as a result of dumping Emacs.
@xref{Pure Storage}.
@cindex @code{defsubr}, Lisp symbol for a primitive
Defining the C function is not enough to make a Lisp primitive
available; you must also create the Lisp symbol for the primitive and
store a suitable subr object in its function cell. The code looks like
this:
@example
defsubr (&@var{sname});
@end example
@noindent
Here @var{sname} is the name you used as the third argument to @code{DEFUN}.
If you add a new primitive to a file that already has Lisp primitives
defined in it, find the function (near the end of the file) named
@code{syms_of_@var{something}}, and add the call to @code{defsubr}
there. If the file doesn't have this function, or if you create a new
file, add to it a @code{syms_of_@var{filename}} (e.g.,
@code{syms_of_myfile}). Then find the spot in @file{emacs.c} where all
of these functions are called, and add a call to
@code{syms_of_@var{filename}} there.
@anchor{Defining Lisp variables in C}
@vindex byte-boolean-vars
@cindex defining Lisp variables in C
@cindex @code{DEFVAR_INT}, @code{DEFVAR_LISP}, @code{DEFVAR_BOOL}
The function @code{syms_of_@var{filename}} is also the place to define
any C variables that are to be visible as Lisp variables.
@code{DEFVAR_LISP} makes a C variable of type @code{Lisp_Object} visible
in Lisp. @code{DEFVAR_INT} makes a C variable of type @code{int}
visible in Lisp with a value that is always an integer.
@code{DEFVAR_BOOL} makes a C variable of type @code{int} visible in Lisp
with a value that is either @code{t} or @code{nil}. Note that variables
defined with @code{DEFVAR_BOOL} are automatically added to the list
@code{byte-boolean-vars} used by the byte compiler.
@cindex defining customization variables in C
If you want to make a Lisp variables that is defined in C behave
like one declared with @code{defcustom}, add an appropriate entry to
@file{cus-start.el}.
@cindex @code{staticpro}, protection from GC
If you define a file-scope C variable of type @code{Lisp_Object},
you must protect it from garbage-collection by calling @code{staticpro}
in @code{syms_of_@var{filename}}, like this:
@example
staticpro (&@var{variable});
@end example
Here is another example function, with more complicated arguments.
This comes from the code in @file{window.c}, and it demonstrates the use
of macros and functions to manipulate Lisp objects.
@smallexample
@group
DEFUN ("coordinates-in-window-p", Fcoordinates_in_window_p,
Scoordinates_in_window_p, 2, 2, 0,
doc: /* Return non-nil if COORDINATES are in WINDOW.
...
@end group
@group
or `right-margin' is returned. */)
(register Lisp_Object coordinates, Lisp_Object window)
@{
struct window *w;
struct frame *f;
int x, y;
Lisp_Object lx, ly;
@end group
@group
CHECK_LIVE_WINDOW (window);
w = XWINDOW (window);
f = XFRAME (w->frame);
CHECK_CONS (coordinates);
lx = Fcar (coordinates);
ly = Fcdr (coordinates);
CHECK_NUMBER_OR_FLOAT (lx);
CHECK_NUMBER_OR_FLOAT (ly);
x = FRAME_PIXEL_X_FROM_CANON_X (f, lx) + FRAME_INTERNAL_BORDER_WIDTH(f);
y = FRAME_PIXEL_Y_FROM_CANON_Y (f, ly) + FRAME_INTERNAL_BORDER_WIDTH(f);
@end group
@group
switch (coordinates_in_window (w, x, y))
@{
case ON_NOTHING: /* NOT in window at all. */
return Qnil;
@end group
...
@group
case ON_MODE_LINE: /* In mode line of window. */
return Qmode_line;
@end group
...
@group
case ON_SCROLL_BAR: /* On scroll-bar of window. */
/* Historically we are supposed to return nil in this case. */
return Qnil;
@end group
@group
default:
abort ();
@}
@}
@end group
@end smallexample
Note that C code cannot call functions by name unless they are defined
in C@. The way to call a function written in Lisp is to use
@code{Ffuncall}, which embodies the Lisp function @code{funcall}. Since
the Lisp function @code{funcall} accepts an unlimited number of
arguments, in C it takes two: the number of Lisp-level arguments, and a
one-dimensional array containing their values. The first Lisp-level
argument is the Lisp function to call, and the rest are the arguments to
pass to it.
The C functions @code{call0}, @code{call1}, @code{call2}, and so on,
provide handy ways to call a Lisp function conveniently with a fixed
number of arguments. They work by calling @code{Ffuncall}.
@file{eval.c} is a very good file to look through for examples;
@file{lisp.h} contains the definitions for some important macros and
functions.
If you define a function which is side-effect free, update the code
in @file{byte-opt.el} that binds @code{side-effect-free-fns} and
@code{side-effect-and-error-free-fns} so that the compiler optimizer
knows about it.
@node Object Internals
@section Object Internals
@cindex object internals
Emacs Lisp provides a rich set of the data types. Some of them, like cons
cells, integers and strings, are common to nearly all Lisp dialects. Some
others, like markers and buffers, are quite special and needed to provide
the basic support to write editor commands in Lisp. To implement such
a variety of object types and provide an efficient way to pass objects between
the subsystems of an interpreter, there is a set of C data structures and
a special type to represent the pointers to all of them, which is known as
@dfn{tagged pointer}.
In C, the tagged pointer is an object of type @code{Lisp_Object}. Any
initialized variable of such a type always holds the value of one of the
following basic data types: integer, symbol, string, cons cell, float,
vectorlike or miscellaneous object. Each of these data types has the
corresponding tag value. All tags are enumerated by @code{enum Lisp_Type}
and placed into a 3-bit bitfield of the @code{Lisp_Object}. The rest of the
bits is the value itself. Integers are immediate, i.e., directly
represented by those @dfn{value bits}, and all other objects are represented
by the C pointers to a corresponding object allocated from the heap. Width
of the @code{Lisp_Object} is platform- and configuration-dependent: usually
it's equal to the width of an underlying platform pointer (i.e., 32-bit on
a 32-bit machine and 64-bit on a 64-bit one), but also there is a special
configuration where @code{Lisp_Object} is 64-bit but all pointers are 32-bit.
The latter trick was designed to overcome the limited range of values for
Lisp integers on a 32-bit system by using 64-bit @code{long long} type for
@code{Lisp_Object}.
The following C data structures are defined in @file{lisp.h} to represent
the basic data types beyond integers:
@table @code
@item struct Lisp_Cons
Cons cell, an object used to construct lists.
@item struct Lisp_String
String, the basic object to represent a sequence of characters.
@item struct Lisp_Vector
Array, a fixed-size set of Lisp objects which may be accessed by an index.
@item struct Lisp_Symbol
Symbol, the unique-named entity commonly used as an identifier.
@item struct Lisp_Float
Floating-point value.
@item union Lisp_Misc
Miscellaneous kinds of objects which don't fit into any of the above.
@end table
These types are the first-class citizens of an internal type system.
Since the tag space is limited, all other types are the subtypes of either
@code{Lisp_Vectorlike} or @code{Lisp_Misc}. Vector subtypes are enumerated
by @code{enum pvec_type}, and nearly all complex objects like windows, buffers,
frames, and processes fall into this category. The rest of special types,
including markers and overlays, are enumerated by @code{enum Lisp_Misc_Type}
and form the set of subtypes of @code{Lisp_Misc}.
Below there is a description of a few subtypes of @code{Lisp_Vectorlike}.
Buffer object represents the text to display and edit. Window is the part
of display structure which shows the buffer or used as a container to
recursively place other windows on the same frame. (Do not confuse Emacs Lisp
window object with the window as an entity managed by the user interface
system like X; in Emacs terminology, the latter is called frame.) Finally,
process object is used to manage the subprocesses.
@menu
* Buffer Internals:: Components of a buffer structure.
* Window Internals:: Components of a window structure.
* Process Internals:: Components of a process structure.
@end menu
@node Buffer Internals
@subsection Buffer Internals
@cindex internals, of buffer
@cindex buffer internals
Two structures (see @file{buffer.h}) are used to represent buffers
in C@. The @code{buffer_text} structure contains fields describing the
text of a buffer; the @code{buffer} structure holds other fields. In
the case of indirect buffers, two or more @code{buffer} structures
reference the same @code{buffer_text} structure.
Here are some of the fields in @code{struct buffer_text}:
@table @code
@item beg
The address of the buffer contents.
@item gpt
@itemx gpt_byte
The character and byte positions of the buffer gap. @xref{Buffer
Gap}.
@item z
@itemx z_byte
The character and byte positions of the end of the buffer text.
@item gap_size
The size of buffer's gap. @xref{Buffer Gap}.
@item modiff
@itemx save_modiff
@itemx chars_modiff
@itemx overlay_modiff
These fields count the number of buffer-modification events performed
in this buffer. @code{modiff} is incremented after each
buffer-modification event, and is never otherwise changed;
@code{save_modiff} contains the value of @code{modiff} the last time
the buffer was visited or saved; @code{chars_modiff} counts only
modifications to the characters in the buffer, ignoring all other
kinds of changes; and @code{overlay_modiff} counts only modifications
to the overlays.
@item beg_unchanged
@itemx end_unchanged
The number of characters at the start and end of the text that are
known to be unchanged since the last complete redisplay.
@item unchanged_modified
@itemx overlay_unchanged_modified
The values of @code{modiff} and @code{overlay_modiff}, respectively,
after the last complete redisplay. If their current values match
@code{modiff} or @code{overlay_modiff}, that means
@code{beg_unchanged} and @code{end_unchanged} contain no useful
information.
@item markers
The markers that refer to this buffer. This is actually a single
marker, and successive elements in its marker @code{chain} are the other
markers referring to this buffer text.
@item intervals
The interval tree which records the text properties of this buffer.
@end table
Some of the fields of @code{struct buffer} are:
@table @code
@item header
A header of type @code{struct vectorlike_header} is common to all
vectorlike objects.
@item own_text
A @code{struct buffer_text} structure that ordinarily holds the buffer
contents. In indirect buffers, this field is not used.
@item text
A pointer to the @code{buffer_text} structure for this buffer. In an
ordinary buffer, this is the @code{own_text} field above. In an
indirect buffer, this is the @code{own_text} field of the base buffer.
@item next
A pointer to the next buffer, in the chain of all buffers, including
killed buffers. This chain is used only for allocation and garbage
collection, in order to collect killed buffers properly.
@item pt
@itemx pt_byte
The character and byte positions of point in a buffer.
@item begv
@itemx begv_byte
The character and byte positions of the beginning of the accessible
range of text in the buffer.
@item zv
@itemx zv_byte
The character and byte positions of the end of the accessible range of
text in the buffer.
@item base_buffer
In an indirect buffer, this points to the base buffer. In an ordinary
buffer, it is null.
@item local_flags
This field contains flags indicating that certain variables are local
in this buffer. Such variables are declared in the C code using
@code{DEFVAR_PER_BUFFER}, and their buffer-local bindings are stored
in fields in the buffer structure itself. (Some of these fields are
described in this table.)
@item modtime
The modification time of the visited file. It is set when the file is
written or read. Before writing the buffer into a file, this field is
compared to the modification time of the file to see if the file has
changed on disk. @xref{Buffer Modification}.
@item auto_save_modified
The time when the buffer was last auto-saved.
@item last_window_start
The @code{window-start} position in the buffer as of the last time the
buffer was displayed in a window.
@item clip_changed
This flag indicates that narrowing has changed in the buffer.
@xref{Narrowing}.
@item prevent_redisplay_optimizations_p
This flag indicates that redisplay optimizations should not be used to
display this buffer.
@item overlay_center
This field holds the current overlay center position. @xref{Managing
Overlays}.
@item overlays_before
@itemx overlays_after
These fields hold, respectively, a list of overlays that end at or
before the current overlay center, and a list of overlays that end
after the current overlay center. @xref{Managing Overlays}.
@code{overlays_before} is sorted in order of decreasing end position,
and @code{overlays_after} is sorted in order of increasing beginning
position.
@c FIXME? the following are now all Lisp_Object BUFFER_INTERNAL_FIELD (foo).
@item name
A Lisp string that names the buffer. It is guaranteed to be unique.
@xref{Buffer Names}.
@item save_length
The length of the file this buffer is visiting, when last read or
saved. This and other fields concerned with saving are not kept in
the @code{buffer_text} structure because indirect buffers are never
saved.
@item directory
The directory for expanding relative file names. This is the value of
the buffer-local variable @code{default-directory} (@pxref{File Name Expansion}).
@item filename
The name of the file visited in this buffer, or @code{nil}. This is
the value of the buffer-local variable @code{buffer-file-name}
(@pxref{Buffer File Name}).
@item undo_list
@itemx backed_up
@itemx auto_save_file_name
@itemx auto_save_file_format
@itemx read_only
@itemx file_format
@itemx file_truename
@itemx invisibility_spec
@itemx display_count
@itemx display_time
These fields store the values of Lisp variables that are automatically
buffer-local (@pxref{Buffer-Local Variables}), whose corresponding
variable names have the additional prefix @code{buffer-} and have
underscores replaced with dashes. For instance, @code{undo_list}
stores the value of @code{buffer-undo-list}.
@item mark
The mark for the buffer. The mark is a marker, hence it is also
included on the list @code{markers}. @xref{The Mark}.
@item local_var_alist
The association list describing the buffer-local variable bindings of
this buffer, not including the built-in buffer-local bindings that
have special slots in the buffer object. (Those slots are omitted
from this table.) @xref{Buffer-Local Variables}.
@item major_mode
Symbol naming the major mode of this buffer, e.g., @code{lisp-mode}.
@item mode_name
Pretty name of the major mode, e.g., @code{"Lisp"}.
@item keymap
@itemx abbrev_table
@itemx syntax_table
@itemx category_table
@itemx display_table
These fields store the buffer's local keymap (@pxref{Keymaps}), abbrev
table (@pxref{Abbrev Tables}), syntax table (@pxref{Syntax Tables}),
category table (@pxref{Categories}), and display table (@pxref{Display
Tables}).
@item downcase_table
@itemx upcase_table
@itemx case_canon_table
These fields store the conversion tables for converting text to lower
case, upper case, and for canonicalizing text for case-fold search.
@xref{Case Tables}.
@item minor_modes
An alist of the minor modes of this buffer.
@item pt_marker
@itemx begv_marker
@itemx zv_marker
These fields are only used in an indirect buffer, or in a buffer that
is the base of an indirect buffer. Each holds a marker that records
@code{pt}, @code{begv}, and @code{zv} respectively, for this buffer
when the buffer is not current.
@item mode_line_format
@itemx header_line_format
@itemx case_fold_search
@itemx tab_width
@itemx fill_column
@itemx left_margin
@itemx auto_fill_function
@itemx truncate_lines
@itemx word_wrap
@itemx ctl_arrow
@itemx bidi_display_reordering
@itemx bidi_paragraph_direction
@itemx selective_display
@itemx selective_display_ellipses
@itemx overwrite_mode
@itemx abbrev_mode
@itemx mark_active
@itemx enable_multibyte_characters
@itemx buffer_file_coding_system
@itemx cache_long_line_scans
@itemx point_before_scroll
@itemx left_fringe_width
@itemx right_fringe_width
@itemx fringes_outside_margins
@itemx scroll_bar_width
@itemx indicate_empty_lines
@itemx indicate_buffer_boundaries
@itemx fringe_indicator_alist
@itemx fringe_cursor_alist
@itemx scroll_up_aggressively
@itemx scroll_down_aggressively
@itemx cursor_type
@itemx cursor_in_non_selected_windows
These fields store the values of Lisp variables that are automatically
buffer-local (@pxref{Buffer-Local Variables}), whose corresponding
variable names have underscores replaced with dashes. For instance,
@code{mode_line_format} stores the value of @code{mode-line-format}.
@item last_selected_window
This is the last window that was selected with this buffer in it, or @code{nil}
if that window no longer displays this buffer.
@end table
@node Window Internals
@subsection Window Internals
@cindex internals, of window
@cindex window internals
The fields of a window (for a complete list, see the definition of
@code{struct window} in @file{window.h}) include:
@table @code
@item frame
The frame that this window is on.
@item mini_p
Non-@code{nil} if this window is a minibuffer window.
@item parent
Internally, Emacs arranges windows in a tree; each group of siblings has
a parent window whose area includes all the siblings. This field points
to a window's parent.
Parent windows do not display buffers, and play little role in display
except to shape their child windows. Emacs Lisp programs usually have
no access to the parent windows; they operate on the windows at the
leaves of the tree, which actually display buffers.
@c FIXME: These two slots and the 'buffer' slot below were replaced
@c with a single slot 'contents' on 2013-03-28. --xfq
@item hchild
@itemx vchild
These fields contain the window's leftmost child and its topmost child
respectively. @code{hchild} is used if the window is subdivided
horizontally by child windows, and @code{vchild} if it is subdivided
vertically. In a live window, only one of @code{hchild}, @code{vchild},
and @code{buffer} (q.v.@:) is non-@code{nil}.
@item next
@itemx prev
The next sibling and previous sibling of this window. @code{next} is
@code{nil} if the window is the right-most or bottom-most in its group;
@code{prev} is @code{nil} if it is the left-most or top-most in its
group.
@item left_col
The left-hand edge of the window, measured in columns, relative to the
leftmost column in the frame (column 0).
@item top_line
The top edge of the window, measured in lines, relative to the topmost
line in the frame (line 0).
@item total_cols
@itemx total_lines
The width and height of the window, measured in columns and lines
respectively. The width includes the scroll bar and fringes, and/or
the separator line on the right of the window (if any).
@item buffer
The buffer that the window is displaying.
@item start
A marker pointing to the position in the buffer that is the first
character displayed in the window.
@item pointm
@cindex window point internals
This is the value of point in the current buffer when this window is
selected; when it is not selected, it retains its previous value.
@item force_start
If this flag is non-@code{nil}, it says that the window has been
scrolled explicitly by the Lisp program. This affects what the next
redisplay does if point is off the screen: instead of scrolling the
window to show the text around point, it moves point to a location that
is on the screen.
@item frozen_window_start_p
This field is set temporarily to 1 to indicate to redisplay that
@code{start} of this window should not be changed, even if point
gets invisible.
@item start_at_line_beg
Non-@code{nil} means current value of @code{start} was the beginning of a line
when it was chosen.
@item use_time
This is the last time that the window was selected. The function
@code{get-lru-window} uses this field.
@item sequence_number
A unique number assigned to this window when it was created.
@item last_modified
The @code{modiff} field of the window's buffer, as of the last time
a redisplay completed in this window.
@item last_overlay_modified
The @code{overlay_modiff} field of the window's buffer, as of the last
time a redisplay completed in this window.
@item last_point
The buffer's value of point, as of the last time a redisplay completed
in this window.
@item last_had_star
A non-@code{nil} value means the window's buffer was modified when the
window was last updated.
@item vertical_scroll_bar
This window's vertical scroll bar.
@item left_margin_cols
@itemx right_margin_cols
The widths of the left and right margins in this window. A value of
@code{nil} means no margin.
@item left_fringe_width
@itemx right_fringe_width
The widths of the left and right fringes in this window. A value of
@code{nil} or @code{t} means use the values of the frame.
@item fringes_outside_margins
A non-@code{nil} value means the fringes outside the display margins;
othersize they are between the margin and the text.
@item window_end_pos
This is computed as @code{z} minus the buffer position of the last glyph
in the current matrix of the window. The value is only valid if
@code{window_end_valid} is not @code{nil}.
@item window_end_bytepos
The byte position corresponding to @code{window_end_pos}.
@item window_end_vpos
The window-relative vertical position of the line containing
@code{window_end_pos}.
@item window_end_valid
This field is set to a non-@code{nil} value if @code{window_end_pos} is truly
valid. This is @code{nil} if nontrivial redisplay is pre-empted, since in that
case the display that @code{window_end_pos} was computed for did not get
onto the screen.
@item cursor
A structure describing where the cursor is in this window.
@item last_cursor
The value of @code{cursor} as of the last redisplay that finished.
@item phys_cursor
A structure describing where the cursor of this window physically is.
@item phys_cursor_type
@c FIXME What is this?
@c itemx phys_cursor_ascent
@itemx phys_cursor_height
@itemx phys_cursor_width
The type, height, and width of the cursor that was last displayed on
this window.
@item phys_cursor_on_p
This field is non-zero if the cursor is physically on.
@item cursor_off_p
Non-zero means the cursor in this window is logically off. This is
used for blinking the cursor.
@item last_cursor_off_p
This field contains the value of @code{cursor_off_p} as of the time of
the last redisplay.
@item must_be_updated_p
This is set to 1 during redisplay when this window must be updated.
@item hscroll
This is the number of columns that the display in the window is scrolled
horizontally to the left. Normally, this is 0.
@item vscroll
Vertical scroll amount, in pixels. Normally, this is 0.
@item dedicated
Non-@code{nil} if this window is dedicated to its buffer.
@item display_table
The window's display table, or @code{nil} if none is specified for it.
@item update_mode_line
Non-@code{nil} means this window's mode line needs to be updated.
@item base_line_number
The line number of a certain position in the buffer, or @code{nil}.
This is used for displaying the line number of point in the mode line.
@item base_line_pos
The position in the buffer for which the line number is known, or
@code{nil} meaning none is known. If it is a buffer, don't display
the line number as long as the window shows that buffer.
@item column_number_displayed
The column number currently displayed in this window's mode line, or @code{nil}
if column numbers are not being displayed.
@item current_matrix
@itemx desired_matrix
Glyph matrices describing the current and desired display of this window.
@end table
@node Process Internals
@subsection Process Internals
@cindex internals, of process
@cindex process internals
The fields of a process (for a complete list, see the definition of
@code{struct Lisp_Process} in @file{process.h}) include:
@table @code
@item name
A string, the name of the process.
@item command
A list containing the command arguments that were used to start this
process. For a network or serial process, it is @code{nil} if the
process is running or @code{t} if the process is stopped.
@item filter
A function used to accept output from the process.
@item sentinel
A function called whenever the state of the process changes.
@item buffer
The associated buffer of the process.
@item pid
An integer, the operating system's process @acronym{ID}.
Pseudo-processes such as network or serial connections use a value of 0.
@item childp
A flag, @code{t} if this is really a child process. For a network or
serial connection, it is a plist based on the arguments to
@code{make-network-process} or @code{make-serial-process}.
@item mark
A marker indicating the position of the end of the last output from this
process inserted into the buffer. This is often but not always the end
of the buffer.
@item kill_without_query
If this is non-zero, killing Emacs while this process is still running
does not ask for confirmation about killing the process.
@item raw_status
The raw process status, as returned by the @code{wait} system call.
@item status
The process status, as @code{process-status} should return it.
@item tick
@itemx update_tick
If these two fields are not equal, a change in the status of the process
needs to be reported, either by running the sentinel or by inserting a
message in the process buffer.
@item pty_flag
Non-@code{nil} if communication with the subprocess uses a pty;
@code{nil} if it uses a pipe.
@item infd
The file descriptor for input from the process.
@item outfd
The file descriptor for output to the process.
@item tty_name
The name of the terminal that the subprocess is using,
or @code{nil} if it is using pipes.
@item decode_coding_system
Coding-system for decoding the input from this process.
@item decoding_buf
A working buffer for decoding.
@item decoding_carryover
Size of carryover in decoding.
@item encode_coding_system
Coding-system for encoding the output to this process.
@item encoding_buf
A working buffer for encoding.
@item inherit_coding_system_flag
Flag to set @code{coding-system} of the process buffer from the
coding system used to decode process output.
@item type
Symbol indicating the type of process: @code{real}, @code{network},
@code{serial}.
@end table
@node C Integer Types
@section C Integer Types
@cindex integer types (C programming language)
Here are some guidelines for use of integer types in the Emacs C
source code. These guidelines sometimes give competing advice; common
sense is advised.
@itemize @bullet
@item
Avoid arbitrary limits. For example, avoid @code{int len = strlen
(s);} unless the length of @code{s} is required for other reasons to
fit in @code{int} range.
@item
Do not assume that signed integer arithmetic wraps around on overflow.
This is no longer true of Emacs porting targets: signed integer
overflow has undefined behavior in practice, and can dump core or
even cause earlier or later code to behave illogically. Unsigned
overflow does wrap around reliably, modulo a power of two.
@item
Prefer signed types to unsigned, as code gets confusing when signed
and unsigned types are combined. Many other guidelines assume that
types are signed; in the rarer cases where unsigned types are needed,
similar advice may apply to the unsigned counterparts (e.g.,
@code{size_t} instead of @code{ptrdiff_t}, or @code{uintptr_t} instead
of @code{intptr_t}).
@item
Prefer @code{int} for Emacs character codes, in the range 0 ..@: 0x3FFFFF@.
More generally, prefer @code{int} for integers known to be in
@code{int} range, e.g., screen column counts.
@item
Prefer @code{ptrdiff_t} for sizes, i.e., for integers bounded by the
maximum size of any individual C object or by the maximum number of
elements in any C array. This is part of Emacs's general preference
for signed types. Using @code{ptrdiff_t} limits objects to
@code{PTRDIFF_MAX} bytes, but larger objects would cause trouble
anyway since they would break pointer subtraction, so this does not
impose an arbitrary limit.
@item
Avoid @code{ssize_t} except when communicating to low-level APIs that
have @code{ssize_t}-related limitations. Although it's equivalent to
@code{ptrdiff_t} on typical platforms, @code{ssize_t} is occasionally
narrower, so using it for size-related calculations could overflow.
Also, @code{ptrdiff_t} is more ubiquitous and better-standardized, has
standard @code{printf} formats, and is the basis for Emacs's internal
size-overflow checking. When using @code{ssize_t}, please note that
POSIX requires support only for values in the range @minus{}1 ..@:
@code{SSIZE_MAX}.
@item
Prefer @code{intptr_t} for internal representations of pointers, or
for integers bounded only by the number of objects that can exist at
any given time or by the total number of bytes that can be allocated.
Currently Emacs sometimes uses other types when @code{intptr_t} would
be better; fixing this is lower priority, as the code works as-is on
Emacs's current porting targets.
@item
Prefer the Emacs-defined type @code{EMACS_INT} for representing values
converted to or from Emacs Lisp fixnums, as fixnum arithmetic is based
on @code{EMACS_INT}.
@item
When representing a system value (such as a file size or a count of
seconds since the Epoch), prefer the corresponding system type (e.g.,
@code{off_t}, @code{time_t}). Do not assume that a system type is
signed, unless this assumption is known to be safe. For example,
although @code{off_t} is always signed, @code{time_t} need not be.
@item
Prefer the Emacs-defined type @code{printmax_t} for representing
values that might be any signed integer that can be printed,
using a @code{printf}-family function.
@item
Prefer @code{intmax_t} for representing values that might be any
signed integer value.
@item
Prefer @code{bool}, @code{false} and @code{true} for booleans.
Using @code{bool} can make programs easier to read and a bit faster than
using @code{int}. Although it is also OK to use @code{int}, @code{0}
and @code{1}, this older style is gradually being phased out. When
using @code{bool}, respect the limitations of the replacement
implementation of @code{bool}, as documented in the source file
@file{lib/stdbool.in.h}. In particular, boolean bitfields should be of type
@code{bool_bf}, not @code{bool}, so that they work correctly even when
compiling Objective C with standard GCC.
@item
In bitfields, prefer @code{unsigned int} or @code{signed int} to
@code{int}, as @code{int} is less portable: it might be signed, and
might not be. Single-bit bit fields should be @code{unsigned int} or
@code{bool_bf} so that their values are 0 or 1.
@end itemize
@c FIXME Mention src/globals.h somewhere in this file?
|