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
|
.\" Copyright (c) 1980, 1983, 1986, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" @(#)intro.2 8.5 (Berkeley) 2/27/95
.\" $FreeBSD$
.\"
.Dd September 8, 2016
.Dt INTRO 2
.Os
.Sh NAME
.Nm intro
.Nd introduction to system calls and error numbers
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In errno.h
.Sh DESCRIPTION
This section provides an overview of the system calls,
their error returns, and other common definitions and concepts.
.\".Pp
.\".Sy System call restart
.\".Pp
.\"(more later...)
.Sh RETURN VALUES
Nearly all of the system calls provide an error number referenced via
the external identifier errno.
This identifier is defined in
.In sys/errno.h
as
.Pp
.Dl extern int * __error();
.Dl #define errno (* __error())
.Pp
The
.Va __error()
function returns a pointer to a field in the thread specific structure for
threads other than the initial thread.
For the initial thread and
non-threaded processes,
.Va __error()
returns a pointer to a global
.Va errno
variable that is compatible with the previous definition.
.Pp
When a system call detects an error,
it returns an integer value
indicating failure (usually -1)
and sets the variable
.Va errno
accordingly.
(This allows interpretation of the failure on receiving
a -1 and to take action accordingly.)
Successful calls never set
.Va errno ;
once set, it remains until another error occurs.
It should only be examined after an error.
Note that a number of system calls overload the meanings of these
error numbers, and that the meanings must be interpreted according
to the type and circumstances of the call.
.Pp
The following is a complete list of the errors and their
names as given in
.In sys/errno.h .
.Bl -hang -width Ds
.It Er 0 Em "Undefined error: 0" .
Not used.
.It Er 1 EPERM Em "Operation not permitted" .
An attempt was made to perform an operation limited to processes
with appropriate privileges or to the owner of a file or other
resources.
.It Er 2 ENOENT Em "No such file or directory" .
A component of a specified pathname did not exist, or the
pathname was an empty string.
.It Er 3 ESRCH Em "No such process" .
No process could be found corresponding to that specified by the given
process ID.
.It Er 4 EINTR Em "Interrupted system call" .
An asynchronous signal (such as
.Dv SIGINT
or
.Dv SIGQUIT )
was caught by the process during the execution of an interruptible
function.
If the signal handler performs a normal return, the
interrupted system call will seem to have returned the error condition.
.It Er 5 EIO Em "Input/output error" .
Some physical input or output error occurred.
This error will not be reported until a subsequent operation on the same file
descriptor and may be lost (over written) by any subsequent errors.
.It Er 6 ENXIO Em "Device not configured" .
Input or output on a special file referred to a device that did not
exist, or
made a request beyond the limits of the device.
This error may also occur when, for example,
a tape drive is not online or no disk pack is
loaded on a drive.
.It Er 7 E2BIG Em "Argument list too long" .
The number of bytes used for the argument and environment
list of the new process exceeded the current limit
.Dv ( NCARGS
in
.In sys/param.h ) .
.It Er 8 ENOEXEC Em "Exec format error" .
A request was made to execute a file
that, although it has the appropriate permissions,
was not in the format required for an
executable file.
.It Er 9 EBADF Em "Bad file descriptor" .
A file descriptor argument was out of range, referred to no open file,
or a read (write) request was made to a file that was only open for
writing (reading).
.Pp
.It Er 10 ECHILD Em "\&No child processes" .
A
.Xr wait 2
or
.Xr waitpid 2
function was executed by a process that had no existing or unwaited-for
child processes.
.It Er 11 EDEADLK Em "Resource deadlock avoided" .
An attempt was made to lock a system resource that
would have resulted in a deadlock situation.
.It Er 12 ENOMEM Em "Cannot allocate memory" .
The new process image required more memory than was allowed by the hardware
or by system-imposed memory management constraints.
A lack of swap space is normally temporary; however,
a lack of core is not.
Soft limits may be increased to their corresponding hard limits.
.It Er 13 EACCES Em "Permission denied" .
An attempt was made to access a file in a way forbidden
by its file access permissions.
.It Er 14 EFAULT Em "Bad address" .
The system detected an invalid address in attempting to
use an argument of a call.
.It Er 15 ENOTBLK Em "Block device required" .
A block device operation was attempted on a non-block device or file.
.It Er 16 EBUSY Em "Device busy" .
An attempt to use a system resource which was in use at the time
in a manner which would have conflicted with the request.
.It Er 17 EEXIST Em "File exists" .
An existing file was mentioned in an inappropriate context,
for instance, as the new link name in a
.Xr link 2
system call.
.It Er 18 EXDEV Em "Cross-device link" .
A hard link to a file on another file system
was attempted.
.It Er 19 ENODEV Em "Operation not supported by device" .
An attempt was made to apply an inappropriate
function to a device,
for example,
trying to read a write-only device such as a printer.
.It Er 20 ENOTDIR Em "Not a directory" .
A component of the specified pathname existed, but it was
not a directory, when a directory was expected.
.It Er 21 EISDIR Em "Is a directory" .
An attempt was made to open a directory with write mode specified.
.It Er 22 EINVAL Em "Invalid argument" .
Some invalid argument was supplied.
(For example,
specifying an undefined signal to a
.Xr signal 3
function
or a
.Xr kill 2
system call).
.It Er 23 ENFILE Em "Too many open files in system" .
Maximum number of open files allowable on the system
has been reached and requests for an open cannot be satisfied
until at least one has been closed.
.It Er 24 EMFILE Em "Too many open files" .
Maximum number of file descriptors allowable in the process
has been reached and requests for an open cannot be satisfied
until at least one has been closed.
The
.Xr getdtablesize 2
system call will obtain the current limit.
.It Er 25 ENOTTY Em "Inappropriate ioctl for device" .
A control function (see
.Xr ioctl 2 )
was attempted for a file or
special device for which the operation was inappropriate.
.It Er 26 ETXTBSY Em "Text file busy" .
The new process was a pure procedure (shared text) file
which was open for writing by another process, or
while the pure procedure file was being executed an
.Xr open 2
call requested write access.
.It Er 27 EFBIG Em "File too large" .
The size of a file exceeded the maximum.
.It Er 28 ENOSPC Em "No space left on device" .
A
.Xr write 2
to an ordinary file, the creation of a
directory or symbolic link, or the creation of a directory
entry failed because no more disk blocks were available
on the file system, or the allocation of an inode for a newly
created file failed because no more inodes were available
on the file system.
.It Er 29 ESPIPE Em "Illegal seek" .
An
.Xr lseek 2
system call was issued on a socket, pipe or
.Tn FIFO .
.It Er 30 EROFS Em "Read-only file system" .
An attempt was made to modify a file or directory
on a file system that was read-only at the time.
.It Er 31 EMLINK Em "Too many links" .
Maximum allowable hard links to a single file has been exceeded (limit
of 32767 hard links per file).
.It Er 32 EPIPE Em "Broken pipe" .
A write on a pipe, socket or
.Tn FIFO
for which there is no process
to read the data.
.It Er 33 EDOM Em "Numerical argument out of domain" .
A numerical input argument was outside the defined domain of the mathematical
function.
.It Er 34 ERANGE Em "Result too large" .
A numerical result of the function was too large to fit in the
available space (perhaps exceeded precision).
.It Er 35 EAGAIN Em "Resource temporarily unavailable" .
This is a temporary condition and later calls to the
same routine may complete normally.
.It Er 36 EINPROGRESS Em "Operation now in progress" .
An operation that takes a long time to complete (such as
a
.Xr connect 2 )
was attempted on a non-blocking object (see
.Xr fcntl 2 ) .
.It Er 37 EALREADY Em "Operation already in progress" .
An operation was attempted on a non-blocking object that already
had an operation in progress.
.It Er 38 ENOTSOCK Em "Socket operation on non-socket" .
Self-explanatory.
.It Er 39 EDESTADDRREQ Em "Destination address required" .
A required address was omitted from an operation on a socket.
.It Er 40 EMSGSIZE Em "Message too long" .
A message sent on a socket was larger than the internal message buffer
or some other network limit.
.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" .
A protocol was specified that does not support the semantics of the
socket type requested.
For example, you cannot use the
.Tn ARPA
Internet
.Tn UDP
protocol with type
.Dv SOCK_STREAM .
.It Er 42 ENOPROTOOPT Em "Protocol not available" .
A bad option or level was specified in a
.Xr getsockopt 2
or
.Xr setsockopt 2
call.
.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" .
The protocol has not been configured into the
system or no implementation for it exists.
.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" .
The support for the socket type has not been configured into the
system or no implementation for it exists.
.It Er 45 EOPNOTSUPP Em "Operation not supported" .
The attempted operation is not supported for the type of object referenced.
Usually this occurs when a file descriptor refers to a file or socket
that cannot support this operation,
for example, trying to
.Em accept
a connection on a datagram socket.
.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" .
The protocol family has not been configured into the
system or no implementation for it exists.
.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" .
An address incompatible with the requested protocol was used.
For example, you should not necessarily expect to be able to use
.Tn NS
addresses with
.Tn ARPA
Internet protocols.
.It Er 48 EADDRINUSE Em "Address already in use" .
Only one usage of each address is normally permitted.
.Pp
.It Er 49 EADDRNOTAVAIL Em "Can't assign requested address" .
Normally results from an attempt to create a socket with an
address not on this machine.
.It Er 50 ENETDOWN Em "Network is down" .
A socket operation encountered a dead network.
.It Er 51 ENETUNREACH Em "Network is unreachable" .
A socket operation was attempted to an unreachable network.
.It Er 52 ENETRESET Em "Network dropped connection on reset" .
The host you were connected to crashed and rebooted.
.It Er 53 ECONNABORTED Em "Software caused connection abort" .
A connection abort was caused internal to your host machine.
.It Er 54 ECONNRESET Em "Connection reset by peer" .
A connection was forcibly closed by a peer.
This normally
results from a loss of the connection on the remote socket
due to a timeout or a reboot.
.It Er 55 ENOBUFS Em "\&No buffer space available" .
An operation on a socket or pipe was not performed because
the system lacked sufficient buffer space or because a queue was full.
.It Er 56 EISCONN Em "Socket is already connected" .
A
.Xr connect 2
request was made on an already connected socket; or,
a
.Xr sendto 2
or
.Xr sendmsg 2
request on a connected socket specified a destination
when already connected.
.It Er 57 ENOTCONN Em "Socket is not connected" .
An request to send or receive data was disallowed because
the socket was not connected and (when sending on a datagram socket)
no address was supplied.
.It Er 58 ESHUTDOWN Em "Can't send after socket shutdown" .
A request to send data was disallowed because the socket
had already been shut down with a previous
.Xr shutdown 2
call.
.It Er 60 ETIMEDOUT Em "Operation timed out" .
A
.Xr connect 2
or
.Xr send 2
request failed because the connected party did not
properly respond after a period of time.
(The timeout
period is dependent on the communication protocol.)
.It Er 61 ECONNREFUSED Em "Connection refused" .
No connection could be made because the target machine actively
refused it.
This usually results from trying to connect
to a service that is inactive on the foreign host.
.It Er 62 ELOOP Em "Too many levels of symbolic links" .
A path name lookup involved more than 32
.Pq Dv MAXSYMLINKS
symbolic links.
.It Er 63 ENAMETOOLONG Em "File name too long" .
A component of a path name exceeded
.Brq Dv NAME_MAX
characters, or an entire
path name exceeded
.Brq Dv PATH_MAX
characters.
(See also the description of
.Dv _PC_NO_TRUNC
in
.Xr pathconf 2 . )
.It Er 64 EHOSTDOWN Em "Host is down" .
A socket operation failed because the destination host was down.
.It Er 65 EHOSTUNREACH Em "No route to host" .
A socket operation was attempted to an unreachable host.
.It Er 66 ENOTEMPTY Em "Directory not empty" .
A directory with entries other than
.Ql .\&
and
.Ql ..\&
was supplied to a remove directory or rename call.
.It Er 67 EPROCLIM Em "Too many processes" .
.It Er 68 EUSERS Em "Too many users" .
The quota system ran out of table entries.
.It Er 69 EDQUOT Em "Disc quota exceeded" .
A
.Xr write 2
to an ordinary file, the creation of a
directory or symbolic link, or the creation of a directory
entry failed because the user's quota of disk blocks was
exhausted, or the allocation of an inode for a newly
created file failed because the user's quota of inodes
was exhausted.
.It Er 70 ESTALE Em "Stale NFS file handle" .
An attempt was made to access an open file (on an
.Tn NFS
file system)
which is now unavailable as referenced by the file descriptor.
This may indicate the file was deleted on the
.Tn NFS
server or some
other catastrophic event occurred.
.It Er 72 EBADRPC Em "RPC struct is bad" .
Exchange of
.Tn RPC
information was unsuccessful.
.It Er 73 ERPCMISMATCH Em "RPC version wrong" .
The version of
.Tn RPC
on the remote peer is not compatible with
the local version.
.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" .
The requested program is not registered on the remote host.
.It Er 75 EPROGMISMATCH Em "Program version wrong" .
The requested version of the program is not available
on the remote host
.Pq Tn RPC .
.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" .
An
.Tn RPC
call was attempted for a procedure which does not exist
in the remote program.
.It Er 77 ENOLCK Em "No locks available" .
A system-imposed limit on the number of simultaneous file
locks was reached.
.It Er 78 ENOSYS Em "Function not implemented" .
Attempted a system call that is not available on this
system.
.It Er 79 EFTYPE Em "Inappropriate file type or format" .
The file was the wrong type for the operation, or a data file had
the wrong format.
.It Er 80 EAUTH Em "Authentication error" .
Attempted to use an invalid authentication ticket to mount a
.Tn NFS
file system.
.It Er 81 ENEEDAUTH Em "Need authenticator" .
An authentication ticket must be obtained before the given
.Tn NFS
file system may be mounted.
.It Er 82 EIDRM Em "Identifier removed" .
An IPC identifier was removed while the current process was waiting on it.
.It Er 83 ENOMSG Em "No message of desired type" .
An IPC message queue does not contain a message of the desired type, or a
message catalog does not contain the requested message.
.It Er 84 EOVERFLOW Em "Value too large to be stored in data type" .
A numerical result of the function was too large to be stored in the caller
provided space.
.It Er 85 ECANCELED Em "Operation canceled" .
The scheduled operation was canceled.
.It Er 86 EILSEQ Em "Illegal byte sequence" .
While decoding a multibyte character the function came along an
invalid or an incomplete sequence of bytes or the given wide
character is invalid.
.It Er 87 ENOATTR Em "Attribute not found" .
The specified extended attribute does not exist.
.It Er 88 EDOOFUS Em "Programming error" .
A function or API is being abused in a way which could only be detected
at run-time.
.It Er 89 EBADMSG Em "Bad message" .
A corrupted message was detected.
.It Er 90 EMULTIHOP Em "Multihop attempted" .
This error code is unused, but present for compatibility with other systems.
.It Er 91 ENOLINK Em "Link has been severed" .
This error code is unused, but present for compatibility with other systems.
.It Er 92 EPROTO Em "Protocol error" .
A device or socket encountered an unrecoverable protocol error.
.It Er 93 ENOTCAPABLE Em "Capabilities insufficient" .
An operation on a capability file descriptor requires greater privilege than
the capability allows.
.It Er 94 ECAPMODE Em "Not permitted in capability mode" .
The system call or operation is not permitted for capability mode processes.
.It Er 95 ENOTRECOVERABLE Em "State not recoverable" .
The state protected by a robust mutex is not recoverable.
.It Er 96 EOWNERDEAD Em "Previous owner died" .
The owner of a robust mutex terminated while holding the mutex lock.
.El
.Sh DEFINITIONS
.Bl -tag -width Ds
.It Process ID .
Each active process in the system is uniquely identified by a non-negative
integer called a process ID.
The range of this ID is from 0 to 99999.
.It Parent process ID
A new process is created by a currently active process (see
.Xr fork 2 ) .
The parent process ID of a process is initially the process ID of its creator.
If the creating process exits,
the parent process ID of each child is set to the ID of the calling process's
reaper (see
.Xr procctl 2 ) ,
normally
.Xr init 8 .
.It Process Group
Each active process is a member of a process group that is identified by
a non-negative integer called the process group ID.
This is the process
ID of the group leader.
This grouping permits the signaling of related
processes (see
.Xr termios 4 )
and the job control mechanisms of
.Xr csh 1 .
.It Session
A session is a set of one or more process groups.
A session is created by a successful call to
.Xr setsid 2 ,
which causes the caller to become the only member of the only process
group in the new session.
.It Session leader
A process that has created a new session by a successful call to
.Xr setsid 2 ,
is known as a session leader.
Only a session leader may acquire a terminal as its controlling terminal (see
.Xr termios 4 ) .
.It Controlling process
A session leader with a controlling terminal is a controlling process.
.It Controlling terminal
A terminal that is associated with a session is known as the controlling
terminal for that session and its members.
.It "Terminal Process Group ID"
A terminal may be acquired by a session leader as its controlling terminal.
Once a terminal is associated with a session, any of the process groups
within the session may be placed into the foreground by setting
the terminal process group ID to the ID of the process group.
This facility is used
to arbitrate between multiple jobs contending for the same terminal;
(see
.Xr csh 1
and
.Xr tty 4 ) .
.It "Orphaned Process Group"
A process group is considered to be
.Em orphaned
if it is not under the control of a job control shell.
More precisely, a process group is orphaned
when none of its members has a parent process that is in the same session
as the group,
but is in a different process group.
Note that when a process exits, the parent process for its children
is normally changed to be
.Xr init 8 ,
which is in a separate session.
Not all members of an orphaned process group are necessarily orphaned
processes (those whose creating process has exited).
The process group of a session leader is orphaned by definition.
.It "Real User ID and Real Group ID"
Each user on the system is identified by a positive integer
termed the real user ID.
.Pp
Each user is also a member of one or more groups.
One of these groups is distinguished from others and
used in implementing accounting facilities.
The positive
integer corresponding to this distinguished group is termed
the real group ID.
.Pp
All processes have a real user ID and real group ID.
These are initialized from the equivalent attributes
of the process that created it.
.It "Effective User Id, Effective Group Id, and Group Access List"
Access to system resources is governed by two values:
the effective user ID, and the group access list.
The first member of the group access list is also known as the
effective group ID.
(In POSIX.1, the group access list is known as the set of supplementary
group IDs, and it is unspecified whether the effective group ID is
a member of the list.)
.Pp
The effective user ID and effective group ID are initially the
process's real user ID and real group ID respectively.
Either
may be modified through execution of a set-user-ID or set-group-ID
file (possibly by one its ancestors) (see
.Xr execve 2 ) .
By convention, the effective group ID (the first member of the group access
list) is duplicated, so that the execution of a set-group-ID program
does not result in the loss of the original (real) group ID.
.Pp
The group access list is a set of group IDs
used only in determining resource accessibility.
Access checks
are performed as described below in ``File Access Permissions''.
.It "Saved Set User ID and Saved Set Group ID"
When a process executes a new file, the effective user ID is set
to the owner of the file if the file is set-user-ID, and the effective
group ID (first element of the group access list) is set to the group
of the file if the file is set-group-ID.
The effective user ID of the process is then recorded as the saved set-user-ID,
and the effective group ID of the process is recorded as the saved set-group-ID.
These values may be used to regain those values as the effective user
or group ID after reverting to the real ID (see
.Xr setuid 2 ) .
(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional,
and are used in setuid and setgid, but this does not work as desired
for the super-user.)
.It Super-user
A process is recognized as a
.Em super-user
process and is granted special privileges if its effective user ID is 0.
.It Descriptor
An integer assigned by the system when a file is referenced
by
.Xr open 2
or
.Xr dup 2 ,
or when a socket is created by
.Xr pipe 2 ,
.Xr socket 2
or
.Xr socketpair 2 ,
which uniquely identifies an access path to that file or socket from
a given process or any of its children.
.It File Name
Names consisting of up to
.Brq Dv NAME_MAX
characters may be used to name
an ordinary file, special file, or directory.
.Pp
These characters may be arbitrary eight-bit values,
excluding
.Dv NUL
.Tn ( ASCII
0) and the
.Ql \&/
character (slash,
.Tn ASCII
47).
.Pp
Note that it is generally unwise to use
.Ql \&* ,
.Ql \&? ,
.Ql \&[
or
.Ql \&]
as part of
file names because of the special meaning attached to these characters
by the shell.
.It Path Name
A path name is a
.Dv NUL Ns -terminated
character string starting with an
optional slash
.Ql \&/ ,
followed by zero or more directory names separated
by slashes, optionally followed by a file name.
The total length of a path name must be less than
.Brq Dv PATH_MAX
characters.
(On some systems, this limit may be infinite.)
.Pp
If a path name begins with a slash, the path search begins at the
.Em root
directory.
Otherwise, the search begins from the current working directory.
A slash by itself names the root directory.
An empty
pathname refers to the current directory.
.It Directory
A directory is a special type of file that contains entries
that are references to other files.
Directory entries are called links.
By convention, a directory
contains at least two links,
.Ql .\&
and
.Ql \&.. ,
referred to as
.Em dot
and
.Em dot-dot
respectively.
Dot refers to the directory itself and
dot-dot refers to its parent directory.
.It "Root Directory and Current Working Directory"
Each process has associated with it a concept of a root directory
and a current working directory for the purpose of resolving path
name searches.
A process's root directory need not be the root
directory of the root file system.
.It File Access Permissions
Every file in the file system has a set of access permissions.
These permissions are used in determining whether a process
may perform a requested operation on the file (such as opening
a file for writing).
Access permissions are established at the
time a file is created.
They may be changed at some later time
through the
.Xr chmod 2
call.
.Pp
File access is broken down according to whether a file may be: read,
written, or executed.
Directory files use the execute
permission to control if the directory may be searched.
.Pp
File access permissions are interpreted by the system as
they apply to three different classes of users: the owner
of the file, those users in the file's group, anyone else.
Every file has an independent set of access permissions for
each of these classes.
When an access check is made, the system
decides if permission should be granted by checking the access
information applicable to the caller.
.Pp
Read, write, and execute/search permissions on
a file are granted to a process if:
.Pp
The process's effective user ID is that of the super-user.
(Note:
even the super-user cannot execute a non-executable file.)
.Pp
The process's effective user ID matches the user ID of the owner
of the file and the owner permissions allow the access.
.Pp
The process's effective user ID does not match the user ID of the
owner of the file, and either the process's effective
group ID matches the group ID
of the file, or the group ID of the file is in
the process's group access list,
and the group permissions allow the access.
.Pp
Neither the effective user ID nor effective group ID
and group access list of the process
match the corresponding user ID and group ID of the file,
but the permissions for ``other users'' allow access.
.Pp
Otherwise, permission is denied.
.It Sockets and Address Families
A socket is an endpoint for communication between processes.
Each socket has queues for sending and receiving data.
.Pp
Sockets are typed according to their communications properties.
These properties include whether messages sent and received
at a socket require the name of the partner, whether communication
is reliable, the format used in naming message recipients, etc.
.Pp
Each instance of the system supports some
collection of socket types; consult
.Xr socket 2
for more information about the types available and
their properties.
.Pp
Each instance of the system supports some number of sets of
communications protocols.
Each protocol set supports addresses
of a certain format.
An Address Family is the set of addresses
for a specific group of protocols.
Each socket has an address
chosen from the address family in which the socket was created.
.El
.Sh SEE ALSO
.Xr intro 3 ,
.Xr perror 3
|