1.\" $NetBSD: intro.2,v 1.55 2010/03/22 19:30:55 joerg Exp $ 2.\" 3.\" Copyright (c) 1980, 1983, 1986, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)intro.2 8.5 (Berkeley) 2/27/95 31.\" 32.Dd July 23, 2009 33.Dt INTRO 2 34.Os 35.Sh NAME 36.Nm intro , 37.Nm errno 38.Nd introduction to system calls and error numbers 39.Sh SYNOPSIS 40.In errno.h 41.Sh DESCRIPTION 42This section provides an overview of the system calls, 43their error returns, and other common definitions and concepts. 44.\".Pp 45.\".Sy System call restart 46.\".Pp 47.\"<more later...> 48.Sh DIAGNOSTICS 49Nearly all of the system calls provide an error number in the external 50variable 51.Va errno . 52.Va errno 53is implemented as a macro which expands to a modifiable lvalue of type 54.Fa int . 55.Pp 56When a system call detects an error, 57it returns an integer value 58indicating failure (usually \-1) 59and sets the variable 60.Va errno 61accordingly. 62(This allows interpretation of the failure on receiving 63a \-1 and to take action accordingly.) 64Successful calls never set 65.Va errno ; 66once set, it remains until another error occurs. 67It should only be examined after an error has been reported, because 68otherwise a leftover value from some previous error may be found 69instead. 70.Po 71Many library functions that are not system calls also set 72.Va errno 73on return, in the same fashion. 74In these cases a nonzero value may be left in 75.Va errno 76even upon successful return if some internal action failed. 77.Pc 78.Pp 79The manual page for each system call will list some of the common 80errno codes that system call can return, but that should not be 81considered an exhaustive list, i.e. 82a properly written program should be able to gracefully recover from 83any error that a system call might return. 84Documenting all the error codes that a system call can return in 85a more specification-like manner would take more resources than 86this project has available. 87.Pp 88Note also that a number of system calls overload the meanings of these 89error numbers, and that in these cases the meanings must be 90interpreted according to the type and circumstances of the call. 91.Pp 92The following is a complete list of the errors and their 93names as given in 94.In errno.h . 95.Bl -hang -width Ds 96.It Er 0 Em "Error 0" . 97Not used. 98.It Er 1 EPERM Em "Operation not permitted" . 99An attempt was made to perform an operation limited to processes 100with appropriate privileges or to the owner of a file or other 101resources. 102.It Er 2 ENOENT Em "No such file or directory" . 103A component of a specified pathname did not exist, or the 104pathname was an empty string. 105.It Er 3 ESRCH Em "No such process" . 106No process could be found corresponding to that specified by the given 107process ID. 108.It Er 4 EINTR Em "Interrupted function call" . 109An asynchronous signal (such as 110.Dv SIGINT 111or 112.Dv SIGQUIT ) 113was caught by the process during the execution of an interruptible 114function. 115If the signal handler performs a normal return, the 116interrupted function call will seem to have returned the error condition. 117.It Er 5 EIO Em "Input/output error" . 118Some physical input or output error occurred. 119This error will not be reported until a subsequent operation on the same file 120descriptor and may be lost (over written) by any subsequent errors. 121.It Er 6 ENXIO Em "Device not configured" . 122Input or output on a special file referred to a device that did not 123exist, or 124made a request beyond the limits of the device. 125This error may also occur when, for example, 126a tape drive is not online or no disk pack is 127loaded on a drive. 128.It Er 7 E2BIG Em "Arg list too long" . 129The number of bytes used for the argument and environment 130list of the new process exceeded the current limit of 131.if t 2\u\s-218\s+2\d 132.if n 2**18 133bytes 134.Pf ( Dv ARG_MAX 135in 136.In sys/syslimits.h ) . 137.It Er 8 ENOEXEC Em "Exec format error" . 138A request was made to execute a file 139that, although it has the appropriate permissions, 140was not in the format required for an 141executable file. 142.It Er 9 EBADF Em "Bad file descriptor" . 143A file descriptor argument was out of range, referred to no open file, 144had been revoked by 145.Xr revoke 2 , 146or a 147.Xr read 2 148(or 149.Xr write 2 ) 150request was made to a file that was 151only open for writing (or reading). 152.It Er 10 ECHILD Em "\&No child processes" . 153A 154.Xr wait 2 155or 156.Xr waitpid 2 157function was executed by a process that had no existing or unwaited-for 158child processes. 159.It Er 11 EDEADLK Em "Resource deadlock avoided" . 160An attempt was made to lock a system resource that 161would have resulted in a deadlock situation. 162.It Er 12 ENOMEM Em "Cannot allocate memory" . 163The new process image required more memory than was allowed by the hardware 164or by system-imposed memory management constraints. 165A lack of swap space is normally temporary; however, 166a lack of core is not. 167Soft limits may be increased to their corresponding hard limits. 168.It Er 13 EACCES Em "Permission denied" . 169An attempt was made to access a file in a way forbidden 170by its file access permissions. 171.It Er 14 EFAULT Em "Bad address" . 172The system detected an invalid address in attempting to 173use an argument of a call. 174The reliable detection of this error cannot be guaranteed and when not detected 175may result in the generation of a signal, indicating an address violation, 176which is sent to the process. 177.It Er 15 ENOTBLK Em "Block device required" . 178A block device operation was attempted on a non-block device or file. 179.It Er 16 EBUSY Em "Resource busy" . 180An attempt to use a system resource which was in use at the time 181in a manner which would have conflicted with the request. 182.It Er 17 EEXIST Em "File exists" . 183An existing file was mentioned in an inappropriate context, 184for instance, as the new link name in a 185.Xr link 2 186function. 187.It Er 18 EXDEV Em "Improper link" . 188A hard link to a file on another file system 189was attempted. 190.It Er 19 ENODEV Em "Operation not supported by device" . 191An attempt was made to apply an inappropriate 192function to a device, 193for example, 194trying to read a write-only device such as a printer. 195.It Er 20 ENOTDIR Em "Not a directory" . 196A component of the specified pathname existed, but it was 197not a directory, when a directory was expected. 198.It Er 21 EISDIR Em "Is a directory" . 199An attempt was made to open a directory with write mode specified. 200.It Er 22 EINVAL Em "Invalid argument" . 201Some invalid argument was supplied. 202(For example, specifying an undefined signal to a 203.Xr signal 3 204or 205.Xr kill 2 206function). 207.It Er 23 ENFILE Em "Too many open files in system" . 208Maximum number of file descriptors allowable on the system 209has been reached and a requests for an open cannot be satisfied 210until at least one has been closed. 211.It Er 24 EMFILE Em "Too many open files" . 212\*[Lt]As released, the limit on the number of 213open files per process is 64.\*[Gt] 214The 215.Xr getrlimit 2 216call with the 217.Ar RLIMIT_NOFILE 218resource will obtain the current limit. 219.It Er 25 ENOTTY Em "Inappropriate ioctl for device" . 220A control function (see 221.Xr ioctl 2 ) 222was attempted for a file or 223special device for which the operation was inappropriate. 224.It Er 26 ETXTBSY Em "Text file busy" . 225The new process was a pure procedure (shared text) file 226which was open for writing by another process, or 227while the pure procedure file was being executed an 228.Xr open 2 229call requested write access. 230.It Er 27 EFBIG Em "File too large" . 231The size of a file exceeded the maximum. 232(The system-wide maximum file size is 233.if t 2\u\s-263\s+2\d 234.if n 2**63 235bytes. 236Each file system may impose a lower limit for files contained within it). 237.It Er 28 ENOSPC Em "Device out of space" . 238A 239.Xr write 2 240to an ordinary file, the creation of a 241directory or symbolic link, or the creation of a directory 242entry failed because no more disk blocks were available 243on the file system, or the allocation of an inode for a newly 244created file failed because no more inodes were available 245on the file system. 246.It Er 29 ESPIPE Em "Illegal seek" . 247An 248.Xr lseek 2 249function was issued on a socket, pipe or 250.Tn FIFO . 251.It Er 30 EROFS Em "Read-only file system" . 252An attempt was made to modify a file or directory 253was made 254on a file system that was read-only at the time. 255.It Er 31 EMLINK Em "Too many links" . 256The number of hard links to a single file has exceeded the maximum. 257(The system-wide maximum number of hard links is 32767. 258Each file system may impose a lower limit for files contained within it). 259.It Er 32 EPIPE Em "Broken pipe" . 260A write on a pipe, socket or 261.Tn FIFO 262for which there is no process 263to read the data. 264.It Er 33 EDOM Em "Numerical argument out of domain" . 265A numerical input argument was outside the defined domain of the mathematical 266function. 267.It Er 34 ERANGE Em "Result too large or too small" . 268The result of the function is too large or too small to be represented 269in the available space. 270.It Er 35 EAGAIN Em "Resource temporarily unavailable" . 271This is a temporary condition and later calls to the 272same routine may complete normally. 273.It Er 36 EINPROGRESS Em "Operation now in progress" . 274An operation that takes a long time to complete (such as 275a 276.Xr connect 2 ) 277was attempted on a non-blocking object (see 278.Xr fcntl 2 ) . 279.It Er 37 EALREADY Em "Operation already in progress" . 280An operation was attempted on a non-blocking object that already 281had an operation in progress. 282.It Er 38 ENOTSOCK Em "Socket operation on non-socket" . 283Self-explanatory. 284.It Er 39 EDESTADDRREQ Em "Destination address required" . 285A required address was omitted from an operation on a socket. 286.It Er 40 EMSGSIZE Em "Message too long" . 287A message sent on a socket was larger than the internal message buffer 288or some other network limit. 289.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" . 290A protocol was specified that does not support the semantics of the 291socket type requested. 292For example, you cannot use the 293.Tn ARPA 294Internet 295.Tn UDP 296protocol with type 297.Dv SOCK_STREAM . 298.It Er 42 ENOPROTOOPT Em "Protocol option not available" . 299A bad option or level was specified in a 300.Xr getsockopt 2 301or 302.Xr setsockopt 2 303call. 304.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" . 305The protocol has not been configured into the 306system or no implementation for it exists. 307.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" . 308The support for the socket type has not been configured into the 309system or no implementation for it exists. 310.It Er 45 EOPNOTSUPP Em "Operation not supported" . 311The attempted operation is not supported for the type of object referenced. 312Usually this occurs when a file descriptor refers to a file or socket 313that cannot support this operation, 314for example, trying to 315.Em accept 316a connection on a datagram socket. 317.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" . 318The protocol family has not been configured into the 319system or no implementation for it exists. 320.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" . 321An address incompatible with the requested protocol was used. 322For example, you shouldn't necessarily expect to be able to use 323.Tn NS 324addresses with 325.Tn ARPA 326Internet protocols. 327.It Er 48 EADDRINUSE Em "Address already in use" . 328Only one usage of each address is normally permitted. 329.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" . 330Normally results from an attempt to create a socket with an 331address not on this machine. 332.It Er 50 ENETDOWN Em "Network is down" . 333A socket operation encountered a dead network. 334.It Er 51 ENETUNREACH Em "Network is unreachable" . 335A socket operation was attempted to an unreachable network. 336.It Er 52 ENETRESET Em "Network dropped connection on reset" . 337The host you were connected to crashed and rebooted. 338.It Er 53 ECONNABORTED Em "Software caused connection abort" . 339A connection abort was caused internal to your host machine. 340.It Er 54 ECONNRESET Em "Connection reset by peer" . 341A connection was forcibly closed by a peer. 342This normally results from a loss of the connection on the remote 343socket due to a timeout or a reboot. 344.It Er 55 ENOBUFS Em "\&No buffer space available" . 345An operation on a socket or pipe was not performed because 346the system lacked sufficient buffer space or because a queue was full. 347.It Er 56 EISCONN Em "Socket is already connected" . 348A 349.Xr connect 2 350request was made on an already connected socket; or, 351a 352.Xr sendto 2 353or 354.Xr sendmsg 2 355request on a connected socket specified a destination 356when already connected. 357.It Er 57 ENOTCONN Em "Socket is not connected" . 358An request to send or receive data was disallowed because 359the socket was not connected and (when sending on a datagram socket) 360no address was supplied. 361.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" . 362A request to send data was disallowed because the socket 363had already been shut down with a previous 364.Xr shutdown 2 365call. 366.It Er 59 ETOOMANYREFS Em "Too many references: can't splice" . 367The resource is used up to capacity. 368.It Er 60 ETIMEDOUT Em "Operation timed out" . 369A 370.Xr connect 2 371or 372.Xr send 2 373request failed because the connected party did not 374properly respond after a period of time. 375(The timeout period is dependent on the communication protocol). 376.It Er 61 ECONNREFUSED Em "Connection refused" . 377No connection could be made because the target machine actively 378refused it. 379This usually results from trying to connect 380to a service that is inactive on the foreign host. 381.It Er 62 ELOOP Em "Too many levels of symbolic links" . 382A path name lookup involved more than 32 383.Pq Dv MAXSYMLINKS 384symbolic links. 385.It Er 63 ENAMETOOLONG Em "File name too long" . 386A component of a path name exceeded 255 387.Pq Dv MAXNAMELEN 388characters, or an entire 389path name exceeded 1023 390.Pq Dv MAXPATHLEN Ns \-1 391characters. 392.It Er 64 EHOSTDOWN Em "Host is down" . 393A socket operation failed because the destination host was down. 394.It Er 65 EHOSTUNREACH Em "No route to host" . 395A socket operation was attempted to an unreachable host. 396.It Er 66 ENOTEMPTY Em "Directory not empty" . 397A directory with entries other than 398.Ql \&. 399and 400.Ql \&.. 401was supplied to a remove directory or rename call. 402.It Er 67 EPROCLIM Em "Too many processes" . 403.It Er 68 EUSERS Em "Too many users" . 404The quota system ran out of table entries. 405.It Er 69 EDQUOT Em "Disc quota exceeded" . 406A 407.Xr write 2 408to an ordinary file, the creation of a 409directory or symbolic link, or the creation of a directory 410entry failed because the user's quota of disk blocks was 411exhausted, or the allocation of an inode for a newly 412created file failed because the user's quota of inodes 413was exhausted. 414.It Er 70 ESTALE Em "Stale NFS file handle" . 415An attempt was made to access an open file (on an 416.Tn NFS 417filesystem) 418which is now unavailable as referenced by the file descriptor. 419This may indicate the file was deleted on the 420.Tn NFS 421server or some other catastrophic event occurred. 422.It Er 71 EREMOTE Em "Too many levels of remote in path" . 423.Tn NFS 424version 3 RPC return code 71. 425.It Er 72 EBADRPC Em "RPC struct is bad" . 426Exchange of 427.Tn RPC 428information was unsuccessful. 429.It Er 73 ERPCMISMATCH Em "RPC version wrong" . 430The version of 431.Tn RPC 432on the remote peer is not compatible with 433the local version. 434.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" . 435The requested program is not registered on the remote host. 436.It Er 75 EPROGMISMATCH Em "Program version wrong" . 437The requested version of the program is not available 438on the remote host 439.Pq Tn RPC . 440.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" . 441An 442.Tn RPC 443call was attempted for a procedure which doesn't exist 444in the remote program. 445.It Er 77 ENOLCK Em "No locks available" . 446A system-imposed limit on the number of simultaneous file 447locks was reached. 448.It Er 78 ENOSYS Em "Function not implemented" . 449Attempted a system call that is not available on this 450system. 451.It Er 79 EFTYPE Em "Inappropriate file type or format" . 452Attempted a file operation on a file of a type for which it was invalid. 453.It Er 80 EAUTH Em "Authentication error" . 454Attempted to use an invalid authentication ticket to mount an 455.Tn NFS 456filesystem. 457.It Er 81 ENEEDAUTH Em "Need authenticator" . 458An authentication ticket must be obtained before the given 459.Tn NFS 460filesystem may be mounted. 461.It Er 82 EIDRM Em "Identifier removed" . 462An IPC identifier was removed while the current process was waiting on it. 463.It Er 83 ENOMSG Em "No message of the desired type" . 464An IPC message queue does not contain a message of the desired type, 465or a message catalog does not contain the requested message. 466.It Er 84 EOVERFLOW Em "Value too large to be stored in data type" . 467A numerical result of the function was too large to be stored in the 468caller-provided space. 469.It Er 85 EILSEQ Em "Illegal byte sequence" . 470A wide character/multibyte character encoding error occurred. 471.It Er 86 ENOTSUP Em "Not supported" . 472An attempt was made to set or change a parameter to an unsupported 473value. 474.It Er 87 ECANCELED Em "Operation canceled" . 475The requested operation was canceled. 476.It Er 88 EBADMSG Em "Bad or corrupt message" . 477A message in the specified message catalog did not satisfy implementation 478defined criteria, or a STREAMS operation encountered an invalid message or 479a file descriptor at the STREAM head. 480.It Er 89 ENODATA Em "No message available" . 481No message is available on the STREAM head read queue 482.It Er 90 ENOSR Em "No STREAM resources" . 483Buffers could not be allocated due to insufficient STREAMs memory resources. 484.It Er 91 ENOSTR Em "Not a STREAM" . 485A STREAM is not associated with the specified file descriptor. 486.It Er 92 ETIME Em "STREAM ioctl timeout" . 487The timer set for a STREAMS 488.Xr ioctl 2 489operation has expired. 490.It Er 93 ENOATTR Em "Attribute not found" . 491The specified extended attribute does not exist. 492.It Er 94 EMULTIHOP Em "Multihop attempted" . 493Components of path require hopping to multiple remote machines and the 494file system does not allow it. 495It occurs when users try to access remote resources which are not directly 496accessible. 497.It Er 95 ENOLINK Em "Link has been severed" . 498Occurs when the link (virtual circuit) connecting to a remote machine is gone. 499.It Er 96 EPROTO Em "Protocol error" . 500Some protocol error occurred. 501This error is device-specific, but is generally not related to a hardware 502failure. 503.El 504.Sh DEFINITIONS 505.Bl -tag -width Ds 506.It Process ID 507Each active process in the system is uniquely identified by a non-negative 508integer called a process ID. 509The range of this ID is from 0 to 30000. 510.It Parent process ID 511A new process is created by a currently active process; (see 512.Xr fork 2 ) . 513The parent process ID of a process is initially the process ID of its creator. 514If the creating process exits, 515the parent process ID of each child is set to the ID of a system process, 516.Xr init 8 . 517.It Process Group 518Each active process is a member of a process group that is identified by 519a non-negative integer called the process group ID. 520This is the process ID of the group leader. 521This grouping permits the signaling of related processes (see 522.Xr termios 4 ) 523and the job control mechanisms of 524.Xr csh 1 . 525.It Session 526A session is a set of one or more process groups. 527A session is created by a successful call to 528.Xr setsid 2 , 529which causes the caller to become the only member of the only process 530group in the new session. 531.It Session leader 532A process that has created a new session by a successful call to 533.Xr setsid 2 , 534is known as a session leader. 535Only a session leader may acquire a terminal as its controlling terminal (see 536.Xr termios 4 ) . 537.It Controlling process 538A session leader with a controlling terminal is a controlling process. 539.It Controlling terminal 540A terminal that is associated with a session is known as the controlling 541terminal for that session and its members. 542.It "Terminal Process Group ID" 543A terminal may be acquired by a session leader as its controlling terminal. 544Once a terminal is associated with a session, any of the process groups 545within the session may be placed into the foreground by setting 546the terminal process group ID to the ID of the process group. 547This facility is used 548to arbitrate between multiple jobs contending for the same terminal. 549(See 550.Xr csh 1 551and 552.Xr tty 4 553for more information on job control.) 554.It "Orphaned Process Group" 555A process group is considered to be 556.Em orphaned 557if it is not under the control of a job control shell. 558More precisely, a process group is orphaned 559when none of its members has a parent process that is in the same session 560as the group, 561but is in a different process group. 562Note that when a process exits, the parent process for its children 563is changed to be 564.Xr init 8 , 565which is in a separate session. 566Not all members of an orphaned process group are necessarily orphaned 567processes (those whose creating process has exited). 568The process group of a session leader is orphaned by definition. 569.It "Real User ID and Real Group ID" 570Each user on the system is identified by a positive integer 571termed the real user ID. 572.Pp 573Each user is also a member of one or more groups. 574One of these groups is distinguished from others and 575used in implementing accounting facilities. 576The positive integer corresponding to this distinguished group is 577termed the real group ID. 578.Pp 579All processes have a real user ID and real group ID. 580These are initialized from the equivalent attributes 581of the process that created it. 582.It "Effective User Id, Effective Group Id, and Group Access List" 583Access to system resources is governed by two values: 584the effective user ID, and the group access list. 585The first member of the group access list is also known as the 586effective group ID. 587(In POSIX.1, the group access list is known as the set of supplementary 588group IDs, and it is unspecified whether the effective group ID is 589a member of the list.) 590.Pp 591The effective user ID and effective group ID are initially the 592process's real user ID and real group ID respectively. 593Either may be modified through execution of a set-user-ID or 594set-group-ID file (possibly by one of its ancestors) (see 595.Xr execve 2 ) . 596By convention, the effective group ID (the first member of the group access 597list) is duplicated, so that the execution of a set-group-ID program 598does not result in the loss of the original (real) group ID. 599.Pp 600The group access list is a set of group IDs 601used only in determining resource accessibility. 602Access checks are performed as described below in 603.Qq File Access Permissions . 604.It "Saved Set User ID and Saved Set Group ID" 605When a process executes a new file, the effective user ID is set 606to the owner of the file if the file is set-user-ID, and the effective 607group ID (first element of the group access list) is set to the group 608of the file if the file is set-group-ID. 609The effective user ID of the process is then recorded as the saved set-user-ID, 610and the effective group ID of the process is recorded as the saved set-group-ID. 611These values may be used to regain those values as the effective user 612or group ID after reverting to the real ID (see 613.Xr setuid 2 ) . 614(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, 615and are used in setuid and setgid, but this does not work as desired 616for the super-user.) 617.It Super-user 618A process is recognized as a 619.Em super-user 620process and is granted special privileges if its effective user ID is 0. 621.It Special Processes 622The processes with process IDs of 0 and 1 are special. 623Process 0 is the scheduler. 624Process 1 is the initialization process 625.Xr init 8 , 626and is the ancestor (parent) of every other process in the system. 627It is used to control the process structure. 628The kernel will allocate other kernel threads to handle certain 629periodic tasks or device related tasks, such as: 630.Bl -tag -width pagedaemon 631.It Sy acctwatch 632System accounting disk watcher, see 633.Xr acct 2 , 634.Xr acct 5 . 635.It Sy aiodoned 636Asynchronous I/O done handler, see 637.Xr uvm 9 . 638.It Sy atabusX 639ATA bus handler, see 640.Xr ata 4 . 641.It Sy cardslotX 642CardBus slot watcher thread, see 643.Xr cardslot 4 . 644.It Sy cryptoret 645The software crypto daemon. 646.It Sy fssbsX 647File system snapshot thread, see 648.Xr fss 4 . 649.It Sy ioflush 650The in-kernel periodic flush the buffer cache to disk task, 651which replaces the old 652.Sy update 653program. 654.It Sy nfsio , nfskqpoll 655NFS handing daemons. 656.It Sy lfs_writer 657Log filesystem writer. 658.It Sy pagedaemon 659The page daemon. 660.It Sy raidX , raidioX , raid_parity , raid_recon , raid_reconip , raid_copyback 661Raid framework related threads, see 662.Xr raid 4 . 663.It Sy scsibusX 664SCSI bus handler, see 665.Xr scsi 4 . 666.It Sy smbiodX , smbkq 667SMBFS handling daemon, see 668.Xr netsmb 4 . 669.It Sy swdmover 670The software data mover I/O thread, see 671.Xr dmoverio 4 . 672.It Sy sysmon 673The systems monitoring framework daemon. 674.It Sy usbX , usbtask 675USB bus handler, see 676.Xr usb 4 . 677.El 678.Pp 679There are more machine-dependent kernel threads allocated by 680different drivers. 681See the specific driver manual pages for more information. 682.It Descriptor 683An integer assigned by the system when a file is referenced 684by 685.Xr open 2 686or 687.Xr dup 2 , 688or when a socket is created by 689.Xr pipe 2 , 690.Xr socket 2 , 691or 692.Xr socketpair 2 , 693which uniquely identifies an access path to that file or socket from 694a given process or any of its children. 695.It File Name 696Names consisting of up to 255 697.Pq Dv MAXNAMELEN 698characters may be used to name 699an ordinary file, special file, or directory. 700.Pp 701These characters may be selected from the set of all 702.Tn ASCII 703character 704excluding 0 (NUL) and the 705.Tn ASCII 706code for 707.Ql \&/ 708(slash). 709(The parity bit, bit 7, must be 0). 710.Pp 711Note that it is generally unwise to use 712.Ql \&* , 713.Ql \&? , 714.Ql \&[ 715or 716.Ql \&] 717as part of 718file names because of the special meaning attached to these characters 719by the shell. 720.It Pathname 721A path name is a 722.Tn NUL Ns -terminated 723character string starting with an 724optional slash 725.Ql \&/ , 726followed by zero or more directory names separated 727by slashes, optionally followed by a file name. 728The total length of a path name must be less than 1024 729.Pq Dv MAXPATHLEN 730characters. 731.Pp 732If a path name begins with a slash, the path search begins at the 733.Em root 734directory. 735Otherwise, the search begins from the current working directory. 736A slash by itself names the root directory. 737An empty string is not a valid pathname. 738.It Directory 739A directory is a special type of file that contains entries 740that are references to other files. 741Directory entries are called links. 742By convention, a directory contains at least two links, 743.Ql \&. 744and 745.Ql \&.. , 746referred to as 747.Em dot 748and 749.Em dot-dot 750respectively. 751Dot refers to the directory itself and dot-dot refers to its parent directory. 752.It "Root Directory and Current Working Directory" 753Each process has associated with it a concept of a root directory 754and a current working directory for the purpose of resolving path 755name searches. 756A process's root directory need not be the root 757directory of the root file system. 758.It File Access Permissions 759Every file in the file system has a set of access permissions. 760These permissions are used in determining whether a process 761may perform a requested operation on the file (such as opening 762a file for writing). 763Access permissions are established at the time a file is created. 764They may be changed at some later time through the 765.Xr chmod 2 766call. 767.Pp 768File access is broken down according to whether a file may be: read, 769written, or executed. 770Directory files use the execute permission to control if the 771directory may be searched. 772.Pp 773File access permissions are interpreted by the system as 774they apply to three different classes of users: the owner 775of the file, those users in the file's group, anyone else. 776Every file has an independent set of access permissions for 777each of these classes. 778When an access check is made, the system decides if permission should be 779granted by checking the access information applicable to the caller. 780.Pp 781Read, write, and execute/search permissions on 782a file are granted to a process if: 783.Pp 784The process's effective user ID is that of the super-user. 785(Note: even the super-user cannot execute a non-executable file). 786.Pp 787The process's effective user ID matches the user ID of the owner 788of the file and the owner permissions allow the access. 789.Pp 790The process's effective user ID does not match the user ID of the 791owner of the file, and either the process's effective 792group ID matches the group ID 793of the file, or the group ID of the file is in 794the process's group access list, 795and the group permissions allow the access. 796.Pp 797Neither the effective user ID nor effective group ID 798and group access list of the process 799match the corresponding user ID and group ID of the file, 800but the permissions for ``other users'' allow access. 801.Pp 802Otherwise, permission is denied. 803.It Sockets and Address Families 804A socket is an endpoint for communication between processes. 805Each socket has queues for sending and receiving data. 806.Pp 807Sockets are typed according to their communications properties. 808These properties include whether messages sent and received 809at a socket require the name of the partner, whether communication 810is reliable, the format used in naming message recipients, etc. 811.Pp 812Each instance of the system supports some 813collection of socket types; consult 814.Xr socket 2 815for more information about the types available and 816their properties. 817.Pp 818Each instance of the system supports some number of sets of 819communications protocols. 820Each protocol set supports addresses of a certain format. 821An Address Family is the set of addresses for a specific group of protocols. 822Each socket has an address 823chosen from the address family in which the socket was created. 824.El 825.Sh SEE ALSO 826.Xr intro 3 , 827.Xr perror 3 828.Sh HISTORY 829An 830.Nm intro 831manual page appeared in 832.At v6 . 833