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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 file descriptors allowable on the system
193 has been reached and a 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 (As released, the limit on the number of
197 open files per process is 64.)
200 system call will obtain the current limit.
201 .It Er 25 ENOTTY Em "Inappropriate ioctl for device" .
202 A control function (see
204 was attempted for a file or
205 special device for which the operation was inappropriate.
206 .It Er 26 ETXTBSY Em "Text file busy" .
207 The new process was a pure procedure (shared text) file
208 which was open for writing by another process, or
209 while the pure procedure file was being executed an
211 call requested write access.
212 .It Er 27 EFBIG Em "File too large" .
213 The size of a file exceeded the maximum.
214 .It Er 28 ENOSPC Em "No space left on device" .
217 to an ordinary file, the creation of a
218 directory or symbolic link, or the creation of a directory
219 entry failed because no more disk blocks were available
220 on the file system, or the allocation of an inode for a newly
221 created file failed because no more inodes were available
223 .It Er 29 ESPIPE Em "Illegal seek" .
226 system call was issued on a socket, pipe or
228 .It Er 30 EROFS Em "Read-only file system" .
229 An attempt was made to modify a file or directory
230 on a file system that was read-only at the time.
231 .It Er 31 EMLINK Em "Too many links" .
232 Maximum allowable hard links to a single file has been exceeded (limit
233 of 32767 hard links per file).
234 .It Er 32 EPIPE Em "Broken pipe" .
235 A write on a pipe, socket or
237 for which there is no process
239 .It Er 33 EDOM Em "Numerical argument out of domain" .
240 A numerical input argument was outside the defined domain of the mathematical
242 .It Er 34 ERANGE Em "Result too large" .
243 A numerical result of the function was too large to fit in the
244 available space (perhaps exceeded precision).
245 .It Er 35 EAGAIN Em "Resource temporarily unavailable" .
246 This is a temporary condition and later calls to the
247 same routine may complete normally.
248 .It Er 36 EINPROGRESS Em "Operation now in progress" .
249 An operation that takes a long time to complete (such as
252 was attempted on a non-blocking object (see
254 .It Er 37 EALREADY Em "Operation already in progress" .
255 An operation was attempted on a non-blocking object that already
256 had an operation in progress.
257 .It Er 38 ENOTSOCK Em "Socket operation on non-socket" .
259 .It Er 39 EDESTADDRREQ Em "Destination address required" .
260 A required address was omitted from an operation on a socket.
261 .It Er 40 EMSGSIZE Em "Message too long" .
262 A message sent on a socket was larger than the internal message buffer
263 or some other network limit.
264 .It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" .
265 A protocol was specified that does not support the semantics of the
266 socket type requested.
267 For example, you cannot use the
273 .It Er 42 ENOPROTOOPT Em "Protocol not available" .
274 A bad option or level was specified in a
279 .It Er 43 EPROTONOSUPPORT Em "Protocol not supported" .
280 The protocol has not been configured into the
281 system or no implementation for it exists.
282 .It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" .
283 The support for the socket type has not been configured into the
284 system or no implementation for it exists.
285 .It Er 45 EOPNOTSUPP Em "Operation not supported" .
286 The attempted operation is not supported for the type of object referenced.
287 Usually this occurs when a file descriptor refers to a file or socket
288 that cannot support this operation,
289 for example, trying to
291 a connection on a datagram socket.
292 .It Er 46 EPFNOSUPPORT Em "Protocol family not supported" .
293 The protocol family has not been configured into the
294 system or no implementation for it exists.
295 .It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" .
296 An address incompatible with the requested protocol was used.
297 For example, you should not necessarily expect to be able to use
302 .It Er 48 EADDRINUSE Em "Address already in use" .
303 Only one usage of each address is normally permitted.
305 .It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" .
306 Normally results from an attempt to create a socket with an
307 address not on this machine.
308 .It Er 50 ENETDOWN Em "Network is down" .
309 A socket operation encountered a dead network.
310 .It Er 51 ENETUNREACH Em "Network is unreachable" .
311 A socket operation was attempted to an unreachable network.
312 .It Er 52 ENETRESET Em "Network dropped connection on reset" .
313 The host you were connected to crashed and rebooted.
314 .It Er 53 ECONNABORTED Em "Software caused connection abort" .
315 A connection abort was caused internal to your host machine.
316 .It Er 54 ECONNRESET Em "Connection reset by peer" .
317 A connection was forcibly closed by a peer.
319 results from a loss of the connection on the remote socket
320 due to a timeout or a reboot.
321 .It Er 55 ENOBUFS Em "\&No buffer space available" .
322 An operation on a socket or pipe was not performed because
323 the system lacked sufficient buffer space or because a queue was full.
324 .It Er 56 EISCONN Em "Socket is already connected" .
327 request was made on an already connected socket; or,
332 request on a connected socket specified a destination
333 when already connected.
334 .It Er 57 ENOTCONN Em "Socket is not connected" .
335 An request to send or receive data was disallowed because
336 the socket was not connected and (when sending on a datagram socket)
337 no address was supplied.
338 .It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" .
339 A request to send data was disallowed because the socket
340 had already been shut down with a previous
343 .It Er 60 ETIMEDOUT Em "Operation timed out" .
348 request failed because the connected party did not
349 properly respond after a period of time.
351 period is dependent on the communication protocol.)
352 .It Er 61 ECONNREFUSED Em "Connection refused" .
353 No connection could be made because the target machine actively
355 This usually results from trying to connect
356 to a service that is inactive on the foreign host.
357 .It Er 62 ELOOP Em "Too many levels of symbolic links" .
358 A path name lookup involved more than 32
361 .It Er 63 ENAMETOOLONG Em "File name too long" .
362 A component of a path name exceeded
364 characters, or an entire
368 (See also the description of
372 .It Er 64 EHOSTDOWN Em "Host is down" .
373 A socket operation failed because the destination host was down.
374 .It Er 65 EHOSTUNREACH Em "No route to host" .
375 A socket operation was attempted to an unreachable host.
376 .It Er 66 ENOTEMPTY Em "Directory not empty" .
377 A directory with entries other than
381 was supplied to a remove directory or rename call.
382 .It Er 67 EPROCLIM Em "Too many processes" .
383 .It Er 68 EUSERS Em "Too many users" .
384 The quota system ran out of table entries.
385 .It Er 69 EDQUOT Em "Disc quota exceeded" .
388 to an ordinary file, the creation of a
389 directory or symbolic link, or the creation of a directory
390 entry failed because the user's quota of disk blocks was
391 exhausted, or the allocation of an inode for a newly
392 created file failed because the user's quota of inodes
394 .It Er 70 ESTALE Em "Stale NFS file handle" .
395 An attempt was made to access an open file (on an
398 which is now unavailable as referenced by the file descriptor.
399 This may indicate the file was deleted on the
402 other catastrophic event occurred.
403 .It Er 72 EBADRPC Em "RPC struct is bad" .
406 information was unsuccessful.
407 .It Er 73 ERPCMISMATCH Em "RPC version wrong" .
410 on the remote peer is not compatible with
412 .It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" .
413 The requested program is not registered on the remote host.
414 .It Er 75 EPROGMISMATCH Em "Program version wrong" .
415 The requested version of the program is not available
418 .It Er 76 EPROCUNAVAIL Em "Bad procedure for program" .
421 call was attempted for a procedure which does not exist
422 in the remote program.
423 .It Er 77 ENOLCK Em "No locks available" .
424 A system-imposed limit on the number of simultaneous file
426 .It Er 78 ENOSYS Em "Function not implemented" .
427 Attempted a system call that is not available on this
429 .It Er 79 EFTYPE Em "Inappropriate file type or format" .
430 The file was the wrong type for the operation, or a data file had
432 .It Er 80 EAUTH Em "Authentication error" .
433 Attempted to use an invalid authentication ticket to mount a
436 .It Er 81 ENEEDAUTH Em "Need authenticator" .
437 An authentication ticket must be obtained before the given
439 file system may be mounted.
440 .It Er 82 EIDRM Em "Identifier removed" .
441 An IPC identifier was removed while the current process was waiting on it.
442 .It Er 83 ENOMSG Em "No message of desired type" .
443 An IPC message queue does not contain a message of the desired type, or a
444 message catalog does not contain the requested message.
445 .It Er 84 EOVERFLOW Em "Value too large to be stored in data type" .
446 A numerical result of the function was too large to be stored in the caller
448 .It Er 85 ECANCELED Em "Operation canceled" .
449 The scheduled operation was canceled.
450 .It Er 86 EILSEQ Em "Illegal byte sequence" .
451 While decoding a multibyte character the function came along an
452 invalid or an incomplete sequence of bytes or the given wide
453 character is invalid.
454 .It Er 87 ENOATTR Em "Attribute not found" .
455 The specified extended attribute does not exist.
456 .It Er 88 EDOOFUS Em "Programming error" .
457 A function or API is being abused in a way which could only be detected
463 Each active process in the system is uniquely identified by a non-negative
464 integer called a process ID.
465 The range of this ID is from 0 to 99999.
466 .It Parent process ID
467 A new process is created by a currently active process (see
469 The parent process ID of a process is initially the process ID of its creator.
470 If the creating process exits,
471 the parent process ID of each child is set to the ID of a system process,
474 Each active process is a member of a process group that is identified by
475 a non-negative integer called the process group ID.
477 ID of the group leader.
478 This grouping permits the signaling of related
481 and the job control mechanisms of
484 A session is a set of one or more process groups.
485 A session is created by a successful call to
487 which causes the caller to become the only member of the only process
488 group in the new session.
490 A process that has created a new session by a successful call to
492 is known as a session leader.
493 Only a session leader may acquire a terminal as its controlling terminal (see
495 .It Controlling process
496 A session leader with a controlling terminal is a controlling process.
497 .It Controlling terminal
498 A terminal that is associated with a session is known as the controlling
499 terminal for that session and its members.
500 .It "Terminal Process Group ID"
501 A terminal may be acquired by a session leader as its controlling terminal.
502 Once a terminal is associated with a session, any of the process groups
503 within the session may be placed into the foreground by setting
504 the terminal process group ID to the ID of the process group.
505 This facility is used
506 to arbitrate between multiple jobs contending for the same terminal;
511 .It "Orphaned Process Group"
512 A process group is considered to be
514 if it is not under the control of a job control shell.
515 More precisely, a process group is orphaned
516 when none of its members has a parent process that is in the same session
518 but is in a different process group.
519 Note that when a process exits, the parent process for its children
522 which is in a separate session.
523 Not all members of an orphaned process group are necessarily orphaned
524 processes (those whose creating process has exited).
525 The process group of a session leader is orphaned by definition.
526 .It "Real User ID and Real Group ID"
527 Each user on the system is identified by a positive integer
528 termed the real user ID.
530 Each user is also a member of one or more groups.
531 One of these groups is distinguished from others and
532 used in implementing accounting facilities.
534 integer corresponding to this distinguished group is termed
537 All processes have a real user ID and real group ID.
538 These are initialized from the equivalent attributes
539 of the process that created it.
540 .It "Effective User Id, Effective Group Id, and Group Access List"
541 Access to system resources is governed by two values:
542 the effective user ID, and the group access list.
543 The first member of the group access list is also known as the
545 (In POSIX.1, the group access list is known as the set of supplementary
546 group IDs, and it is unspecified whether the effective group ID is
547 a member of the list.)
549 The effective user ID and effective group ID are initially the
550 process's real user ID and real group ID respectively.
552 may be modified through execution of a set-user-ID or set-group-ID
553 file (possibly by one its ancestors) (see
555 By convention, the effective group ID (the first member of the group access
556 list) is duplicated, so that the execution of a set-group-ID program
557 does not result in the loss of the original (real) group ID.
559 The group access list is a set of group IDs
560 used only in determining resource accessibility.
562 are performed as described below in ``File Access Permissions''.
563 .It "Saved Set User ID and Saved Set Group ID"
564 When a process executes a new file, the effective user ID is set
565 to the owner of the file if the file is set-user-ID, and the effective
566 group ID (first element of the group access list) is set to the group
567 of the file if the file is set-group-ID.
568 The effective user ID of the process is then recorded as the saved set-user-ID,
569 and the effective group ID of the process is recorded as the saved set-group-ID.
570 These values may be used to regain those values as the effective user
571 or group ID after reverting to the real ID (see
573 (In POSIX.1, the saved set-user-ID and saved set-group-ID are optional,
574 and are used in setuid and setgid, but this does not work as desired
577 A process is recognized as a
579 process and is granted special privileges if its effective user ID is 0.
581 An integer assigned by the system when a file is referenced
586 or when a socket is created by
591 which uniquely identifies an access path to that file or socket from
592 a given process or any of its children.
594 Names consisting of up to
596 characters may be used to name
597 an ordinary file, special file, or directory.
599 These characters may be arbitrary eight-bit values,
609 Note that it is generally unwise to use
616 file names because of the special meaning attached to these characters
620 .Dv NUL Ns -terminated
621 character string starting with an
624 followed by zero or more directory names separated
625 by slashes, optionally followed by a file name.
626 The total length of a path name must be less than
629 (On some systems, this limit may be infinite.)
631 If a path name begins with a slash, the path search begins at the
634 Otherwise, the search begins from the current working directory.
635 A slash by itself names the root directory.
637 pathname refers to the current directory.
639 A directory is a special type of file that contains entries
640 that are references to other files.
641 Directory entries are called links.
642 By convention, a directory
643 contains at least two links,
652 Dot refers to the directory itself and
653 dot-dot refers to its parent directory.
654 .It "Root Directory and Current Working Directory"
655 Each process has associated with it a concept of a root directory
656 and a current working directory for the purpose of resolving path
658 A process's root directory need not be the root
659 directory of the root file system.
660 .It File Access Permissions
661 Every file in the file system has a set of access permissions.
662 These permissions are used in determining whether a process
663 may perform a requested operation on the file (such as opening
665 Access permissions are established at the
666 time a file is created.
667 They may be changed at some later time
672 File access is broken down according to whether a file may be: read,
673 written, or executed.
674 Directory files use the execute
675 permission to control if the directory may be searched.
677 File access permissions are interpreted by the system as
678 they apply to three different classes of users: the owner
679 of the file, those users in the file's group, anyone else.
680 Every file has an independent set of access permissions for
681 each of these classes.
682 When an access check is made, the system
683 decides if permission should be granted by checking the access
684 information applicable to the caller.
686 Read, write, and execute/search permissions on
687 a file are granted to a process if:
689 The process's effective user ID is that of the super-user.
691 even the super-user cannot execute a non-executable file.)
693 The process's effective user ID matches the user ID of the owner
694 of the file and the owner permissions allow the access.
696 The process's effective user ID does not match the user ID of the
697 owner of the file, and either the process's effective
698 group ID matches the group ID
699 of the file, or the group ID of the file is in
700 the process's group access list,
701 and the group permissions allow the access.
703 Neither the effective user ID nor effective group ID
704 and group access list of the process
705 match the corresponding user ID and group ID of the file,
706 but the permissions for ``other users'' allow access.
708 Otherwise, permission is denied.
709 .It Sockets and Address Families
710 A socket is an endpoint for communication between processes.
711 Each socket has queues for sending and receiving data.
713 Sockets are typed according to their communications properties.
714 These properties include whether messages sent and received
715 at a socket require the name of the partner, whether communication
716 is reliable, the format used in naming message recipients, etc.
718 Each instance of the system supports some
719 collection of socket types; consult
721 for more information about the types available and
724 Each instance of the system supports some number of sets of
725 communications protocols.
726 Each protocol set supports addresses
728 An Address Family is the set of addresses
729 for a specific group of protocols.
730 Each socket has an address
731 chosen from the address family in which the socket was created.