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