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28 .\" @(#)intro.2 8.5 (Berkeley) 2/27/95
35 .Nd introduction to system calls and error numbers
41 This section provides an overview of the system calls,
42 their error returns, and other common definitions and concepts.
44 .\".Sy System call restart
48 Nearly all of the system calls provide an error number referenced via
49 the external identifier errno.
50 This identifier is defined in
54 .Dl extern int * __error();
55 .Dl #define errno (* __error())
59 function returns a pointer to a field in the thread specific structure for
60 threads other than the initial thread.
61 For the initial thread and
62 non-threaded processes,
64 returns a pointer to a global
66 variable that is compatible with the previous definition.
68 When a system call detects an error,
69 it returns an integer value
70 indicating failure (usually -1)
74 (This allows interpretation of the failure on receiving
75 a -1 and to take action accordingly.)
76 Successful calls never set
78 once set, it remains until another error occurs.
79 It should only be examined after an error.
80 Note that a number of system calls overload the meanings of these
81 error numbers, and that the meanings must be interpreted according
82 to the type and circumstances of the call.
84 The following is a complete list of the errors and their
88 .It Er 0 Em "Undefined error: 0" .
90 .It Er 1 EPERM Em "Operation not permitted" .
91 An attempt was made to perform an operation limited to processes
92 with appropriate privileges or to the owner of a file or other
94 .It Er 2 ENOENT Em "No such file or directory" .
95 A component of a specified pathname did not exist, or the
96 pathname was an empty string.
97 .It Er 3 ESRCH Em "No such process" .
98 No process could be found corresponding to that specified by the given
100 .It Er 4 EINTR Em "Interrupted system call" .
101 An asynchronous signal (such as
105 was caught by the process during the execution of an interruptible
107 If the signal handler performs a normal return, the
108 interrupted system call will seem to have returned the error condition.
109 .It Er 5 EIO Em "Input/output error" .
110 Some physical input or output error occurred.
111 This error will not be reported until a subsequent operation on the same file
112 descriptor and may be lost (over written) by any subsequent errors.
113 .It Er 6 ENXIO Em "Device not configured" .
114 Input or output on a special file referred to a device that did not
116 made a request beyond the limits of the device.
117 This error may also occur when, for example,
118 a tape drive is not online or no disk pack is
120 .It Er 7 E2BIG Em "Argument list too long" .
121 The number of bytes used for the argument and environment
122 list of the new process exceeded the current limit
126 .It Er 8 ENOEXEC Em "Exec format error" .
127 A request was made to execute a file
128 that, although it has the appropriate permissions,
129 was not in the format required for an
131 .It Er 9 EBADF Em "Bad file descriptor" .
132 A file descriptor argument was out of range, referred to no open file,
133 or a read (write) request was made to a file that was only open for
135 .It Er 10 ECHILD Em "\&No child processes" .
140 function was executed by a process that had no existing or unwaited-for
142 .It Er 11 EDEADLK Em "Resource deadlock avoided" .
143 An attempt was made to lock a system resource that
144 would have resulted in a deadlock situation.
145 .It Er 12 ENOMEM Em "Cannot allocate memory" .
146 The new process image required more memory than was allowed by the hardware
147 or by system-imposed memory management constraints.
148 A lack of swap space is normally temporary; however,
149 a lack of core is not.
150 Soft limits may be increased to their corresponding hard limits.
151 .It Er 13 EACCES Em "Permission denied" .
152 An attempt was made to access a file in a way forbidden
153 by its file access permissions.
154 .It Er 14 EFAULT Em "Bad address" .
155 The system detected an invalid address in attempting to
156 use an argument of a call.
157 .It Er 15 ENOTBLK Em "Block device required" .
158 A block device operation was attempted on a non-block device or file.
159 .It Er 16 EBUSY Em "Device busy" .
160 An attempt to use a system resource which was in use at the time
161 in a manner which would have conflicted with the request.
162 .It Er 17 EEXIST Em "File exists" .
163 An existing file was mentioned in an inappropriate context,
164 for instance, as the new link name in a
167 .It Er 18 EXDEV Em "Cross-device link" .
168 A hard link to a file on another file system
170 .It Er 19 ENODEV Em "Operation not supported by device" .
171 An attempt was made to apply an inappropriate
172 function to a device,
174 trying to read a write-only device such as a printer.
175 .It Er 20 ENOTDIR Em "Not a directory" .
176 A component of the specified pathname existed, but it was
177 not a directory, when a directory was expected.
178 .It Er 21 EISDIR Em "Is a directory" .
179 An attempt was made to open a directory with write mode specified.
180 .It Er 22 EINVAL Em "Invalid argument" .
181 Some invalid argument was supplied.
183 specifying an undefined signal to a
189 .It Er 23 ENFILE Em "Too many open files in system" .
190 Maximum number of open files allowable on the system
191 has been reached and requests for an open cannot be satisfied
192 until at least one has been closed.
193 .It Er 24 EMFILE Em "Too many open files" .
194 Maximum number of file descriptors allowable in the process
195 has been reached and requests for an open cannot be satisfied
196 until at least one has been closed.
199 system call will obtain the current limit.
200 .It Er 25 ENOTTY Em "Inappropriate ioctl for device" .
201 A control function (see
203 was attempted for a file or
204 special device for which the operation was inappropriate.
205 .It Er 26 ETXTBSY Em "Text file busy" .
206 The new process was a pure procedure (shared text) file
207 which was open for writing by another process, or
208 while the pure procedure file was being executed an
210 call requested write access.
211 .It Er 27 EFBIG Em "File too large" .
212 The size of a file exceeded the maximum.
213 .It Er 28 ENOSPC Em "No space left on device" .
216 to an ordinary file, the creation of a
217 directory or symbolic link, or the creation of a directory
218 entry failed because no more disk blocks were available
219 on the file system, or the allocation of an inode for a newly
220 created file failed because no more inodes were available
222 .It Er 29 ESPIPE Em "Illegal seek" .
225 system call was issued on a socket, pipe or
227 .It Er 30 EROFS Em "Read-only file system" .
228 An attempt was made to modify a file or directory
229 on a file system that was read-only at the time.
230 .It Er 31 EMLINK Em "Too many links" .
231 Maximum allowable hard links to a single file has been exceeded (limit
232 of 32767 hard links per file).
233 .It Er 32 EPIPE Em "Broken pipe" .
234 A write on a pipe, socket or
236 for which there is no process
238 .It Er 33 EDOM Em "Numerical argument out of domain" .
239 A numerical input argument was outside the defined domain of the mathematical
241 .It Er 34 ERANGE Em "Result too large" .
242 A numerical result of the function was too large to fit in the
243 available space (perhaps exceeded precision).
244 .It Er 35 EAGAIN Em "Resource temporarily unavailable" .
245 This is a temporary condition and later calls to the
246 same routine may complete normally.
247 .It Er 36 EINPROGRESS Em "Operation now in progress" .
248 An operation that takes a long time to complete (such as
251 was attempted on a non-blocking object (see
253 .It Er 37 EALREADY Em "Operation already in progress" .
254 An operation was attempted on a non-blocking object that already
255 had an operation in progress.
256 .It Er 38 ENOTSOCK Em "Socket operation on non-socket" .
258 .It Er 39 EDESTADDRREQ Em "Destination address required" .
259 A required address was omitted from an operation on a socket.
260 .It Er 40 EMSGSIZE Em "Message too long" .
261 A message sent on a socket was larger than the internal message buffer
262 or some other network limit.
263 .It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" .
264 A protocol was specified that does not support the semantics of the
265 socket type requested.
266 For example, you cannot use the
272 .It Er 42 ENOPROTOOPT Em "Protocol not available" .
273 A bad option or level was specified in a
278 .It Er 43 EPROTONOSUPPORT Em "Protocol not supported" .
279 The protocol has not been configured into the
280 system or no implementation for it exists.
281 .It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" .
282 The support for the socket type has not been configured into the
283 system or no implementation for it exists.
284 .It Er 45 EOPNOTSUPP Em "Operation not supported" .
285 The attempted operation is not supported for the type of object referenced.
286 Usually this occurs when a file descriptor refers to a file or socket
287 that cannot support this operation,
288 for example, trying to
290 a connection on a datagram socket.
291 .It Er 46 EPFNOSUPPORT Em "Protocol family not supported" .
292 The protocol family has not been configured into the
293 system or no implementation for it exists.
294 .It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" .
295 An address incompatible with the requested protocol was used.
296 For example, you should not necessarily expect to be able to use
301 .It Er 48 EADDRINUSE Em "Address already in use" .
302 Only one usage of each address is normally permitted.
303 .It Er 49 EADDRNOTAVAIL Em "Can't assign requested address" .
304 Normally results from an attempt to create a socket with an
305 address not on this machine.
306 .It Er 50 ENETDOWN Em "Network is down" .
307 A socket operation encountered a dead network.
308 .It Er 51 ENETUNREACH Em "Network is unreachable" .
309 A socket operation was attempted to an unreachable network.
310 .It Er 52 ENETRESET Em "Network dropped connection on reset" .
311 The host you were connected to crashed and rebooted.
312 .It Er 53 ECONNABORTED Em "Software caused connection abort" .
313 A connection abort was caused internal to your host machine.
314 .It Er 54 ECONNRESET Em "Connection reset by peer" .
315 A connection was forcibly closed by a peer.
317 results from a loss of the connection on the remote socket
318 due to a timeout or a reboot.
319 .It Er 55 ENOBUFS Em "\&No buffer space available" .
320 An operation on a socket or pipe was not performed because
321 the system lacked sufficient buffer space or because a queue was full.
322 .It Er 56 EISCONN Em "Socket is already connected" .
325 request was made on an already connected socket; or,
330 request on a connected socket specified a destination
331 when already connected.
332 .It Er 57 ENOTCONN Em "Socket is not connected" .
333 An request to send or receive data was disallowed because
334 the socket was not connected and (when sending on a datagram socket)
335 no address was supplied.
336 .It Er 58 ESHUTDOWN Em "Can't send after socket shutdown" .
337 A request to send data was disallowed because the socket
338 had already been shut down with a previous
341 .It Er 60 ETIMEDOUT Em "Operation timed out" .
346 request failed because the connected party did not
347 properly respond after a period of time.
349 period is dependent on the communication protocol.)
350 .It Er 61 ECONNREFUSED Em "Connection refused" .
351 No connection could be made because the target machine actively
353 This usually results from trying to connect
354 to a service that is inactive on the foreign host.
355 .It Er 62 ELOOP Em "Too many levels of symbolic links" .
356 A path name lookup involved more than 32
359 .It Er 63 ENAMETOOLONG Em "File name too long" .
360 A component of a path name exceeded
362 characters, or an entire
366 (See also the description of
370 .It Er 64 EHOSTDOWN Em "Host is down" .
371 A socket operation failed because the destination host was down.
372 .It Er 65 EHOSTUNREACH Em "No route to host" .
373 A socket operation was attempted to an unreachable host.
374 .It Er 66 ENOTEMPTY Em "Directory not empty" .
375 A directory with entries other than
379 was supplied to a remove directory or rename call.
380 .It Er 67 EPROCLIM Em "Too many processes" .
381 .It Er 68 EUSERS Em "Too many users" .
382 The quota system ran out of table entries.
383 .It Er 69 EDQUOT Em "Disc quota exceeded" .
386 to an ordinary file, the creation of a
387 directory or symbolic link, or the creation of a directory
388 entry failed because the user's quota of disk blocks was
389 exhausted, or the allocation of an inode for a newly
390 created file failed because the user's quota of inodes
392 .It Er 70 ESTALE Em "Stale NFS file handle" .
393 An attempt was made to access an open file (on an
396 which is now unavailable as referenced by the file descriptor.
397 This may indicate the file was deleted on the
400 other catastrophic event occurred.
401 .It Er 72 EBADRPC Em "RPC struct is bad" .
404 information was unsuccessful.
405 .It Er 73 ERPCMISMATCH Em "RPC version wrong" .
408 on the remote peer is not compatible with
410 .It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" .
411 The requested program is not registered on the remote host.
412 .It Er 75 EPROGMISMATCH Em "Program version wrong" .
413 The requested version of the program is not available
416 .It Er 76 EPROCUNAVAIL Em "Bad procedure for program" .
419 call was attempted for a procedure which does not exist
420 in the remote program.
421 .It Er 77 ENOLCK Em "No locks available" .
422 A system-imposed limit on the number of simultaneous file
424 .It Er 78 ENOSYS Em "Function not implemented" .
425 Attempted a system call that is not available on this
427 .It Er 79 EFTYPE Em "Inappropriate file type or format" .
428 The file was the wrong type for the operation, or a data file had
430 .It Er 80 EAUTH Em "Authentication error" .
431 Attempted to use an invalid authentication ticket to mount a
434 .It Er 81 ENEEDAUTH Em "Need authenticator" .
435 An authentication ticket must be obtained before the given
437 file system may be mounted.
438 .It Er 82 EIDRM Em "Identifier removed" .
439 An IPC identifier was removed while the current process was waiting on it.
440 .It Er 83 ENOMSG Em "No message of desired type" .
441 An IPC message queue does not contain a message of the desired type, or a
442 message catalog does not contain the requested message.
443 .It Er 84 EOVERFLOW Em "Value too large to be stored in data type" .
444 A numerical result of the function was too large to be stored in the caller
446 .It Er 85 ECANCELED Em "Operation canceled" .
447 The scheduled operation was canceled.
448 .It Er 86 EILSEQ Em "Illegal byte sequence" .
449 While decoding a multibyte character the function came along an
450 invalid or an incomplete sequence of bytes or the given wide
451 character is invalid.
452 .It Er 87 ENOATTR Em "Attribute not found" .
453 The specified extended attribute does not exist.
454 .It Er 88 EDOOFUS Em "Programming error" .
455 A function or API is being abused in a way which could only be detected
457 .It Er 89 EBADMSG Em "Bad message" .
458 A corrupted message was detected.
459 .It Er 90 EMULTIHOP Em "Multihop attempted" .
460 This error code is unused, but present for compatibility with other systems.
461 .It Er 91 ENOLINK Em "Link has been severed" .
462 This error code is unused, but present for compatibility with other systems.
463 .It Er 92 EPROTO Em "Protocol error" .
464 A device or socket encountered an unrecoverable protocol error.
465 .It Er 93 ENOTCAPABLE Em "Capabilities insufficient" .
466 An operation on a capability file descriptor requires greater privilege than
467 the capability allows.
468 .It Er 94 ECAPMODE Em "Not permitted in capability mode" .
469 The system call or operation is not permitted for capability mode processes.
470 .It Er 95 ENOTRECOVERABLE Em "State not recoverable" .
471 The state protected by a robust mutex is not recoverable.
472 .It Er 96 EOWNERDEAD Em "Previous owner died" .
473 The owner of a robust mutex terminated while holding the mutex lock.
474 .It Er 97 EINTEGRITY Em "Integrity check failed" .
475 An integrity check such as a check-hash or a cross-correlation failed.
476 The integrity error falls in the kernel I/O stack between
478 that identifies errors in parameters to a system call and
480 that identifies errors with the underlying storage media.
481 It is typically raised by intermediate kernel layers such as a
482 filesystem or an in-kernel GEOM subsystem when they detect inconsistencies.
483 Uses include allowing the
485 command to return a different exit value to automate the running of
487 during a system boot.
492 Each active process in the system is uniquely identified by a non-negative
493 integer called a process ID.
494 The range of this ID is from 0 to 99999.
495 .It Parent process ID
496 A new process is created by a currently active process (see
498 The parent process ID of a process is initially the process ID of its creator.
499 If the creating process exits,
500 the parent process ID of each child is set to the ID of the calling process's
506 Each active process is a member of a process group that is identified by
507 a non-negative integer called the process group ID.
509 ID of the group leader.
510 This grouping permits the signaling of related
513 and the job control mechanisms of
516 A session is a set of one or more process groups.
517 A session is created by a successful call to
519 which causes the caller to become the only member of the only process
520 group in the new session.
522 A process that has created a new session by a successful call to
524 is known as a session leader.
525 Only a session leader may acquire a terminal as its controlling terminal (see
527 .It Controlling process
528 A session leader with a controlling terminal is a controlling process.
529 .It Controlling terminal
530 A terminal that is associated with a session is known as the controlling
531 terminal for that session and its members.
532 .It "Terminal Process Group ID"
533 A terminal may be acquired by a session leader as its controlling terminal.
534 Once a terminal is associated with a session, any of the process groups
535 within the session may be placed into the foreground by setting
536 the terminal process group ID to the ID of the process group.
537 This facility is used
538 to arbitrate between multiple jobs contending for the same terminal;
543 .It "Orphaned Process Group"
544 A process group is considered to be
546 if it is not under the control of a job control shell.
547 More precisely, a process group is orphaned
548 when none of its members has a parent process that is in the same session
550 but is in a different process group.
551 Note that when a process exits, the parent process for its children
552 is normally changed to be
554 which is in a separate session.
555 Not all members of an orphaned process group are necessarily orphaned
556 processes (those whose creating process has exited).
557 The process group of a session leader is orphaned by definition.
558 .It "Real User ID and Real Group ID"
559 Each user on the system is identified by a positive integer
560 termed the real user ID.
562 Each user is also a member of one or more groups.
563 One of these groups is distinguished from others and
564 used in implementing accounting facilities.
566 integer corresponding to this distinguished group is termed
569 All processes have a real user ID and real group ID.
570 These are initialized from the equivalent attributes
571 of the process that created it.
572 .It "Effective User Id, Effective Group Id, and Group Access List"
573 Access to system resources is governed by two values:
574 the effective user ID, and the group access list.
575 The first member of the group access list is also known as the
577 (In POSIX.1, the group access list is known as the set of supplementary
578 group IDs, and it is unspecified whether the effective group ID is
579 a member of the list.)
581 The effective user ID and effective group ID are initially the
582 process's real user ID and real group ID respectively.
584 may be modified through execution of a set-user-ID or set-group-ID
585 file (possibly by one its ancestors) (see
587 By convention, the effective group ID (the first member of the group access
588 list) is duplicated, so that the execution of a set-group-ID program
589 does not result in the loss of the original (real) group ID.
591 The group access list is a set of group IDs
592 used only in determining resource accessibility.
594 are performed as described below in ``File Access Permissions''.
595 .It "Saved Set User ID and Saved Set Group ID"
596 When a process executes a new file, the effective user ID is set
597 to the owner of the file if the file is set-user-ID, and the effective
598 group ID (first element of the group access list) is set to the group
599 of the file if the file is set-group-ID.
600 The effective user ID of the process is then recorded as the saved set-user-ID,
601 and the effective group ID of the process is recorded as the saved set-group-ID.
602 These values may be used to regain those values as the effective user
603 or group ID after reverting to the real ID (see
605 (In POSIX.1, the saved set-user-ID and saved set-group-ID are optional,
606 and are used in setuid and setgid, but this does not work as desired
609 A process is recognized as a
611 process and is granted special privileges if its effective user ID is 0.
613 An integer assigned by the system when a file is referenced
618 or when a socket is created by
623 which uniquely identifies an access path to that file or socket from
624 a given process or any of its children.
626 Names consisting of up to
628 characters may be used to name
629 an ordinary file, special file, or directory.
631 These characters may be arbitrary eight-bit values,
641 Note that it is generally unwise to use
648 file names because of the special meaning attached to these characters
652 .Dv NUL Ns -terminated
653 character string starting with an
656 followed by zero or more directory names separated
657 by slashes, optionally followed by a file name.
658 The total length of a path name must be less than
661 (On some systems, this limit may be infinite.)
663 If a path name begins with a slash, the path search begins at the
666 Otherwise, the search begins from the current working directory.
667 A slash by itself names the root directory.
669 pathname refers to the current directory.
671 A directory is a special type of file that contains entries
672 that are references to other files.
673 Directory entries are called links.
674 By convention, a directory
675 contains at least two links,
684 Dot refers to the directory itself and
685 dot-dot refers to its parent directory.
686 .It "Root Directory and Current Working Directory"
687 Each process has associated with it a concept of a root directory
688 and a current working directory for the purpose of resolving path
690 A process's root directory need not be the root
691 directory of the root file system.
692 .It File Access Permissions
693 Every file in the file system has a set of access permissions.
694 These permissions are used in determining whether a process
695 may perform a requested operation on the file (such as opening
697 Access permissions are established at the
698 time a file is created.
699 They may be changed at some later time
704 File access is broken down according to whether a file may be: read,
705 written, or executed.
706 Directory files use the execute
707 permission to control if the directory may be searched.
709 File access permissions are interpreted by the system as
710 they apply to three different classes of users: the owner
711 of the file, those users in the file's group, anyone else.
712 Every file has an independent set of access permissions for
713 each of these classes.
714 When an access check is made, the system
715 decides if permission should be granted by checking the access
716 information applicable to the caller.
718 Read, write, and execute/search permissions on
719 a file are granted to a process if:
721 The process's effective user ID is that of the super-user.
723 even the super-user cannot execute a non-executable file.)
725 The process's effective user ID matches the user ID of the owner
726 of the file and the owner permissions allow the access.
728 The process's effective user ID does not match the user ID of the
729 owner of the file, and either the process's effective
730 group ID matches the group ID
731 of the file, or the group ID of the file is in
732 the process's group access list,
733 and the group permissions allow the access.
735 Neither the effective user ID nor effective group ID
736 and group access list of the process
737 match the corresponding user ID and group ID of the file,
738 but the permissions for ``other users'' allow access.
740 Otherwise, permission is denied.
741 .It Sockets and Address Families
742 A socket is an endpoint for communication between processes.
743 Each socket has queues for sending and receiving data.
745 Sockets are typed according to their communications properties.
746 These properties include whether messages sent and received
747 at a socket require the name of the partner, whether communication
748 is reliable, the format used in naming message recipients, etc.
750 Each instance of the system supports some
751 collection of socket types; consult
753 for more information about the types available and
756 Each instance of the system supports some number of sets of
757 communications protocols.
758 Each protocol set supports addresses
760 An Address Family is the set of addresses
761 for a specific group of protocols.
762 Each socket has an address
763 chosen from the address family in which the socket was created.