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32 .\" @(#)intro.2 8.5 (Berkeley) 2/27/95
33 .\" $FreeBSD: src/lib/libc/sys/intro.2,v 1.21.2.7 2003/02/24 01:01:48 trhodes Exp $
40 .Nd introduction to system calls and error numbers
46 This section provides an overview of the system calls,
47 their error returns, and other common definitions and concepts.
49 .\".Sy System call restart
53 Nearly all of the system calls provide an error number referenced via
54 the external identifier errno.
55 This identifier is defined in
59 .Dl extern int * __error();
60 .Dl #define errno (* __error())
64 function returns a pointer to a field in the thread specific structure for
65 threads other than the initial thread.
66 For the initial thread and
67 non-threaded processes,
69 returns a pointer to a global
71 variable that is compatible with the previous definition.
73 When a system call detects an error,
74 it returns an integer value
75 indicating failure (usually -1)
79 <This allows interpretation of the failure on receiving
80 a -1 and to take action accordingly.>
81 Successful calls never set
83 once set, it remains until another error occurs.
84 It should only be examined after an error.
85 Note that a number of system calls overload the meanings of these
86 error numbers, and that the meanings must be interpreted according
87 to the type and circumstances of the call.
89 The following is a complete list of the errors and their
93 .It Er 0 Em "Undefined error: 0" .
95 .It Er 1 EPERM Em "Operation not permitted" .
96 An attempt was made to perform an operation limited to processes
97 with appropriate privileges or to the owner of a file or other
99 .It Er 2 ENOENT Em "No such file or directory" .
100 A component of a specified pathname did not exist, or the
101 pathname was an empty string.
102 .It Er 3 ESRCH Em "No such process" .
103 No process could be found corresponding to that specified by the given
105 .It Er 4 EINTR Em "Interrupted system call" .
106 An asynchronous signal (such as
110 was caught by the process during the execution of an interruptible
112 If the signal handler performs a normal return, the
113 interrupted function call will seem to have returned the error condition.
114 .It Er 5 EIO Em "Input/output error" .
115 Some physical input or output error occurred.
116 This error will not be reported until a subsequent operation on the same file
117 descriptor and may be lost (over written) by any subsequent errors.
118 .It Er 6 ENXIO Em "Device not configured" .
119 Input or output on a special file referred to a device that did not
121 made a request beyond the limits of the device.
122 This error may also occur when, for example,
123 a tape drive is not online or no disk pack is
125 .It Er 7 E2BIG Em "Argument list too long" .
126 The number of bytes used for the argument and environment
127 list of the new process exceeded the current limit
131 .Aq Pa sys/param.h ) .
132 .It Er 8 ENOEXEC Em "Exec format error" .
133 A request was made to execute a file
134 that, although it has the appropriate permissions,
135 was not in the format required for an
137 .It Er 9 EBADF Em "Bad file descriptor" .
138 A file descriptor argument was out of range, referred to no open file,
139 or a read (write) request was made to a file that was only open for
142 .It Er 10 ECHILD Em "\&No child processes" .
147 function was executed by a process that had no existing or unwaited-for
149 .It Er 11 EDEADLK Em "Resource deadlock avoided" .
150 An attempt was made to lock a system resource that
151 would have resulted in a deadlock situation.
152 .It Er 12 ENOMEM Em "Cannot allocate memory" .
153 The new process image required more memory than was allowed by the hardware
154 or by system-imposed memory management constraints.
155 A lack of swap space is normally temporary; however,
156 a lack of core is not.
157 Soft limits may be increased to their corresponding hard limits.
158 .It Er 13 EACCES Em "Permission denied" .
159 An attempt was made to access a file in a way forbidden
160 by its file access permissions.
161 .It Er 14 EFAULT Em "Bad address" .
162 The system detected an invalid address in attempting to
163 use an argument of a call.
164 .It Er 15 ENOTBLK Em "Block device required" .
165 A block device operation was attempted on a non-block device or file.
166 .It Er 16 EBUSY Em "Device busy" .
167 An attempt to use a system resource which was in use at the time
168 in a manner which would have conflicted with the request.
169 .It Er 17 EEXIST Em "File exists" .
170 An existing file was mentioned in an inappropriate context,
171 for instance, as the new link name in a
174 .It Er 18 EXDEV Em "Cross-device link" .
175 A hard link to a file on another file system
177 .It Er 19 ENODEV Em "Operation not supported by device" .
178 An attempt was made to apply an inappropriate
179 function to a device,
181 trying to read a write-only device such as a printer.
182 .It Er 20 ENOTDIR Em "Not a directory" .
183 A component of the specified pathname existed, but it was
184 not a directory, when a directory was expected.
185 .It Er 21 EISDIR Em "Is a directory" .
186 An attempt was made to open a directory with write mode specified.
187 .It Er 22 EINVAL Em "Invalid argument" .
188 Some invalid argument was supplied.
190 specifying an undefined signal to a
195 .It Er 23 ENFILE Em "Too many open files in system" .
196 Maximum number of file descriptors allowable on the system
197 has been reached and a requests for an open cannot be satisfied
198 until at least one has been closed.
199 .It Er 24 EMFILE Em "Too many open files" .
200 <As released, the limit on the number of
201 open files per process is 64.>
204 function will obtain the current limit.
205 .It Er 25 ENOTTY Em "Inappropriate ioctl for device" .
206 A control function (see
208 was attempted for a file or
209 special device for which the operation was inappropriate.
210 .It Er 26 ETXTBSY Em "Text file busy" .
211 The new process was a pure procedure (shared text) file
212 which was open for writing by another process, or
213 while the pure procedure file was being executed an
215 call requested write access.
216 .It Er 27 EFBIG Em "File too large" .
217 The size of a file exceeded the maximum (about
218 .if t 2\u\s-231\s+2\d
221 .It Er 28 ENOSPC Em "No space left on device" .
224 to an ordinary file, the creation of a
225 directory or symbolic link, or the creation of a directory
226 entry failed because no more disk blocks were available
227 on the file system, or the allocation of an inode for a newly
228 created file failed because no more inodes were available
230 .It Er 29 ESPIPE Em "Illegal seek" .
233 function was issued on a socket, pipe or
235 .It Er 30 EROFS Em "Read-only file system" .
236 An attempt was made to modify a file or directory
237 on a file system that was read-only at the time.
238 .It Er 31 EMLINK Em "Too many links" .
239 Maximum allowable hard links to a single file has been exceeded (limit
240 of 32767 hard links per file).
241 .It Er 32 EPIPE Em "Broken pipe" .
242 A write on a pipe, socket or
244 for which there is no process
246 .It Er 33 EDOM Em "Numerical argument out of domain" .
247 A numerical input argument was outside the defined domain of the mathematical
249 .It Er 34 ERANGE Em "Result too large" .
250 A numerical result of the function was too large to fit in the
251 available space (perhaps exceeded precision).
252 .It Er 35 EAGAIN Em "Resource temporarily unavailable" .
253 This is a temporary condition and later calls to the
254 same routine may complete normally.
255 .It Er 36 EINPROGRESS Em "Operation now in progress" .
256 An operation that takes a long time to complete (such as
259 was attempted on a non-blocking object (see
261 .It Er 37 EALREADY Em "Operation already in progress" .
262 An operation was attempted on a non-blocking object that already
263 had an operation in progress.
264 .It Er 38 ENOTSOCK Em "Socket operation on non-socket" .
266 .It Er 39 EDESTADDRREQ Em "Destination address required" .
267 A required address was omitted from an operation on a socket.
268 .It Er 40 EMSGSIZE Em "Message too long" .
269 A message sent on a socket was larger than the internal message buffer
270 or some other network limit.
271 .It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" .
272 A protocol was specified that does not support the semantics of the
273 socket type requested.
274 For example, you cannot use the
280 .It Er 42 ENOPROTOOPT Em "Protocol not available" .
281 A bad option or level was specified in a
286 .It Er 43 EPROTONOSUPPORT Em "Protocol not supported" .
287 The protocol has not been configured into the
288 system or no implementation for it exists.
289 .It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" .
290 The support for the socket type has not been configured into the
291 system or no implementation for it exists.
292 .It Er 45 EOPNOTSUPP Em "Operation not supported" .
293 The attempted operation is not supported for the type of object referenced.
294 Usually this occurs when a file descriptor refers to a file or socket
295 that cannot support this operation,
296 for example, trying to
298 a connection on a datagram socket.
299 .It Er 46 EPFNOSUPPORT Em "Protocol family not supported" .
300 The protocol family has not been configured into the
301 system or no implementation for it exists.
302 .It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" .
303 An address incompatible with the requested protocol was used.
304 For example, you shouldn't necessarily expect to be able to use
309 .It Er 48 EADDRINUSE Em "Address already in use" .
310 Only one usage of each address is normally permitted.
312 .It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" .
313 Normally results from an attempt to create a socket with an
314 address not on this machine.
315 .It Er 50 ENETDOWN Em "Network is down" .
316 A socket operation encountered a dead network.
317 .It Er 51 ENETUNREACH Em "Network is unreachable" .
318 A socket operation was attempted to an unreachable network.
319 .It Er 52 ENETRESET Em "Network dropped connection on reset" .
320 The host you were connected to crashed and rebooted.
321 .It Er 53 ECONNABORTED Em "Software caused connection abort" .
322 A connection abort was caused internal to your host machine.
323 .It Er 54 ECONNRESET Em "Connection reset by peer" .
324 A connection was forcibly closed by a peer. This normally
325 results from a loss of the connection on the remote socket
326 due to a timeout or a reboot.
327 .It Er 55 ENOBUFS Em "\&No buffer space available" .
328 An operation on a socket or pipe was not performed because
329 the system lacked sufficient buffer space or because a queue was full.
330 .It Er 56 EISCONN Em "Socket is already connected" .
333 request was made on an already connected socket; or,
338 request on a connected socket specified a destination
339 when already connected.
340 .It Er 57 ENOTCONN Em "Socket is not connected" .
341 An request to send or receive data was disallowed because
342 the socket was not connected and (when sending on a datagram socket)
343 no address was supplied.
344 .It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" .
345 A request to send data was disallowed because the socket
346 had already been shut down with a previous
349 .It Er 60 ETIMEDOUT Em "Operation timed out" .
354 request failed because the connected party did not
355 properly respond after a period of time. (The timeout
356 period is dependent on the communication protocol.)
357 .It Er 61 ECONNREFUSED Em "Connection refused" .
358 No connection could be made because the target machine actively
359 refused it. This usually results from trying to connect
360 to a service that is inactive on the foreign host.
361 .It Er 62 ELOOP Em "Too many levels of symbolic links" .
362 A path name lookup involved more than 32
365 .It Er 63 ENAMETOOLONG Em "File name too long" .
366 A component of a path name exceeded 255
368 characters, or an entire
369 path name exceeded 1023
370 .Pq Dv MAXPATHLEN Ns -1
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 doesn't 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 filesystem 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.
458 Each active process in the system is uniquely identified by a non-negative
459 integer called a process ID. The range of this ID is from 0 to 99999.
460 .It Parent process ID
461 A new process is created by a currently active process; (see
463 The parent process ID of a process is initially the process ID of its creator.
464 If the creating process exits,
465 the parent process ID of each child is set to the ID of a system process,
468 Each active process is a member of a process group that is identified by
469 a non-negative integer called the process group ID. This is the process
470 ID of the group leader. This grouping permits the signaling of related
473 and the job control mechanisms of
476 A session is a set of one or more process groups.
477 A session is created by a successful call to
479 which causes the caller to become the only member of the only process
480 group in the new session.
482 A process that has created a new session by a successful call to
484 is known as a session leader.
485 Only a session leader may acquire a terminal as its controlling terminal (see
487 .It Controlling process
488 A session leader with a controlling terminal is a controlling process.
489 .It Controlling terminal
490 A terminal that is associated with a session is known as the controlling
491 terminal for that session and its members.
492 .It "Terminal Process Group ID"
493 A terminal may be acquired by a session leader as its controlling terminal.
494 Once a terminal is associated with a session, any of the process groups
495 within the session may be placed into the foreground by setting
496 the terminal process group ID to the ID of the process group.
497 This facility is used
498 to arbitrate between multiple jobs contending for the same terminal;
503 .It "Orphaned Process Group"
504 A process group is considered to be
506 if it is not under the control of a job control shell.
507 More precisely, a process group is orphaned
508 when none of its members has a parent process that is in the same session
510 but is in a different process group.
511 Note that when a process exits, the parent process for its children
514 which is in a separate session.
515 Not all members of an orphaned process group are necessarily orphaned
516 processes (those whose creating process has exited).
517 The process group of a session leader is orphaned by definition.
518 .It "Real User ID and Real Group ID"
519 Each user on the system is identified by a positive integer
520 termed the real user ID.
522 Each user is also a member of one or more groups.
523 One of these groups is distinguished from others and
524 used in implementing accounting facilities. The positive
525 integer corresponding to this distinguished group is termed
528 All processes have a real user ID and real group ID.
529 These are initialized from the equivalent attributes
530 of the process that created it.
531 .It "Effective User Id, Effective Group Id, and Group Access List"
532 Access to system resources is governed by two values:
533 the effective user ID, and the group access list.
534 The first member of the group access list is also known as the
536 (In POSIX.1, the group access list is known as the set of supplementary
537 group IDs, and it is unspecified whether the effective group ID is
538 a member of the list.)
540 The effective user ID and effective group ID are initially the
541 process's real user ID and real group ID respectively. Either
542 may be modified through execution of a set-user-ID or set-group-ID
543 file (possibly by one its ancestors) (see
545 By convention, the effective group ID (the first member of the group access
546 list) is duplicated, so that the execution of a set-group-ID program
547 does not result in the loss of the original (real) group ID.
549 The group access list is a set of group IDs
550 used only in determining resource accessibility. Access checks
551 are performed as described below in ``File Access Permissions''.
552 .It "Saved Set User ID and Saved Set Group ID"
553 When a process executes a new file, the effective user ID is set
554 to the owner of the file if the file is set-user-ID, and the effective
555 group ID (first element of the group access list) is set to the group
556 of the file if the file is set-group-ID.
557 The effective user ID of the process is then recorded as the saved set-user-ID,
558 and the effective group ID of the process is recorded as the saved set-group-ID.
559 These values may be used to regain those values as the effective user
560 or group ID after reverting to the real ID (see
562 (In POSIX.1, the saved set-user-ID and saved set-group-ID are optional,
563 and are used in setuid and setgid, but this does not work as desired
566 A process is recognized as a
568 process and is granted special privileges if its effective user ID is 0.
569 .It Special Processes
570 The processes with process IDs of 0, 1, and 2 are special.
571 Process 0 is the scheduler. Process 1 is the initialization process
573 and is the ancestor of every other process in the system.
574 It is used to control the process structure.
575 Process 2 is the paging daemon.
577 An integer assigned by the system when a file is referenced
582 or when a socket is created by
587 which uniquely identifies an access path to that file or socket from
588 a given process or any of its children.
590 Names consisting of up to 255
592 characters may be used to name
593 an ordinary file, special file, or directory.
595 These characters may be selected from the set of all
598 excluding 0 (NUL) and the
604 Note that it is generally unwise to use
611 file names because of the special meaning attached to these characters
615 .Tn NUL Ns -terminated
616 character string starting with an
619 followed by zero or more directory names separated
620 by slashes, optionally followed by a file name.
621 The total length of a path name must be less than 1024
625 If a path name begins with a slash, the path search begins at the
628 Otherwise, the search begins from the current working directory.
629 A slash by itself names the root directory. An empty
630 pathname refers to the current directory.
632 A directory is a special type of file that contains entries
633 that are references to other files.
634 Directory entries are called links. By convention, a directory
635 contains at least two links,
643 respectively. Dot refers to the directory itself and
644 dot-dot refers to its parent directory.
645 .It "Root Directory and Current Working Directory"
646 Each process has associated with it a concept of a root directory
647 and a current working directory for the purpose of resolving path
648 name searches. A process's root directory need not be the root
649 directory of the root file system.
650 .It File Access Permissions
651 Every file in the file system has a set of access permissions.
652 These permissions are used in determining whether a process
653 may perform a requested operation on the file (such as opening
654 a file for writing). Access permissions are established at the
655 time a file is created. They may be changed at some later time
660 File access is broken down according to whether a file may be: read,
661 written, or executed. Directory files use the execute
662 permission to control if the directory may be searched.
664 File access permissions are interpreted by the system as
665 they apply to three different classes of users: the owner
666 of the file, those users in the file's group, anyone else.
667 Every file has an independent set of access permissions for
668 each of these classes. When an access check is made, the system
669 decides if permission should be granted by checking the access
670 information applicable to the caller.
672 Read, write, and execute/search permissions on
673 a file are granted to a process if:
675 The process's effective user ID is that of the super-user.
677 even the super-user cannot execute a non-executable file.)
679 The process's effective user ID matches the user ID of the owner
680 of the file and the owner permissions allow the access.
682 The process's effective user ID does not match the user ID of the
683 owner of the file, and either the process's effective
684 group ID matches the group ID
685 of the file, or the group ID of the file is in
686 the process's group access list,
687 and the group permissions allow the access.
689 Neither the effective user ID nor effective group ID
690 and group access list of the process
691 match the corresponding user ID and group ID of the file,
692 but the permissions for ``other users'' allow access.
694 Otherwise, permission is denied.
695 .It Sockets and Address Families
696 A socket is an endpoint for communication between processes.
697 Each socket has queues for sending and receiving data.
699 Sockets are typed according to their communications properties.
700 These properties include whether messages sent and received
701 at a socket require the name of the partner, whether communication
702 is reliable, the format used in naming message recipients, etc.
704 Each instance of the system supports some
705 collection of socket types; consult
707 for more information about the types available and
710 Each instance of the system supports some number of sets of
711 communications protocols. Each protocol set supports addresses
712 of a certain format. An Address Family is the set of addresses
713 for a specific group of protocols. Each socket has an address
714 chosen from the address family in which the socket was created.