summaryrefslogtreecommitdiff
path: root/init/init.c
blob: c63bba1ffb1c1b3b45da7bbffb268c0f5182f914 (plain)
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
/* vi: set sw=4 ts=4: */
/*
 * Mini init implementation for busybox
 *
 * Copyright (C) 1995, 1996 by Bruce Perens <bruce@pixar.com>.
 * Copyright (C) 1999-2004 by Erik Andersen <andersen@codepoet.org>
 * Adjusted by so many folks, it's impossible to keep track.
 *
 * Licensed under GPLv2 or later, see file LICENSE in this source tree.
 */

//config:config INIT
//config:	bool "init"
//config:	default y
//config:	select FEATURE_SYSLOG
//config:	help
//config:	  init is the first program run when the system boots.
//config:
//config:config FEATURE_USE_INITTAB
//config:	bool "Support reading an inittab file"
//config:	default y
//config:	depends on INIT
//config:	help
//config:	  Allow init to read an inittab file when the system boot.
//config:
//config:config FEATURE_KILL_REMOVED
//config:	bool "Support killing processes that have been removed from inittab"
//config:	default n
//config:	depends on FEATURE_USE_INITTAB
//config:	help
//config:	  When respawn entries are removed from inittab and a SIGHUP is
//config:	  sent to init, this option will make init kill the processes
//config:	  that have been removed.
//config:
//config:config FEATURE_KILL_DELAY
//config:	int "How long to wait between TERM and KILL (0 - send TERM only)" if FEATURE_KILL_REMOVED
//config:	range 0 1024
//config:	default 0
//config:	depends on FEATURE_KILL_REMOVED
//config:	help
//config:	  With nonzero setting, init sends TERM, forks, child waits N
//config:	  seconds, sends KILL and exits. Setting it too high is unwise
//config:	  (child will hang around for too long and could actually kill
//config:	  the wrong process!)
//config:
//config:config FEATURE_INIT_SCTTY
//config:	bool "Run commands with leading dash with controlling tty"
//config:	default y
//config:	depends on INIT
//config:	help
//config:	  If this option is enabled, init will try to give a controlling
//config:	  tty to any command which has leading hyphen (often it's "-/bin/sh").
//config:	  More precisely, init will do "ioctl(STDIN_FILENO, TIOCSCTTY, 0)".
//config:	  If device attached to STDIN_FILENO can be a ctty but is not yet
//config:	  a ctty for other session, it will become this process' ctty.
//config:	  This is not the traditional init behavour, but is often what you want
//config:	  in an embedded system where the console is only accessed during
//config:	  development or for maintenance.
//config:	  NB: using cttyhack applet may work better.
//config:
//config:config FEATURE_INIT_SYSLOG
//config:	bool "Enable init to write to syslog"
//config:	default y
//config:	depends on INIT
//config:
//config:config FEATURE_EXTRA_QUIET
//config:	bool "Be _extra_ quiet on boot"
//config:	default y
//config:	depends on INIT
//config:	help
//config:	  Prevent init from logging some messages to the console during boot.
//config:
//config:config FEATURE_INIT_COREDUMPS
//config:	bool "Support dumping core for child processes (debugging only)"
//config:	default y
//config:	depends on INIT
//config:	help
//config:	  If this option is enabled and the file /.init_enable_core
//config:	  exists, then init will call setrlimit() to allow unlimited
//config:	  core file sizes. If this option is disabled, processes
//config:	  will not generate any core files.
//config:
//config:config FEATURE_INITRD
//config:	bool "Support running init from within an initrd (not initramfs)"
//config:	default y
//config:	depends on INIT
//config:	help
//config:	  Legacy support for running init under the old-style initrd. Allows
//config:	  the name linuxrc to act as init, and it doesn't assume init is PID 1.
//config:
//config:	  This does not apply to initramfs, which runs /init as PID 1 and
//config:	  requires no special support.
//config:
//config:config INIT_TERMINAL_TYPE
//config:	string "Initial terminal type"
//config:	default "linux"
//config:	depends on INIT
//config:	help
//config:	  This is the initial value set by init for the TERM environment
//config:	  variable. This variable is used by programs which make use of
//config:	  extended terminal capabilities.
//config:
//config:	  Note that on Linux, init attempts to detect serial terminal and
//config:	  sets TERM to "vt102" if one is found.

//applet:IF_INIT(APPLET(init, BB_DIR_SBIN, BB_SUID_DROP))
//applet:IF_FEATURE_INITRD(APPLET_ODDNAME(linuxrc, init, BB_DIR_ROOT, BB_SUID_DROP, linuxrc))

//kbuild:lib-$(CONFIG_INIT) += init.o

#define DEBUG_SEGV_HANDLER 0

#include "libbb.h"
#include <syslog.h>
#include <paths.h>
#include <sys/resource.h>
#ifdef __linux__
# include <linux/vt.h>
# include <sys/sysinfo.h>
#endif
#include "reboot.h" /* reboot() constants */

#if DEBUG_SEGV_HANDLER
# undef _GNU_SOURCE
# define _GNU_SOURCE 1
# undef __USE_GNU
# define __USE_GNU 1
# include <execinfo.h>
# include <sys/ucontext.h>
#endif

/* Used only for sanitizing purposes in set_sane_term() below. On systems where
 * the baud rate is stored in a separate field, we can safely disable them. */
#ifndef CBAUD
# define CBAUD 0
# define CBAUDEX 0
#endif

/* Was a CONFIG_xxx option. A lot of people were building
 * not fully functional init by switching it on! */
#define DEBUG_INIT 0

#define CONSOLE_NAME_SIZE 32

/* Default sysinit script. */
#ifndef INIT_SCRIPT
# define INIT_SCRIPT  "/etc/init.d/rcS"
#endif

/* Each type of actions can appear many times. They will be
 * handled in order. RESTART is an exception, only 1st is used.
 */
/* Start these actions first and wait for completion */
#define SYSINIT     0x01
/* Start these after SYSINIT and wait for completion */
#define WAIT        0x02
/* Start these after WAIT and *dont* wait for completion */
#define ONCE        0x04
/*
 * NB: while SYSINIT/WAIT/ONCE are being processed,
 * SIGHUP ("reread /etc/inittab") will be processed only after
 * each group of actions. If new inittab adds, say, a SYSINIT action,
 * it will not be run, since init is already "past SYSINIT stage".
 */
/* Start these after ONCE are started, restart on exit */
#define RESPAWN     0x08
/* Like RESPAWN, but wait for <Enter> to be pressed on tty */
#define ASKFIRST    0x10
/*
 * Start these on SIGINT, and wait for completion.
 * Then go back to respawning RESPAWN and ASKFIRST actions.
 * NB: kernel sends SIGINT to us if Ctrl-Alt-Del was pressed.
 */
#define CTRLALTDEL  0x20
/*
 * Start these before killing all processes in preparation for
 * running RESTART actions or doing low-level halt/reboot/poweroff
 * (initiated by SIGUSR1/SIGTERM/SIGUSR2).
 * Wait for completion before proceeding.
 */
#define SHUTDOWN    0x40
/*
 * exec() on SIGQUIT. SHUTDOWN actions are started and waited for,
 * then all processes are killed, then init exec's 1st RESTART action,
 * replacing itself by it. If no RESTART action specified,
 * SIGQUIT has no effect.
 */
#define RESTART     0x80


/* A linked list of init_actions, to be read from inittab */
struct init_action {
	struct init_action *next;
	pid_t pid;
	uint8_t action_type;
	char terminal[CONSOLE_NAME_SIZE];
	char command[1];
};

static struct init_action *init_action_list = NULL;

static const char *log_console = VC_5;

enum {
	L_LOG = 0x1,
	L_CONSOLE = 0x2,
};

/* Print a message to the specified device.
 * "where" may be bitwise-or'd from L_LOG | L_CONSOLE
 * NB: careful, we can be called after vfork!
 */
#define dbg_message(...) do { if (DEBUG_INIT) message(__VA_ARGS__); } while (0)
static void message(int where, const char *fmt, ...)
	__attribute__ ((format(printf, 2, 3)));
static void message(int where, const char *fmt, ...)
{
	va_list arguments;
	unsigned l;
	char msg[128];

	msg[0] = '\r';
	va_start(arguments, fmt);
	l = 1 + vsnprintf(msg + 1, sizeof(msg) - 2, fmt, arguments);
	if (l > sizeof(msg) - 2)
		l = sizeof(msg) - 2;
	va_end(arguments);

#if ENABLE_FEATURE_INIT_SYSLOG
	msg[l] = '\0';
	if (where & L_LOG) {
		/* Log the message to syslogd */
		openlog(applet_name, 0, LOG_DAEMON);
		/* don't print "\r" */
		syslog(LOG_INFO, "%s", msg + 1);
		closelog();
	}
	msg[l++] = '\n';
	msg[l] = '\0';
#else
	{
		static int log_fd = -1;

		msg[l++] = '\n';
		msg[l] = '\0';
		/* Take full control of the log tty, and never close it.
		 * It's mine, all mine!  Muhahahaha! */
		if (log_fd < 0) {
			if (!log_console) {
				log_fd = STDERR_FILENO;
			} else {
				log_fd = device_open(log_console, O_WRONLY | O_NONBLOCK | O_NOCTTY);
				if (log_fd < 0) {
					bb_error_msg("can't log to %s", log_console);
					where = L_CONSOLE;
				} else {
					close_on_exec_on(log_fd);
				}
			}
		}
		if (where & L_LOG) {
			full_write(log_fd, msg, l);
			if (log_fd == STDERR_FILENO)
				return; /* don't print dup messages */
		}
	}
#endif

	if (where & L_CONSOLE) {
		/* Send console messages to console so people will see them. */
		full_write(STDERR_FILENO, msg, l);
	}
}

static void console_init(void)
{
#ifdef VT_OPENQRY
	int vtno;
#endif
	char *s;

	s = getenv("CONSOLE");
	if (!s)
		s = getenv("console");
	if (s) {
		int fd = open(s, O_RDWR | O_NONBLOCK | O_NOCTTY);
		if (fd >= 0) {
			dup2(fd, STDIN_FILENO);
			dup2(fd, STDOUT_FILENO);
			xmove_fd(fd, STDERR_FILENO);
		}
		dbg_message(L_LOG, "console='%s'", s);
	} else {
		/* Make sure fd 0,1,2 are not closed
		 * (so that they won't be used by future opens) */
		bb_sanitize_stdio();
// Users report problems
//		/* Make sure init can't be blocked by writing to stderr */
//		fcntl(STDERR_FILENO, F_SETFL, fcntl(STDERR_FILENO, F_GETFL) | O_NONBLOCK);
	}

	s = getenv("TERM");
#ifdef VT_OPENQRY
	if (ioctl(STDIN_FILENO, VT_OPENQRY, &vtno) != 0) {
		/* Not a linux terminal, probably serial console.
		 * Force the TERM setting to vt102
		 * if TERM is set to linux (the default) */
		if (!s || strcmp(s, "linux") == 0)
			putenv((char*)"TERM=vt102");
		if (!ENABLE_FEATURE_INIT_SYSLOG)
			log_console = NULL;
	} else
#endif
	if (!s)
		putenv((char*)"TERM=" CONFIG_INIT_TERMINAL_TYPE);
}

/* Set terminal settings to reasonable defaults.
 * NB: careful, we can be called after vfork! */
static void set_sane_term(void)
{
	struct termios tty;

	tcgetattr(STDIN_FILENO, &tty);

	/* set control chars */
	tty.c_cc[VINTR] = 3;	/* C-c */
	tty.c_cc[VQUIT] = 28;	/* C-\ */
	tty.c_cc[VERASE] = 127;	/* C-? */
	tty.c_cc[VKILL] = 21;	/* C-u */
	tty.c_cc[VEOF] = 4;	/* C-d */
	tty.c_cc[VSTART] = 17;	/* C-q */
	tty.c_cc[VSTOP] = 19;	/* C-s */
	tty.c_cc[VSUSP] = 26;	/* C-z */

#ifdef __linux__
	/* use line discipline 0 */
	tty.c_line = 0;
#endif

	/* Make it be sane */
#ifndef CRTSCTS
# define CRTSCTS 0
#endif
	/* added CRTSCTS to fix Debian bug 528560 */
	tty.c_cflag &= CBAUD | CBAUDEX | CSIZE | CSTOPB | PARENB | PARODD | CRTSCTS;
	tty.c_cflag |= CREAD | HUPCL | CLOCAL;

	/* input modes */
	tty.c_iflag = ICRNL | IXON | IXOFF;

	/* output modes */
	tty.c_oflag = OPOST | ONLCR;

	/* local modes */
	tty.c_lflag = ISIG | ICANON | ECHO | ECHOE | ECHOK | ECHOCTL | ECHOKE | IEXTEN;

	tcsetattr_stdin_TCSANOW(&tty);
}

/* Open the new terminal device.
 * NB: careful, we can be called after vfork! */
static int open_stdio_to_tty(const char* tty_name)
{
	/* empty tty_name means "use init's tty", else... */
	if (tty_name[0]) {
		int fd;

		close(STDIN_FILENO);
		/* fd can be only < 0 or 0: */
		fd = device_open(tty_name, O_RDWR);
		if (fd) {
			message(L_LOG | L_CONSOLE, "can't open %s: %s",
				tty_name, strerror(errno));
			return 0; /* failure */
		}
		dup2(STDIN_FILENO, STDOUT_FILENO);
		dup2(STDIN_FILENO, STDERR_FILENO);
	}
	set_sane_term();
	return 1; /* success */
}

static void reset_sighandlers_and_unblock_sigs(void)
{
	bb_signals(0
		+ (1 << SIGUSR1)
		+ (1 << SIGUSR2)
		+ (1 << SIGTERM)
		+ (1 << SIGQUIT)
		+ (1 << SIGINT)
		+ (1 << SIGHUP)
		+ (1 << SIGTSTP)
		+ (1 << SIGSTOP)
		, SIG_DFL);
	sigprocmask_allsigs(SIG_UNBLOCK);
}

/* Wrapper around exec:
 * Takes string.
 * If chars like '>' detected, execs '[-]/bin/sh -c "exec ......."'.
 * Otherwise splits words on whitespace, deals with leading dash,
 * and uses plain exec().
 * NB: careful, we can be called after vfork!
 */
static void init_exec(const char *command)
{
	/* +8 allows to write VLA sizes below more efficiently: */
	unsigned command_size = strlen(command) + 8;
	/* strlen(command) + strlen("exec ")+1: */
	char buf[command_size];
	/* strlen(command) / 2 + 4: */
	char *cmd[command_size / 2];
	int dash;

	dash = (command[0] == '-' /* maybe? && command[1] == '/' */);
	command += dash;

	/* See if any special /bin/sh requiring characters are present */
	if (strpbrk(command, "~`!$^&*()=|\\{}[];\"'<>?") != NULL) {
		sprintf(buf, "exec %s", command); /* excluding "-" */
		/* NB: LIBBB_DEFAULT_LOGIN_SHELL define has leading dash */
		cmd[0] = (char*)(LIBBB_DEFAULT_LOGIN_SHELL + !dash);
		cmd[1] = (char*)"-c";
		cmd[2] = buf;
		cmd[3] = NULL;
		command = LIBBB_DEFAULT_LOGIN_SHELL + 1;
	} else {
		/* Convert command (char*) into cmd (char**, one word per string) */
		char *word, *next;
		int i = 0;
		next = strcpy(buf, command - dash); /* command including "-" */
		command = next + dash;
		while ((word = strsep(&next, " \t")) != NULL) {
			if (*word != '\0') { /* not two spaces/tabs together? */
				cmd[i] = word;
				i++;
			}
		}
		cmd[i] = NULL;
	}
	/* If we saw leading "-", it is interactive shell.
	 * Try harder to give it a controlling tty.
	 */
	if (ENABLE_FEATURE_INIT_SCTTY && dash) {
		/* _Attempt_ to make stdin a controlling tty. */
		ioctl(STDIN_FILENO, TIOCSCTTY, 0 /*only try, don't steal*/);
	}
	/* Here command never contains the dash, cmd[0] might */
	BB_EXECVP(command, cmd);
	message(L_LOG | L_CONSOLE, "can't run '%s': %s", command, strerror(errno));
	/* returns if execvp fails */
}

/* Used only by run_actions */
static pid_t run(const struct init_action *a)
{
	pid_t pid;

	/* Careful: don't be affected by a signal in vforked child */
	sigprocmask_allsigs(SIG_BLOCK);
	if (BB_MMU && (a->action_type & ASKFIRST))
		pid = fork();
	else
		pid = vfork();
	if (pid < 0)
		message(L_LOG | L_CONSOLE, "can't fork");
	if (pid) {
		sigprocmask_allsigs(SIG_UNBLOCK);
		return pid; /* Parent or error */
	}

	/* Child */

	/* Reset signal handlers that were set by the parent process */
	reset_sighandlers_and_unblock_sigs();

	/* Create a new session and make ourself the process group leader */
	setsid();

	/* Open the new terminal device */
	if (!open_stdio_to_tty(a->terminal))
		_exit(EXIT_FAILURE);

	/* NB: on NOMMU we can't wait for input in child, so
	 * "askfirst" will work the same as "respawn". */
	if (BB_MMU && (a->action_type & ASKFIRST)) {
		static const char press_enter[] ALIGN1 =
#ifdef CUSTOMIZED_BANNER
#include CUSTOMIZED_BANNER
#endif
			"\nPlease press Enter to activate this console. ";
		char c;
		/*
		 * Save memory by not exec-ing anything large (like a shell)
		 * before the user wants it. This is critical if swap is not
		 * enabled and the system has low memory. Generally this will
		 * be run on the second virtual console, and the first will
		 * be allowed to start a shell or whatever an init script
		 * specifies.
		 */
		dbg_message(L_LOG, "waiting for enter to start '%s'"
					"(pid %d, tty '%s')\n",
				a->command, getpid(), a->terminal);
		full_write(STDOUT_FILENO, press_enter, sizeof(press_enter) - 1);
		while (safe_read(STDIN_FILENO, &c, 1) == 1 && c != '\n')
			continue;
	}

	/*
	 * When a file named /.init_enable_core exists, setrlimit is called
	 * before processes are spawned to set core file size as unlimited.
	 * This is for debugging only.  Don't use this is production, unless
	 * you want core dumps lying about....
	 */
	if (ENABLE_FEATURE_INIT_COREDUMPS) {
		if (access("/.init_enable_core", F_OK) == 0) {
			struct rlimit limit;
			limit.rlim_cur = RLIM_INFINITY;
			limit.rlim_max = RLIM_INFINITY;
			setrlimit(RLIMIT_CORE, &limit);
		}
	}

	/* Log the process name and args */
	message(L_LOG, "starting pid %d, tty '%s': '%s'",
			getpid(), a->terminal, a->command);

	/* Now run it.  The new program will take over this PID,
	 * so nothing further in init.c should be run. */
	init_exec(a->command);
	/* We're still here?  Some error happened. */
	_exit(-1);
}

static struct init_action *mark_terminated(pid_t pid)
{
	struct init_action *a;

	if (pid > 0) {
		update_utmp(pid, DEAD_PROCESS,
				/*tty_name:*/ NULL,
				/*username:*/ NULL,
				/*hostname:*/ NULL
		);
		for (a = init_action_list; a; a = a->next) {
			if (a->pid == pid) {
				a->pid = 0;
				return a;
			}
		}
	}
	return NULL;
}

static void waitfor(pid_t pid)
{
	/* waitfor(run(x)): protect against failed fork inside run() */
	if (pid <= 0)
		return;

	/* Wait for any child (prevent zombies from exiting orphaned processes)
	 * but exit the loop only when specified one has exited. */
	while (1) {
		pid_t wpid = wait(NULL);
		mark_terminated(wpid);
		/* Unsafe. SIGTSTP handler might have wait'ed it already */
		/*if (wpid == pid) break;*/
		/* More reliable: */
		if (kill(pid, 0))
			break;
	}
}

/* Run all commands of a particular type */
static void run_actions(int action_type)
{
	struct init_action *a;

	for (a = init_action_list; a; a = a->next) {
		if (!(a->action_type & action_type))
			continue;

		if (a->action_type & (SYSINIT | WAIT | ONCE | CTRLALTDEL | SHUTDOWN)) {
			pid_t pid = run(a);
			if (a->action_type & (SYSINIT | WAIT | CTRLALTDEL | SHUTDOWN))
				waitfor(pid);
		}
		if (a->action_type & (RESPAWN | ASKFIRST)) {
			/* Only run stuff with pid == 0. If pid != 0,
			 * it is already running
			 */
			if (a->pid == 0)
				a->pid = run(a);
		}
	}
}

static void new_init_action(uint8_t action_type, const char *command, const char *cons)
{
	struct init_action *a, **nextp;

	/* Scenario:
	 * old inittab:
	 * ::shutdown:umount -a -r
	 * ::shutdown:swapoff -a
	 * new inittab:
	 * ::shutdown:swapoff -a
	 * ::shutdown:umount -a -r
	 * On reload, we must ensure entries end up in correct order.
	 * To achieve that, if we find a matching entry, we move it
	 * to the end.
	 */
	nextp = &init_action_list;
	while ((a = *nextp) != NULL) {
		/* Don't enter action if it's already in the list.
		 * This prevents losing running RESPAWNs.
		 */
		if (strcmp(a->command, command) == 0
		 && strcmp(a->terminal, cons) == 0
		) {
			/* Remove from list */
			*nextp = a->next;
			/* Find the end of the list */
			while (*nextp != NULL)
				nextp = &(*nextp)->next;
			a->next = NULL;
			goto append;
		}
		nextp = &a->next;
	}

	a = xzalloc(sizeof(*a) + strlen(command));

	/* Append to the end of the list */
 append:
	*nextp = a;
	a->action_type = action_type;
	strcpy(a->command, command);
	safe_strncpy(a->terminal, cons, sizeof(a->terminal));
	dbg_message(L_LOG | L_CONSOLE, "command='%s' action=%x tty='%s'\n",
		a->command, a->action_type, a->terminal);
}

/* NOTE that if CONFIG_FEATURE_USE_INITTAB is NOT defined,
 * then parse_inittab() simply adds in some default
 * actions (i.e., runs INIT_SCRIPT and then starts a pair
 * of "askfirst" shells).  If CONFIG_FEATURE_USE_INITTAB
 * _is_ defined, but /etc/inittab is missing, this
 * results in the same set of default behaviors.
 */
static void parse_inittab(void)
{
#if ENABLE_FEATURE_USE_INITTAB
	char *token[4];
	parser_t *parser = config_open2("/etc/inittab", fopen_for_read);

	if (parser == NULL)
#endif
	{
		/* No inittab file - set up some default behavior */
		/* Sysinit */
		new_init_action(SYSINIT, INIT_SCRIPT, "");
		/* Askfirst shell on tty1-4 */
		new_init_action(ASKFIRST, bb_default_login_shell, "");
//TODO: VC_1 instead of ""? "" is console -> ctty problems -> angry users
		new_init_action(ASKFIRST, bb_default_login_shell, VC_2);
		new_init_action(ASKFIRST, bb_default_login_shell, VC_3);
		new_init_action(ASKFIRST, bb_default_login_shell, VC_4);
		/* Reboot on Ctrl-Alt-Del */
		new_init_action(CTRLALTDEL, "reboot", "");
		/* Umount all filesystems on halt/reboot */
		new_init_action(SHUTDOWN, "umount -a -r", "");
		/* Swapoff on halt/reboot */
		new_init_action(SHUTDOWN, "swapoff -a", "");
		/* Restart init when a QUIT is received */
		new_init_action(RESTART, "init", "");
		return;
	}

#if ENABLE_FEATURE_USE_INITTAB
	/* optional_tty:ignored_runlevel:action:command
	 * Delims are not to be collapsed and need exactly 4 tokens
	 */
	while (config_read(parser, token, 4, 0, "#:",
				PARSE_NORMAL & ~(PARSE_TRIM | PARSE_COLLAPSE))) {
		/* order must correspond to SYSINIT..RESTART constants */
		static const char actions[] ALIGN1 =
			"sysinit\0""wait\0""once\0""respawn\0""askfirst\0"
			"ctrlaltdel\0""shutdown\0""restart\0";
		int action;
		char *tty = token[0];

		if (!token[3]) /* less than 4 tokens */
			goto bad_entry;
		action = index_in_strings(actions, token[2]);
		if (action < 0 || !token[3][0]) /* token[3]: command */
			goto bad_entry;
		/* turn .*TTY -> /dev/TTY */
		if (tty[0]) {
			tty = concat_path_file("/dev/", skip_dev_pfx(tty));
		}
		new_init_action(1 << action, token[3], tty);
		if (tty[0])
			free(tty);
		continue;
 bad_entry:
		message(L_LOG | L_CONSOLE, "Bad inittab entry at line %d",
				parser->lineno);
	}
	config_close(parser);
#endif
}

static void pause_and_low_level_reboot(unsigned magic) NORETURN;
static void pause_and_low_level_reboot(unsigned magic)
{
	pid_t pid;

	/* Allow time for last message to reach serial console, etc */
	sleep(1);

	/* We have to fork here, since the kernel calls do_exit(EXIT_SUCCESS)
	 * in linux/kernel/sys.c, which can cause the machine to panic when
	 * the init process exits... */
	pid = vfork();
	if (pid == 0) { /* child */
		reboot(magic);
		_exit(EXIT_SUCCESS);
	}
	while (1)
		sleep(1);
}

static void run_shutdown_and_kill_processes(void)
{
	/* Run everything to be run at "shutdown".  This is done _prior_
	 * to killing everything, in case people wish to use scripts to
	 * shut things down gracefully... */
	run_actions(SHUTDOWN);

	message(L_CONSOLE | L_LOG, "The system is going down NOW!");

	/* Send signals to every process _except_ pid 1 */
	kill(-1, SIGTERM);
	message(L_CONSOLE | L_LOG, "Sent SIG%s to all processes", "TERM");
	sync();
	sleep(1);

	kill(-1, SIGKILL);
	message(L_CONSOLE, "Sent SIG%s to all processes", "KILL");
	sync();
	/*sleep(1); - callers take care about making a pause */
}

/* Signal handling by init:
 *
 * For process with PID==1, on entry kernel sets all signals to SIG_DFL
 * and unmasks all signals. However, for process with PID==1,
 * default action (SIG_DFL) on any signal is to ignore it,
 * even for special signals SIGKILL and SIGCONT.
 * Also, any signal can be caught or blocked.
 * (but SIGSTOP is still handled specially, at least in 2.6.20)
 *
 * We install two kinds of handlers, "immediate" and "delayed".
 *
 * Immediate handlers execute at any time, even while, say, sysinit
 * is running.
 *
 * Delayed handlers just set a flag variable. The variable is checked
 * in the main loop and acted upon.
 *
 * halt/poweroff/reboot and restart have immediate handlers.
 * They only traverse linked list of struct action's, never modify it,
 * this should be safe to do even in signal handler. Also they
 * never return.
 *
 * SIGSTOP and SIGTSTP have immediate handlers. They just wait
 * for SIGCONT to happen.
 *
 * SIGHUP has a delayed handler, because modifying linked list
 * of struct action's from a signal handler while it is manipulated
 * by the program may be disastrous.
 *
 * Ctrl-Alt-Del has a delayed handler. Not a must, but allowing
 * it to happen even somewhere inside "sysinit" would be a bit awkward.
 *
 * There is a tiny probability that SIGHUP and Ctrl-Alt-Del will collide
 * and only one will be remembered and acted upon.
 */

/* The SIGUSR[12]/SIGTERM handler */
static void halt_reboot_pwoff(int sig) NORETURN;
static void halt_reboot_pwoff(int sig)
{
	const char *m;
	unsigned rb;

	/* We may call run() and it unmasks signals,
	 * including the one masked inside this signal handler.
	 * Testcase which would start multiple reboot scripts:
	 *  while true; do reboot; done
	 * Preventing it:
	 */
	reset_sighandlers_and_unblock_sigs();

	run_shutdown_and_kill_processes();

	m = "halt";
	rb = RB_HALT_SYSTEM;
	if (sig == SIGTERM) {
		m = "reboot";
		rb = RB_AUTOBOOT;
	} else if (sig == SIGUSR2) {
		m = "poweroff";
		rb = RB_POWER_OFF;
	}
	message(L_CONSOLE, "Requesting system %s", m);
	pause_and_low_level_reboot(rb);
	/* not reached */
}

/* Handler for QUIT - exec "restart" action,
 * else (no such action defined) do nothing */
static void restart_handler(int sig UNUSED_PARAM)
{
	struct init_action *a;

	for (a = init_action_list; a; a = a->next) {
		if (!(a->action_type & RESTART))
			continue;

		/* Starting from here, we won't return.
		 * Thus don't need to worry about preserving errno
		 * and such.
		 */

		reset_sighandlers_and_unblock_sigs();

		run_shutdown_and_kill_processes();

#ifdef RB_ENABLE_CAD
		/* Allow Ctrl-Alt-Del to reboot the system.
		 * This is how kernel sets it up for init, we follow suit.
		 */
		reboot(RB_ENABLE_CAD); /* misnomer */
#endif

		if (open_stdio_to_tty(a->terminal)) {
			dbg_message(L_CONSOLE, "Trying to re-exec %s", a->command);
			/* Theoretically should be safe.
			 * But in practice, kernel bugs may leave
			 * unkillable processes, and wait() may block forever.
			 * Oh well. Hoping "new" init won't be too surprised
			 * by having children it didn't create.
			 */
			//while (wait(NULL) > 0)
			//	continue;
			init_exec(a->command);
		}
		/* Open or exec failed */
		pause_and_low_level_reboot(RB_HALT_SYSTEM);
		/* not reached */
	}
}

/* The SIGSTOP/SIGTSTP handler
 * NB: inside it, all signals except SIGCONT are masked
 * via appropriate setup in sigaction().
 */
static void stop_handler(int sig UNUSED_PARAM)
{
	smallint saved_bb_got_signal;
	int saved_errno;

	saved_bb_got_signal = bb_got_signal;
	saved_errno = errno;
	signal(SIGCONT, record_signo);

	while (1) {
		pid_t wpid;

		if (bb_got_signal == SIGCONT)
			break;
		/* NB: this can accidentally wait() for a process
		 * which we waitfor() elsewhere! waitfor() must have
		 * code which is resilient against this.
		 */
		wpid = wait_any_nohang(NULL);
		mark_terminated(wpid);
		sleep(1);
	}

	signal(SIGCONT, SIG_DFL);
	errno = saved_errno;
	bb_got_signal = saved_bb_got_signal;
}

#if ENABLE_FEATURE_USE_INITTAB
static void reload_inittab(void)
{
	struct init_action *a, **nextp;

	message(L_LOG, "reloading /etc/inittab");

	/* Disable old entries */
	for (a = init_action_list; a; a = a->next)
		a->action_type = 0;

	/* Append new entries, or modify existing entries
	 * (incl. setting a->action_type) if cmd and device name
	 * match new ones. End result: only entries with
	 * a->action_type == 0 are stale.
	 */
	parse_inittab();

#if ENABLE_FEATURE_KILL_REMOVED
	/* Kill stale entries */
	/* Be nice and send SIGTERM first */
	for (a = init_action_list; a; a = a->next)
		if (a->action_type == 0 && a->pid != 0)
			kill(a->pid, SIGTERM);
	if (CONFIG_FEATURE_KILL_DELAY) {
		/* NB: parent will wait in NOMMU case */
		if ((BB_MMU ? fork() : vfork()) == 0) { /* child */
			sleep(CONFIG_FEATURE_KILL_DELAY);
			for (a = init_action_list; a; a = a->next)
				if (a->action_type == 0 && a->pid != 0)
					kill(a->pid, SIGKILL);
			_exit(EXIT_SUCCESS);
		}
	}
#endif

	/* Remove stale entries and SYSINIT entries.
	 * We never rerun SYSINIT entries anyway,
	 * removing them too saves a few bytes
	 */
	nextp = &init_action_list;
	while ((a = *nextp) != NULL) {
		/*
		 * Why pid == 0 check?
		 * Process can be removed from inittab and added *later*.
		 * If we delete its entry but process still runs,
		 * duplicate is spawned when the entry is re-added.
		 */
		if ((a->action_type & ~SYSINIT) == 0 && a->pid == 0) {
			*nextp = a->next;
			free(a);
		} else {
			nextp = &a->next;
		}
	}

	/* Not needed: */
	/* run_actions(RESPAWN | ASKFIRST); */
	/* - we return to main loop, which does this automagically */
}
#endif

static int check_delayed_sigs(void)
{
	int sigs_seen = 0;

	while (1) {
		smallint sig = bb_got_signal;

		if (!sig)
			return sigs_seen;
		bb_got_signal = 0;
		sigs_seen = 1;
#if ENABLE_FEATURE_USE_INITTAB
		if (sig == SIGHUP)
			reload_inittab();
#endif
		if (sig == SIGINT)
			run_actions(CTRLALTDEL);
	}
}

#if DEBUG_SEGV_HANDLER
static
void handle_sigsegv(int sig, siginfo_t *info, void *ucontext)
{
	long ip;
	ucontext_t *uc;

	uc = ucontext;
	ip = uc->uc_mcontext.gregs[REG_EIP];
	fdprintf(2, "signal:%d address:0x%lx ip:0x%lx\n",
			sig,
			/* this is void*, but using %p would print "(null)"
			 * even for ptrs which are not exactly 0, but, say, 0x123:
			 */
			(long)info->si_addr,
			ip);
	{
		/* glibc extension */
		void *array[50];
		int size;
		size = backtrace(array, 50);
		backtrace_symbols_fd(array, size, 2);
	}
	for (;;) sleep(9999);
}
#endif

int init_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int init_main(int argc UNUSED_PARAM, char **argv)
{
	if (argv[1] && strcmp(argv[1], "-q") == 0) {
		return kill(1, SIGHUP);
	}

#if DEBUG_SEGV_HANDLER
	{
		struct sigaction sa;
		memset(&sa, 0, sizeof(sa));
		sa.sa_sigaction = handle_sigsegv;
		sa.sa_flags = SA_SIGINFO;
		sigaction(SIGSEGV, &sa, NULL);
		sigaction(SIGILL, &sa, NULL);
		sigaction(SIGFPE, &sa, NULL);
		sigaction(SIGBUS, &sa, NULL);
	}
#endif

	if (!DEBUG_INIT) {
		/* Expect to be invoked as init with PID=1 or be invoked as linuxrc */
		if (getpid() != 1
		 && (!ENABLE_FEATURE_INITRD || applet_name[0] != 'l') /* not linuxrc? */
		) {
			bb_error_msg_and_die("must be run as PID 1");
		}
#ifdef RB_DISABLE_CAD
		/* Turn off rebooting via CTL-ALT-DEL - we get a
		 * SIGINT on CAD so we can shut things down gracefully... */
		reboot(RB_DISABLE_CAD); /* misnomer */
#endif
	}

	/* If, say, xmalloc would ever die, we don't want to oops kernel
	 * by exiting.
	 * NB: we set die_sleep *after* PID 1 check and bb_show_usage.
	 * Otherwise, for example, "init u" ("please rexec yourself"
	 * command for sysvinit) will show help text (which isn't too bad),
	 * *and sleep forever* (which is bad!)
	 */
	die_sleep = 30 * 24*60*60;

	/* Figure out where the default console should be */
	console_init();
	set_sane_term();
	xchdir("/");
	setsid();

	/* Make sure environs is set to something sane */
	putenv((char *) "HOME=/");
	putenv((char *) bb_PATH_root_path);
	putenv((char *) "SHELL=/bin/sh");
	putenv((char *) "USER=root"); /* needed? why? */

	if (argv[1])
		xsetenv("RUNLEVEL", argv[1]);

#if !ENABLE_FEATURE_EXTRA_QUIET
	/* Hello world */
	message(L_CONSOLE | L_LOG, "init started: %s", bb_banner);
#endif

#if 0
/* It's 2013, does anyone really still depend on this? */
/* If you do, consider adding swapon to sysinot actions then! */
/* struct sysinfo is linux-specific */
# ifdef __linux__
	/* Make sure there is enough memory to do something useful. */
	/*if (ENABLE_SWAPONOFF) - WRONG: we may have non-bbox swapon*/ {
		struct sysinfo info;

		if (sysinfo(&info) == 0
		 && (info.mem_unit ? info.mem_unit : 1) * (long long)info.totalram < 1024*1024
		) {
			message(L_CONSOLE, "Low memory, forcing swapon");
			/* swapon -a requires /proc typically */
			new_init_action(SYSINIT, "mount -t proc proc /proc", "");
			/* Try to turn on swap */
			new_init_action(SYSINIT, "swapon -a", "");
			run_actions(SYSINIT);   /* wait and removing */
		}
	}
# endif
#endif

	/* Check if we are supposed to be in single user mode */
	if (argv[1]
	 && (strcmp(argv[1], "single") == 0 || strcmp(argv[1], "-s") == 0 || LONE_CHAR(argv[1], '1'))
	) {
		/* ??? shouldn't we set RUNLEVEL="b" here? */
		/* Start a shell on console */
		new_init_action(RESPAWN, bb_default_login_shell, "");
	} else {
		/* Not in single user mode - see what inittab says */

		/* NOTE that if CONFIG_FEATURE_USE_INITTAB is NOT defined,
		 * then parse_inittab() simply adds in some default
		 * actions(i.e., INIT_SCRIPT and a pair
		 * of "askfirst" shells */
		parse_inittab();
	}

#if ENABLE_SELINUX
	if (getenv("SELINUX_INIT") == NULL) {
		int enforce = 0;

		putenv((char*)"SELINUX_INIT=YES");
		if (selinux_init_load_policy(&enforce) == 0) {
			BB_EXECVP(argv[0], argv);
		} else if (enforce > 0) {
			/* SELinux in enforcing mode but load_policy failed */
			message(L_CONSOLE, "can't load SELinux Policy. "
				"Machine is in enforcing mode. Halting now.");
			return EXIT_FAILURE;
		}
	}
#endif

	/* Make the command line just say "init"  - thats all, nothing else */
	strncpy(argv[0], "init", strlen(argv[0]));
	/* Wipe argv[1]-argv[N] so they don't clutter the ps listing */
	while (*++argv)
		nuke_str(*argv);

	/* Set up signal handlers */
	if (!DEBUG_INIT) {
		struct sigaction sa;

		bb_signals(0
			+ (1 << SIGUSR1) /* halt */
			+ (1 << SIGTERM) /* reboot */
			+ (1 << SIGUSR2) /* poweroff */
			, halt_reboot_pwoff);
		signal(SIGQUIT, restart_handler); /* re-exec another init */

		/* Stop handler must allow only SIGCONT inside itself */
		memset(&sa, 0, sizeof(sa));
		sigfillset(&sa.sa_mask);
		sigdelset(&sa.sa_mask, SIGCONT);
		sa.sa_handler = stop_handler;
		/* NB: sa_flags doesn't have SA_RESTART.
		 * It must be able to interrupt wait().
		 */
		sigaction_set(SIGTSTP, &sa); /* pause */
		/* Does not work as intended, at least in 2.6.20.
		 * SIGSTOP is simply ignored by init:
		 */
		sigaction_set(SIGSTOP, &sa); /* pause */

		/* SIGINT (Ctrl-Alt-Del) must interrupt wait(),
		 * setting handler without SA_RESTART flag.
		 */
		bb_signals_recursive_norestart((1 << SIGINT), record_signo);
	}

	/* Set up "reread /etc/inittab" handler.
	 * Handler is set up without SA_RESTART, it will interrupt syscalls.
	 */
	if (!DEBUG_INIT && ENABLE_FEATURE_USE_INITTAB)
		bb_signals_recursive_norestart((1 << SIGHUP), record_signo);

	/* Now run everything that needs to be run */
	/* First run the sysinit command */
	run_actions(SYSINIT);
	check_delayed_sigs();
	/* Next run anything that wants to block */
	run_actions(WAIT);
	check_delayed_sigs();
	/* Next run anything to be run only once */
	run_actions(ONCE);

	/* Now run the looping stuff for the rest of forever.
	 */
	while (1) {
		int maybe_WNOHANG;

		maybe_WNOHANG = check_delayed_sigs();

		/* (Re)run the respawn/askfirst stuff */
		run_actions(RESPAWN | ASKFIRST);
		maybe_WNOHANG |= check_delayed_sigs();

		/* Don't consume all CPU time - sleep a bit */
		sleep(1);
		maybe_WNOHANG |= check_delayed_sigs();

		/* Wait for any child process(es) to exit.
		 *
		 * If check_delayed_sigs above reported that a signal
		 * was caught, wait will be nonblocking. This ensures
		 * that if SIGHUP has reloaded inittab, respawn and askfirst
		 * actions will not be delayed until next child death.
		 */
		if (maybe_WNOHANG)
			maybe_WNOHANG = WNOHANG;
		while (1) {
			pid_t wpid;
			struct init_action *a;

			/* If signals happen _in_ the wait, they interrupt it,
			 * bb_signals_recursive_norestart set them up that way
			 */
			wpid = waitpid(-1, NULL, maybe_WNOHANG);
			if (wpid <= 0)
				break;

			a = mark_terminated(wpid);
			if (a) {
				message(L_LOG, "process '%s' (pid %d) exited. "
						"Scheduling for restart.",
						a->command, wpid);
			}
			/* See if anyone else is waiting to be reaped */
			maybe_WNOHANG = WNOHANG;
		}
	} /* while (1) */
}

//usage:#define linuxrc_trivial_usage NOUSAGE_STR
//usage:#define linuxrc_full_usage ""

//usage:#define init_trivial_usage
//usage:       ""
//usage:#define init_full_usage "\n\n"
//usage:       "Init is the first process started during boot. It never exits."
//usage:	IF_FEATURE_USE_INITTAB(
//usage:   "\n""It (re)spawns children according to /etc/inittab."
//usage:	)
//usage:	IF_NOT_FEATURE_USE_INITTAB(
//usage:   "\n""This version of init doesn't use /etc/inittab,"
//usage:   "\n""has fixed set of processed to run."
//usage:	)
//usage:
//usage:#define init_notes_usage
//usage:	"This version of init is designed to be run only by the kernel.\n"
//usage:	"\n"
//usage:	"BusyBox init doesn't support multiple runlevels. The runlevels field of\n"
//usage:	"the /etc/inittab file is completely ignored by BusyBox init. If you want\n"
//usage:	"runlevels, use sysvinit.\n"
//usage:	"\n"
//usage:	"BusyBox init works just fine without an inittab. If no inittab is found,\n"
//usage:	"it has the following default behavior:\n"
//usage:	"\n"
//usage:	"	::sysinit:/etc/init.d/rcS\n"
//usage:	"	::askfirst:/bin/sh\n"
//usage:	"	::ctrlaltdel:/sbin/reboot\n"
//usage:	"	::shutdown:/sbin/swapoff -a\n"
//usage:	"	::shutdown:/bin/umount -a -r\n"
//usage:	"	::restart:/sbin/init\n"
//usage:	"	tty2::askfirst:/bin/sh\n"
//usage:	"	tty3::askfirst:/bin/sh\n"
//usage:	"	tty4::askfirst:/bin/sh\n"
//usage:	"\n"
//usage:	"If you choose to use an /etc/inittab file, the inittab entry format is as follows:\n"
//usage:	"\n"
//usage:	"	<id>:<runlevels>:<action>:<process>\n"
//usage:	"\n"
//usage:	"	<id>:\n"
//usage:	"\n"
//usage:	"		WARNING: This field has a non-traditional meaning for BusyBox init!\n"
//usage:	"		The id field is used by BusyBox init to specify the controlling tty for\n"
//usage:	"		the specified process to run on. The contents of this field are\n"
//usage:	"		appended to \"/dev/\" and used as-is. There is no need for this field to\n"
//usage:	"		be unique, although if it isn't you may have strange results. If this\n"
//usage:	"		field is left blank, then the init's stdin/out will be used.\n"
//usage:	"\n"
//usage:	"	<runlevels>:\n"
//usage:	"\n"
//usage:	"		The runlevels field is completely ignored.\n"
//usage:	"\n"
//usage:	"	<action>:\n"
//usage:	"\n"
//usage:	"		Valid actions include: sysinit, respawn, askfirst, wait,\n"
//usage:	"		once, restart, ctrlaltdel, and shutdown.\n"
//usage:	"\n"
//usage:	"		The available actions can be classified into two groups: actions\n"
//usage:	"		that are run only once, and actions that are re-run when the specified\n"
//usage:	"		process exits.\n"
//usage:	"\n"
//usage:	"		Run only-once actions:\n"
//usage:	"\n"
//usage:	"			'sysinit' is the first item run on boot. init waits until all\n"
//usage:	"			sysinit actions are completed before continuing. Following the\n"
//usage:	"			completion of all sysinit actions, all 'wait' actions are run.\n"
//usage:	"			'wait' actions, like 'sysinit' actions, cause init to wait until\n"
//usage:	"			the specified task completes. 'once' actions are asynchronous,\n"
//usage:	"			therefore, init does not wait for them to complete. 'restart' is\n"
//usage:	"			the action taken to restart the init process. By default this should\n"
//usage:	"			simply run /sbin/init, but can be a script which runs pivot_root or it\n"
//usage:	"			can do all sorts of other interesting things. The 'ctrlaltdel' init\n"
//usage:	"			actions are run when the system detects that someone on the system\n"
//usage:	"			console has pressed the CTRL-ALT-DEL key combination. Typically one\n"
//usage:	"			wants to run 'reboot' at this point to cause the system to reboot.\n"
//usage:	"			Finally the 'shutdown' action specifies the actions to taken when\n"
//usage:	"			init is told to reboot. Unmounting filesystems and disabling swap\n"
//usage:	"			is a very good here.\n"
//usage:	"\n"
//usage:	"		Run repeatedly actions:\n"
//usage:	"\n"
//usage:	"			'respawn' actions are run after the 'once' actions. When a process\n"
//usage:	"			started with a 'respawn' action exits, init automatically restarts\n"
//usage:	"			it. Unlike sysvinit, BusyBox init does not stop processes from\n"
//usage:	"			respawning out of control. The 'askfirst' actions acts just like\n"
//usage:	"			respawn, except that before running the specified process it\n"
//usage:	"			displays the line \"Please press Enter to activate this console.\"\n"
//usage:	"			and then waits for the user to press enter before starting the\n"
//usage:	"			specified process.\n"
//usage:	"\n"
//usage:	"		Unrecognized actions (like initdefault) will cause init to emit an\n"
//usage:	"		error message, and then go along with its business. All actions are\n"
//usage:	"		run in the order they appear in /etc/inittab.\n"
//usage:	"\n"
//usage:	"	<process>:\n"
//usage:	"\n"
//usage:	"		Specifies the process to be executed and its command line.\n"
//usage:	"\n"
//usage:	"Example /etc/inittab file:\n"
//usage:	"\n"
//usage:	"	# This is run first except when booting in single-user mode\n"
//usage:	"	#\n"
//usage:	"	::sysinit:/etc/init.d/rcS\n"
//usage:	"	\n"
//usage:	"	# /bin/sh invocations on selected ttys\n"
//usage:	"	#\n"
//usage:	"	# Start an \"askfirst\" shell on the console (whatever that may be)\n"
//usage:	"	::askfirst:-/bin/sh\n"
//usage:	"	# Start an \"askfirst\" shell on /dev/tty2-4\n"
//usage:	"	tty2::askfirst:-/bin/sh\n"
//usage:	"	tty3::askfirst:-/bin/sh\n"
//usage:	"	tty4::askfirst:-/bin/sh\n"
//usage:	"	\n"
//usage:	"	# /sbin/getty invocations for selected ttys\n"
//usage:	"	#\n"
//usage:	"	tty4::respawn:/sbin/getty 38400 tty4\n"
//usage:	"	tty5::respawn:/sbin/getty 38400 tty5\n"
//usage:	"	\n"
//usage:	"	\n"
//usage:	"	# Example of how to put a getty on a serial line (for a terminal)\n"
//usage:	"	#\n"
//usage:	"	#::respawn:/sbin/getty -L ttyS0 9600 vt100\n"
//usage:	"	#::respawn:/sbin/getty -L ttyS1 9600 vt100\n"
//usage:	"	#\n"
//usage:	"	# Example how to put a getty on a modem line\n"
//usage:	"	#::respawn:/sbin/getty 57600 ttyS2\n"
//usage:	"	\n"
//usage:	"	# Stuff to do when restarting the init process\n"
//usage:	"	::restart:/sbin/init\n"
//usage:	"	\n"
//usage:	"	# Stuff to do before rebooting\n"
//usage:	"	::ctrlaltdel:/sbin/reboot\n"
//usage:	"	::shutdown:/bin/umount -a -r\n"
//usage:	"	::shutdown:/sbin/swapoff -a\n"