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28 .\" @(#)intro.2 8.5 (Berkeley) 2/27/95
29 .\" $FreeBSD: src/lib/libc/sys/intro.2,v 1.48 2007/01/09 00:28:14 imp Exp $
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
52 This identifier is defined in
56 .Dl extern __thread int errno;
57 .Dl static __inline int * __error(void);
58 .Dl #define errno (* __error())
60 This means there exists a thread-local
62 variable, though it is shadowed by the inline
64 function to allow compilation of source code which
65 erroneously itself declares
69 which collides with the thread-local declaration.
72 function returns a pointer the thread specific
77 it will compile to a no-op, effectively producing
78 the same code as if the define wouldn't exist.
80 When a system call detects an error,
81 it returns an integer value
82 indicating failure (usually -1)
86 (This allows interpretation of the failure on receiving
87 a -1 and to take action accordingly.)
88 Successful calls never set
90 once set, it remains until another error occurs.
91 It should only be examined after an error.
92 Note that a number of system calls overload the meanings of these
93 error numbers, and that the meanings must be interpreted according
94 to the type and circumstances of the call.
96 The following is a complete list of the errors and their
100 .It Er 0 Em "Undefined error: 0" .
102 .It Er 1 EPERM Em "Operation not permitted" .
103 An attempt was made to perform an operation limited to processes
104 with appropriate privileges or to the owner of a file or other
106 .It Er 2 ENOENT Em "No such file or directory" .
107 A component of a specified pathname did not exist, or the
108 pathname was an empty string.
109 .It Er 3 ESRCH Em "No such process" .
110 No process could be found corresponding to that specified by the given
112 .It Er 4 EINTR Em "Interrupted system call" .
113 An asynchronous signal (such as
117 was caught by the process during the execution of an interruptible
119 If the signal handler performs a normal return, the
120 interrupted system call will seem to have returned the error condition.
121 .It Er 5 EIO Em "Input/output error" .
122 Some physical input or output error occurred.
123 This error will not be reported until a subsequent operation on the same file
124 descriptor and may be lost (over written) by any subsequent errors.
125 .It Er 6 ENXIO Em "Device not configured" .
126 Input or output on a special file referred to a device that did not
128 made a request beyond the limits of the device.
129 This error may also occur when, for example,
130 a tape drive is not online or no disk pack is
132 .It Er 7 E2BIG Em "Argument list too long" .
133 The number of bytes used for the argument and environment
134 list of the new process exceeded the current limit
138 .It Er 8 ENOEXEC Em "Exec format error" .
139 A request was made to execute a file
140 that, although it has the appropriate permissions,
141 was not in the format required for an
143 .It Er 9 EBADF Em "Bad file descriptor" .
144 A file descriptor argument was out of range, referred to no open file,
145 or a read (write) request was made to a file that was only open for
147 .It Er 10 ECHILD Em "\&No child processes" .
152 function was executed by a process that had no existing or unwaited-for
154 .It Er 11 EDEADLK Em "Resource deadlock avoided" .
155 An attempt was made to lock a system resource that
156 would have resulted in a deadlock situation.
157 .It Er 12 ENOMEM Em "Cannot allocate memory" .
158 The new process image required more memory than was allowed by the hardware
159 or by system-imposed memory management constraints.
160 A lack of swap space is normally temporary; however,
161 a lack of core is not.
162 Soft limits may be increased to their corresponding hard limits.
163 .It Er 13 EACCES Em "Permission denied" .
164 An attempt was made to access a file in a way forbidden
165 by its file access permissions.
166 .It Er 14 EFAULT Em "Bad address" .
167 The system detected an invalid address in attempting to
168 use an argument of a call.
169 .It Er 15 ENOTBLK Em "Block device required" .
170 A block device operation was attempted on a non-block device or file.
171 .It Er 16 EBUSY Em "Device busy" .
172 An attempt to use a system resource which was in use at the time
173 in a manner which would have conflicted with the request.
174 .It Er 17 EEXIST Em "File exists" .
175 An existing file was mentioned in an inappropriate context,
176 for instance, as the new link name in a
179 .It Er 18 EXDEV Em "Cross-device link" .
180 A hard link to a file on another file system
182 .It Er 19 ENODEV Em "Operation not supported by device" .
183 An attempt was made to apply an inappropriate
184 function to a device,
186 trying to read a write-only device such as a printer.
187 .It Er 20 ENOTDIR Em "Not a directory" .
188 A component of the specified pathname existed, but it was
189 not a directory, when a directory was expected.
190 .It Er 21 EISDIR Em "Is a directory" .
191 An attempt was made to open a directory with write mode specified.
192 .It Er 22 EINVAL Em "Invalid argument" .
193 Some invalid argument was supplied.
195 specifying an undefined signal to a
201 .It Er 23 ENFILE Em "Too many open files in system" .
202 Maximum number of file descriptors allowable on the system
203 has been reached and a requests for an open cannot be satisfied
204 until at least one has been closed.
205 .It Er 24 EMFILE Em "Too many open files" .
206 (As released, the limit on the number of
207 open files per process is 64.)
210 system call will obtain the current limit.
211 .It Er 25 ENOTTY Em "Inappropriate ioctl for device" .
212 A control function (see
214 was attempted for a file or
215 special device for which the operation was inappropriate.
216 .It Er 26 ETXTBSY Em "Text file busy" .
217 The new process was a pure procedure (shared text) file
218 which was open for writing by another process, or
219 while the pure procedure file was being executed an
221 call requested write access.
222 .It Er 27 EFBIG Em "File too large" .
223 The size of a file exceeded the maximum.
224 .It Er 28 ENOSPC Em "No space left on device" .
227 to an ordinary file, the creation of a
228 directory or symbolic link, or the creation of a directory
229 entry failed because no more disk blocks were available
230 on the file system, or the allocation of an inode for a newly
231 created file failed because no more inodes were available
233 .It Er 29 ESPIPE Em "Illegal seek" .
236 system call was issued on a socket, pipe or
238 .It Er 30 EROFS Em "Read-only file system" .
239 An attempt was made to modify a file or directory
240 on a file system that was read-only at the time.
241 .It Er 31 EMLINK Em "Too many links" .
242 Maximum allowable hard links to a single file has been exceeded (limit
243 of 32767 hard links per file).
244 .It Er 32 EPIPE Em "Broken pipe" .
245 A write on a pipe, socket or
247 for which there is no process
249 .It Er 33 EDOM Em "Numerical argument out of domain" .
250 A numerical input argument was outside the defined domain of the mathematical
252 .It Er 34 ERANGE Em "Result too large" .
253 A numerical result of the function was too large to fit in the
254 available space (perhaps exceeded precision).
255 .It Er 35 EAGAIN Em "Resource temporarily unavailable" .
256 This is a temporary condition and later calls to the
257 same routine may complete normally.
258 .It Er 36 EINPROGRESS Em "Operation now in progress" .
259 An operation that takes a long time to complete (such as
262 was attempted on a non-blocking object (see
264 .It Er 37 EALREADY Em "Operation already in progress" .
265 An operation was attempted on a non-blocking object that already
266 had an operation in progress.
267 .It Er 38 ENOTSOCK Em "Socket operation on non-socket" .
269 .It Er 39 EDESTADDRREQ Em "Destination address required" .
270 A required address was omitted from an operation on a socket.
271 .It Er 40 EMSGSIZE Em "Message too long" .
272 A message sent on a socket was larger than the internal message buffer
273 or some other network limit.
274 .It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" .
275 A protocol was specified that does not support the semantics of the
276 socket type requested.
277 For example, you cannot use the
283 .It Er 42 ENOPROTOOPT Em "Protocol not available" .
284 A bad option or level was specified in a
289 .It Er 43 EPROTONOSUPPORT Em "Protocol not supported" .
290 The protocol has not been configured into the
291 system or no implementation for it exists.
292 .It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" .
293 The support for the socket type has not been configured into the
294 system or no implementation for it exists.
295 .It Er 45 EOPNOTSUPP Em "Operation not supported" .
296 The attempted operation is not supported for the type of object referenced.
297 Usually this occurs when a file descriptor refers to a file or socket
298 that cannot support this operation,
299 for example, trying to
301 a connection on a datagram socket.
302 .It Er 46 EPFNOSUPPORT Em "Protocol family not supported" .
303 The protocol family has not been configured into the
304 system or no implementation for it exists.
305 .It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" .
306 An address incompatible with the requested protocol was used.
307 For example, you should not necessarily expect to be able to use
312 .It Er 48 EADDRINUSE Em "Address already in use" .
313 Only one usage of each address is normally permitted.
314 .It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" .
315 Normally results from an attempt to create a socket with an
316 address not on this machine.
317 .It Er 50 ENETDOWN Em "Network is down" .
318 A socket operation encountered a dead network.
319 .It Er 51 ENETUNREACH Em "Network is unreachable" .
320 A socket operation was attempted to an unreachable network.
321 .It Er 52 ENETRESET Em "Network dropped connection on reset" .
322 The host you were connected to crashed and rebooted.
323 .It Er 53 ECONNABORTED Em "Software caused connection abort" .
324 A connection abort was caused internal to your host machine.
325 .It Er 54 ECONNRESET Em "Connection reset by peer" .
326 A connection was forcibly closed by a peer.
328 results from a loss of the connection on the remote socket
329 due to a timeout or a reboot.
330 .It Er 55 ENOBUFS Em "\&No buffer space available" .
331 An operation on a socket or pipe was not performed because
332 the system lacked sufficient buffer space or because a queue was full.
333 .It Er 56 EISCONN Em "Socket is already connected" .
336 request was made on an already connected socket; or,
341 request on a connected socket specified a destination
342 when already connected.
343 .It Er 57 ENOTCONN Em "Socket is not connected" .
344 An request to send or receive data was disallowed because
345 the socket was not connected and (when sending on a datagram socket)
346 no address was supplied.
347 .It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" .
348 A request to send data was disallowed because the socket
349 had already been shut down with a previous
352 .It Er 60 ETIMEDOUT Em "Operation timed out" .
357 request failed because the connected party did not
358 properly respond after a period of time.
360 period is dependent on the communication protocol.)
361 .It Er 61 ECONNREFUSED Em "Connection refused" .
362 No connection could be made because the target machine actively
364 This usually results from trying to connect
365 to a service that is inactive on the foreign host.
366 .It Er 62 ELOOP Em "Too many levels of symbolic links" .
367 A path name lookup involved more than 32
370 .It Er 63 ENAMETOOLONG Em "File name too long" .
371 A component of a path name exceeded
373 characters, or an entire
377 (See also the description of
381 .It Er 64 EHOSTDOWN Em "Host is down" .
382 A socket operation failed because the destination host was down.
383 .It Er 65 EHOSTUNREACH Em "No route to host" .
384 A socket operation was attempted to an unreachable host.
385 .It Er 66 ENOTEMPTY Em "Directory not empty" .
386 A directory with entries other than
390 was supplied to a remove directory or rename call.
391 .It Er 67 EPROCLIM Em "Too many processes" .
392 .It Er 68 EUSERS Em "Too many users" .
393 The quota system ran out of table entries.
394 .It Er 69 EDQUOT Em "Disc quota exceeded" .
397 to an ordinary file, the creation of a
398 directory or symbolic link, or the creation of a directory
399 entry failed because the user's quota of disk blocks was
400 exhausted, or the allocation of an inode for a newly
401 created file failed because the user's quota of inodes
403 .It Er 70 ESTALE Em "Stale NFS file handle" .
404 An attempt was made to access an open file (on an
407 which is now unavailable as referenced by the file descriptor.
408 This may indicate the file was deleted on the
411 other catastrophic event occurred.
412 .It Er 72 EBADRPC Em "RPC struct is bad" .
415 information was unsuccessful.
416 .It Er 73 ERPCMISMATCH Em "RPC version wrong" .
419 on the remote peer is not compatible with
421 .It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" .
422 The requested program is not registered on the remote host.
423 .It Er 75 EPROGMISMATCH Em "Program version wrong" .
424 The requested version of the program is not available
427 .It Er 76 EPROCUNAVAIL Em "Bad procedure for program" .
430 call was attempted for a procedure which does not exist
431 in the remote program.
432 .It Er 77 ENOLCK Em "No locks available" .
433 A system-imposed limit on the number of simultaneous file
435 .It Er 78 ENOSYS Em "Function not implemented" .
436 Attempted a system call that is not available on this
438 .It Er 79 EFTYPE Em "Inappropriate file type or format" .
439 The file was the wrong type for the operation, or a data file had
441 .It Er 80 EAUTH Em "Authentication error" .
442 Attempted to use an invalid authentication ticket to mount a
445 .It Er 81 ENEEDAUTH Em "Need authenticator" .
446 An authentication ticket must be obtained before the given
448 file system may be mounted.
449 .It Er 82 EIDRM Em "Identifier removed" .
450 An IPC identifier was removed while the current process was waiting on it.
451 .It Er 83 ENOMSG Em "No message of desired type" .
452 An IPC message queue does not contain a message of the desired type, or a
453 message catalog does not contain the requested message.
454 .It Er 84 EOVERFLOW Em "Value too large to be stored in data type" .
455 A numerical result of the function was too large to be stored in the caller
457 .It Er 85 ECANCELED Em "Operation canceled" .
458 The scheduled operation was canceled.
459 .It Er 86 EILSEQ Em "Illegal byte sequence" .
460 While decoding a multibyte character the function came along an
461 invalid or an incomplete sequence of bytes or the given wide
462 character is invalid.
463 .It Er 87 ENOATTR Em "Attribute not found" .
464 The specified extended attribute does not exist.
465 .It Er 88 EDOOFUS Em "Programming error" .
466 A function or API is being abused in a way which could only be detected
472 Each active process in the system is uniquely identified by a non-negative
473 integer called a process ID.
474 The range of this ID is from 0 to 99999.
475 .It Parent process ID
476 A new process is created by a currently active process (see
478 The parent process ID of a process is initially the process ID of its creator.
479 If the creating process exits,
480 the parent process ID of each child is set to the ID of a system process,
483 Each active process is a member of a process group that is identified by
484 a non-negative integer called the process group ID.
486 ID of the group leader.
487 This grouping permits the signaling of related
490 and the job control mechanisms of
493 A session is a set of one or more process groups.
494 A session is created by a successful call to
496 which causes the caller to become the only member of the only process
497 group in the new session.
499 A process that has created a new session by a successful call to
501 is known as a session leader.
502 Only a session leader may acquire a terminal as its controlling terminal (see
504 .It Controlling process
505 A session leader with a controlling terminal is a controlling process.
506 .It Controlling terminal
507 A terminal that is associated with a session is known as the controlling
508 terminal for that session and its members.
509 .It "Terminal Process Group ID"
510 A terminal may be acquired by a session leader as its controlling terminal.
511 Once a terminal is associated with a session, any of the process groups
512 within the session may be placed into the foreground by setting
513 the terminal process group ID to the ID of the process group.
514 This facility is used
515 to arbitrate between multiple jobs contending for the same terminal;
520 .It "Orphaned Process Group"
521 A process group is considered to be
523 if it is not under the control of a job control shell.
524 More precisely, a process group is orphaned
525 when none of its members has a parent process that is in the same session
527 but is in a different process group.
528 Note that when a process exits, the parent process for its children
531 which is in a separate session.
532 Not all members of an orphaned process group are necessarily orphaned
533 processes (those whose creating process has exited).
534 The process group of a session leader is orphaned by definition.
535 .It "Real User ID and Real Group ID"
536 Each user on the system is identified by a positive integer
537 termed the real user ID.
539 Each user is also a member of one or more groups.
540 One of these groups is distinguished from others and
541 used in implementing accounting facilities.
543 integer corresponding to this distinguished group is termed
546 All processes have a real user ID and real group ID.
547 These are initialized from the equivalent attributes
548 of the process that created it.
549 .It "Effective User Id, Effective Group Id, and Group Access List"
550 Access to system resources is governed by two values:
551 the effective user ID, and the group access list.
552 The first member of the group access list is also known as the
554 (In POSIX.1, the group access list is known as the set of supplementary
555 group IDs, and it is unspecified whether the effective group ID is
556 a member of the list.)
558 The effective user ID and effective group ID are initially the
559 process's real user ID and real group ID respectively.
561 may be modified through execution of a set-user-ID or set-group-ID
562 file (possibly by one its ancestors) (see
564 By convention, the effective group ID (the first member of the group access
565 list) is duplicated, so that the execution of a set-group-ID program
566 does not result in the loss of the original (real) group ID.
568 The group access list is a set of group IDs
569 used only in determining resource accessibility.
571 are performed as described below in ``File Access Permissions''.
572 .It "Saved Set User ID and Saved Set Group ID"
573 When a process executes a new file, the effective user ID is set
574 to the owner of the file if the file is set-user-ID, and the effective
575 group ID (first element of the group access list) is set to the group
576 of the file if the file is set-group-ID.
577 The effective user ID of the process is then recorded as the saved set-user-ID,
578 and the effective group ID of the process is recorded as the saved set-group-ID.
579 These values may be used to regain those values as the effective user
580 or group ID after reverting to the real ID (see
582 (In POSIX.1, the saved set-user-ID and saved set-group-ID are optional,
583 and are used in setuid and setgid, but this does not work as desired
586 A process is recognized as a
588 process and is granted special privileges if its effective user ID is 0.
590 An integer assigned by the system when a file is referenced
595 or when a socket is created by
600 which uniquely identifies an access path to that file or socket from
601 a given process or any of its children.
603 Names consisting of up to
605 characters may be used to name
606 an ordinary file, special file, or directory.
608 These characters may be arbitrary eight-bit values,
618 Note that it is generally unwise to use
625 file names because of the special meaning attached to these characters
629 .Dv NUL Ns -terminated
630 character string starting with an
633 followed by zero or more directory names separated
634 by slashes, optionally followed by a file name.
635 The total length of a path name must be less than
638 (On some systems, this limit may be infinite.)
640 If a path name begins with a slash, the path search begins at the
643 Otherwise, the search begins from the current working directory.
644 A slash by itself names the root directory.
646 pathname refers to the current directory.
648 A directory is a special type of file that contains entries
649 that are references to other files.
650 Directory entries are called links.
651 By convention, a directory
652 contains at least two links,
661 Dot refers to the directory itself and
662 dot-dot refers to its parent directory.
663 .It "Root Directory and Current Working Directory"
664 Each process has associated with it a concept of a root directory
665 and a current working directory for the purpose of resolving path
667 A process's root directory need not be the root
668 directory of the root file system.
669 .It File Access Permissions
670 Every file in the file system has a set of access permissions.
671 These permissions are used in determining whether a process
672 may perform a requested operation on the file (such as opening
674 Access permissions are established at the
675 time a file is created.
676 They may be changed at some later time
681 File access is broken down according to whether a file may be: read,
682 written, or executed.
683 Directory files use the execute
684 permission to control if the directory may be searched.
686 File access permissions are interpreted by the system as
687 they apply to three different classes of users: the owner
688 of the file, those users in the file's group, anyone else.
689 Every file has an independent set of access permissions for
690 each of these classes.
691 When an access check is made, the system
692 decides if permission should be granted by checking the access
693 information applicable to the caller.
695 Read, write, and execute/search permissions on
696 a file are granted to a process if:
698 The process's effective user ID is that of the super-user.
700 even the super-user cannot execute a non-executable file.)
702 The process's effective user ID matches the user ID of the owner
703 of the file and the owner permissions allow the access.
705 The process's effective user ID does not match the user ID of the
706 owner of the file, and either the process's effective
707 group ID matches the group ID
708 of the file, or the group ID of the file is in
709 the process's group access list,
710 and the group permissions allow the access.
712 Neither the effective user ID nor effective group ID
713 and group access list of the process
714 match the corresponding user ID and group ID of the file,
715 but the permissions for ``other users'' allow access.
717 Otherwise, permission is denied.
718 .It Sockets and Address Families
719 A socket is an endpoint for communication between processes.
720 Each socket has queues for sending and receiving data.
722 Sockets are typed according to their communications properties.
723 These properties include whether messages sent and received
724 at a socket require the name of the partner, whether communication
725 is reliable, the format used in naming message recipients, etc.
727 Each instance of the system supports some
728 collection of socket types; consult
730 for more information about the types available and
733 Each instance of the system supports some number of sets of
734 communications protocols.
735 Each protocol set supports addresses of a certain format.
736 An Address Family is the set of addresses for a specific group of protocols.
737 Each socket has an address
738 chosen from the address family in which the socket was created.