1.\" $NetBSD: intro.2,v 1.27 2002/02/08 01:28:18 ross 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. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)intro.2 8.5 (Berkeley) 2/27/95 35.\" 36.Dd March 14, 2000 37.Dt INTRO 2 38.Os 39.Sh NAME 40.Nm intro , 41.Nm errno 42.Nd introduction to system calls and error numbers 43.Sh SYNOPSIS 44.Fd #include \*[Lt]errno.h\*[Gt] 45.Sh DESCRIPTION 46This section provides an overview of the system calls, 47their error returns, and other common definitions and concepts. 48.\".Pp 49.\".Sy System call restart 50.\".Pp 51.\"<more later...> 52.Sh DIAGNOSTICS 53Nearly all of the system calls provide an error number in the external 54variable 55.Va errno . 56.Va errno 57is implemented as a macro which expands to a modifiable lvalue of type 58.Fa int . 59.Pp 60When a system call detects an error, 61it returns an integer value 62indicating failure (usually -1) 63and sets the variable 64.Va errno 65accordingly. 66\*[Lt]This allows interpretation of the failure on receiving 67a -1 and to take action accordingly.\*[Gt] 68Successful calls never set 69.Va errno ; 70once set, it remains until another error occurs. 71It should only be examined after an error. 72Note that a number of system calls overload the meanings of these 73error numbers, and that the meanings must be interpreted according 74to the type and circumstances of the call. 75.Pp 76The manual page for each system call will list some of the common 77errno codes that system call can return, but that should not be 78considered an exhaustive list, i.e. 79a properly written program should be able to gracefully recover from 80any error that a system call might return. 81Documenting all the error codes that a system call can return in 82a more specification-like manner would take more resources than 83this project has available. 84.Pp 85The following is a complete list of the errors and their 86names as given in 87.Aq Pa errno.h . 88.Bl -hang -width Ds 89.It Er 0 Em "Error 0" . 90Not used. 91.It Er 1 EPERM Em "Operation not permitted" . 92An attempt was made to perform an operation limited to processes 93with appropriate privileges or to the owner of a file or other 94resources. 95.It Er 2 ENOENT Em "No such file or directory" . 96A component of a specified pathname did not exist, or the 97pathname was an empty string. 98.It Er 3 ESRCH Em "No such process" . 99No process could be found corresponding to that specified by the given 100process ID. 101.It Er 4 EINTR Em "Interrupted function call" . 102An asynchronous signal (such as 103.Dv SIGINT 104or 105.Dv SIGQUIT ) 106was caught by the process during the execution of an interruptible 107function. 108If the signal handler performs a normal return, the 109interrupted function call will seem to have returned the error condition. 110.It Er 5 EIO Em "Input/output error" . 111Some physical input or output error occurred. 112This error will not be reported until a subsequent operation on the same file 113descriptor and may be lost (over written) by any subsequent errors. 114.It Er 6 ENXIO Em "\&No such device or address" . 115Input or output on a special file referred to a device that did not 116exist, or 117made a request beyond the limits of the device. 118This error may also occur when, for example, 119a tape drive is not online or no disk pack is 120loaded on a drive. 121.It Er 7 E2BIG Em "Arg list too long" . 122The number of bytes used for the argument and environment 123list of the new process exceeded the current limit of 124.if t 2\u\s-218\s+2\d 125.if n 2**18 126bytes 127.Pf ( Dv ARG_MAX 128in 129.Aq Pa sys/syslimits.h ) . 130.It Er 8 ENOEXEC Em "Exec format error" . 131A request was made to execute a file 132that, although it has the appropriate permissions, 133was not in the format required for an 134executable file. 135.It Er 9 EBADF Em "Bad file descriptor" . 136A file descriptor argument was out of range, referred to no open file, 137had been revoked by 138.Xr revoke 2 , 139or a 140.Xr read 2 141(or 142.Xr write 2 ) 143request was made to a file that was 144only open for writing (or reading). 145.It Er 10 ECHILD Em "\&No child processes" . 146A 147.Xr wait 2 148or 149.Xr waitpid 2 150function was executed by a process that had no existing or unwaited-for 151child processes. 152.It Er 11 EDEADLK Em "Resource deadlock avoided" . 153An attempt was made to lock a system resource that 154would have resulted in a deadlock situation. 155.It Er 12 ENOMEM Em "Cannot allocate memory" . 156The new process image required more memory than was allowed by the hardware 157or by system-imposed memory management constraints. 158A lack of swap space is normally temporary; however, 159a lack of core is not. 160Soft limits may be increased to their corresponding hard limits. 161.It Er 13 EACCES Em "Permission denied" . 162An attempt was made to access a file in a way forbidden 163by its file access permissions. 164.It Er 14 EFAULT Em "Bad address" . 165The system detected an invalid address in attempting to 166use an argument of a call. 167The reliable detection of this error cannot be guaranteed and when not detected 168may result in the generation of a signal, indicating an address violation, 169which is sent to the process. 170.It Er 15 ENOTBLK Em "Not a block device" . 171A block device operation was attempted on a non-block device or file. 172.It Er 16 EBUSY Em "Resource busy" . 173An attempt to use a system resource which was in use at the time 174in a manner which would have conflicted with the request. 175.It Er 17 EEXIST Em "File exists" . 176An existing file was mentioned in an inappropriate context, 177for instance, as the new link name in a 178.Xr link 2 179function. 180.It Er 18 EXDEV Em "Improper link" . 181A hard link to a file on another file system 182was attempted. 183.It Er 19 ENODEV Em "Operation not supported by device" . 184An attempt was made to apply an inappropriate 185function to a device, 186for example, 187trying to read a write-only device such as a printer. 188.It Er 20 ENOTDIR Em "Not a directory" . 189A component of the specified pathname existed, but it was 190not a directory, when a directory was expected. 191.It Er 21 EISDIR Em "Is a directory" . 192An attempt was made to open a directory with write mode specified. 193.It Er 22 EINVAL Em "Invalid argument" . 194Some invalid argument was supplied. 195(For example, specifying an undefined signal to a 196.Xr signal 3 197or 198.Xr kill 2 199function). 200.It Er 23 ENFILE Em "Too many open files in system" . 201Maximum number of file descriptors allowable on the system 202has been reached and a requests for an open cannot be satisfied 203until at least one has been closed. 204.It Er 24 EMFILE Em "Too many open files" . 205\*[Lt]As released, the limit on the number of 206open files per process is 64.\*[Gt] 207The 208.Xr getrlimit 2 209call with the 210.Ar RLIMIT_NOFILE 211resource will obtain the current limit. 212.It Er 25 ENOTTY Em "Inappropriate ioctl for device" . 213A control function (see 214.Xr ioctl 2 ) 215was attempted for a file or 216special device for which the operation was inappropriate. 217.It Er 26 ETXTBSY Em "Text file busy" . 218The new process was a pure procedure (shared text) file 219which was open for writing by another process, or 220while the pure procedure file was being executed an 221.Xr open 2 222call requested write access. 223.It Er 27 EFBIG Em "File too large" . 224The size of a file exceeded the maximum. 225(The system-wide maximum file size is 226.if t 2\u\s-263\s+2\d 227.if n 2**63 228bytes. 229Each file system may impose a lower limit for files contained within it). 230.It Er 28 ENOSPC Em "Device out of space" . 231A 232.Xr write 2 233to an ordinary file, the creation of a 234directory or symbolic link, or the creation of a directory 235entry failed because no more disk blocks were available 236on the file system, or the allocation of an inode for a newly 237created file failed because no more inodes were available 238on the file system. 239.It Er 29 ESPIPE Em "Illegal seek" . 240An 241.Xr lseek 2 242function was issued on a socket, pipe or 243.Tn FIFO . 244.It Er 30 EROFS Em "Read-only file system" . 245An attempt was made to modify a file or directory 246was made 247on a file system that was read-only at the time. 248.It Er 31 EMLINK Em "Too many links" . 249The number of hard links to a single file has exceeded the maximum. 250(The system-wide maximum number of hard links is 32767. 251Each file system may impose a lower limit for files contained within it). 252.It Er 32 EPIPE Em "Broken pipe" . 253A write on a pipe, socket or 254.Tn FIFO 255for which there is no process 256to read the data. 257.It Er 33 EDOM Em "Numerical argument out of domain" . 258A numerical input argument was outside the defined domain of the mathematical 259function. 260.It Er 34 ERANGE Em "Numerical result out of range" . 261A numerical result of the function was too large to fit in the 262available space (perhaps exceeded precision). 263.It Er 35 EAGAIN Em "Resource temporarily unavailable" . 264This is a temporary condition and later calls to the 265same routine may complete normally. 266.It Er 36 EINPROGRESS Em "Operation now in progress" . 267An operation that takes a long time to complete (such as 268a 269.Xr connect 2 ) 270was attempted on a non-blocking object (see 271.Xr fcntl 2 ) . 272.It Er 37 EALREADY Em "Operation already in progress" . 273An operation was attempted on a non-blocking object that already 274had an operation in progress. 275.It Er 38 ENOTSOCK Em "Socket operation on non-socket" . 276Self-explanatory. 277.It Er 39 EDESTADDRREQ Em "Destination address required" . 278A required address was omitted from an operation on a socket. 279.It Er 40 EMSGSIZE Em "Message too long" . 280A message sent on a socket was larger than the internal message buffer 281or some other network limit. 282.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" . 283A protocol was specified that does not support the semantics of the 284socket type requested. 285For example, you cannot use the 286.Tn ARPA 287Internet 288.Tn UDP 289protocol with type 290.Dv SOCK_STREAM . 291.It Er 42 ENOPROTOOPT Em "Protocol not available" . 292A bad option or level was specified in a 293.Xr getsockopt 2 294or 295.Xr setsockopt 2 296call. 297.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" . 298The protocol has not been configured into the 299system or no implementation for it exists. 300.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" . 301The support for the socket type has not been configured into the 302system or no implementation for it exists. 303.It Er 45 EOPNOTSUPP Em "Operation not supported" . 304The attempted operation is not supported for the type of object referenced. 305Usually this occurs when a file descriptor refers to a file or socket 306that cannot support this operation, 307for example, trying to 308.Em accept 309a connection on a datagram socket. 310.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" . 311The protocol family has not been configured into the 312system or no implementation for it exists. 313.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" . 314An address incompatible with the requested protocol was used. 315For example, you shouldn't necessarily expect to be able to use 316.Tn NS 317addresses with 318.Tn ARPA 319Internet protocols. 320.It Er 48 EADDRINUSE Em "Address already in use" . 321Only one usage of each address is normally permitted. 322.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" . 323Normally results from an attempt to create a socket with an 324address not on this machine. 325.It Er 50 ENETDOWN Em "Network is down" . 326A socket operation encountered a dead network. 327.It Er 51 ENETUNREACH Em "Network is unreachable" . 328A socket operation was attempted to an unreachable network. 329.It Er 52 ENETRESET Em "Network dropped connection on reset" . 330The host you were connected to crashed and rebooted. 331.It Er 53 ECONNABORTED Em "Software caused connection abort" . 332A connection abort was caused internal to your host machine. 333.It Er 54 ECONNRESET Em "Connection reset by peer" . 334A connection was forcibly closed by a peer. 335This normally results from a loss of the connection on the remote 336socket due to a timeout or a reboot. 337.It Er 55 ENOBUFS Em "\&No buffer space available" . 338An operation on a socket or pipe was not performed because 339the system lacked sufficient buffer space or because a queue was full. 340.It Er 56 EISCONN Em "Socket is already connected" . 341A 342.Xr connect 2 343request was made on an already connected socket; or, 344a 345.Xr sendto 2 346or 347.Xr sendmsg 2 348request on a connected socket specified a destination 349when already connected. 350.It Er 57 ENOTCONN Em "Socket is not connected" . 351An request to send or receive data was disallowed because 352the socket was not connected and (when sending on a datagram socket) 353no address was supplied. 354.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" . 355A request to send data was disallowed because the socket 356had already been shut down with a previous 357.Xr shutdown 2 358call. 359.It Er 60 ETIMEDOUT Em "Operation timed out" . 360A 361.Xr connect 2 362or 363.Xr send 2 364request failed because the connected party did not 365properly respond after a period of time. 366(The timeout period is dependent on the communication protocol). 367.It Er 61 ECONNREFUSED Em "Connection refused" . 368No connection could be made because the target machine actively 369refused it. 370This usually results from trying to connect 371to a service that is inactive on the foreign host. 372.It Er 62 ELOOP Em "Too many levels of symbolic links" . 373A path name lookup involved more than 8 symbolic links. 374.It Er 63 ENAMETOOLONG Em "File name too long" . 375A component of a path name exceeded 255 376.Pq Dv MAXNAMELEN 377characters, or an entire 378path name exceeded 1023 379.Pq Dv MAXPATHLEN Ns -1 380characters. 381.It Er 64 EHOSTDOWN Em "Host is down" . 382A socket operation failed because the destination host was down. 383.It Er 65 EHOSTUNREACH Em "No route to host" . 384A socket operation was attempted to an unreachable host. 385.It Er 66 ENOTEMPTY Em "Directory not empty" . 386A directory with entries other than 387.Ql \&. 388and 389.Ql \&.. 390was 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" . 393The quota system ran out of table entries. 394.It Er 69 EDQUOT Em "Disc quota exceeded" . 395A 396.Xr write 2 397to an ordinary file, the creation of a 398directory or symbolic link, or the creation of a directory 399entry failed because the user's quota of disk blocks was 400exhausted, or the allocation of an inode for a newly 401created file failed because the user's quota of inodes 402was exhausted. 403.It Er 70 ESTALE Em "Stale NFS file handle" . 404An attempt was made to access an open file (on an 405.Tn NFS 406filesystem) 407which is now unavailable as referenced by the file descriptor. 408This may indicate the file was deleted on the 409.Tn NFS 410server or some other catastrophic event occurred. 411.It Er 72 EBADRPC Em "RPC struct is bad" . 412Exchange of 413.Tn RPC 414information was unsuccessful. 415.It Er 73 ERPCMISMATCH Em "RPC version wrong" . 416The version of 417.Tn RPC 418on the remote peer is not compatible with 419the local version. 420.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" . 421The requested program is not registered on the remote host. 422.It Er 75 EPROGMISMATCH Em "Program version wrong" . 423The requested version of the program is not available 424on the remote host 425.Pq Tn RPC . 426.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" . 427An 428.Tn RPC 429call was attempted for a procedure which doesn't exist 430in the remote program. 431.It Er 77 ENOLCK Em "No locks available" . 432A system-imposed limit on the number of simultaneous file 433locks was reached. 434.It Er 78 ENOSYS Em "Function not implemented" . 435Attempted a system call that is not available on this 436system. 437.It Er 79 EFTYPE Em "Inappropriate file type or format" . 438Attempted a file operation on a file of a type for which it was invalid. 439.It Er 80 EAUTH Em "Authentication error" . 440Attempted to use an invalid authentication ticket to mount an 441.Tn NFS 442filesystem. 443.It Er 81 ENEEDAUTH Em "Need authenticator" . 444An authentication ticket must be obtained before the given 445.Tn NFS 446filesystem may be mounted. 447.It Er 82 EIDRM Em "Identifier removed" . 448An IPC identifier was removed while the current process was waiting on it. 449.It Er 83 ENOMSG Em "No message of the desired type" . 450An IPC message queue does not contain a message of the desired type, 451or a message catalog does not contain the requested message. 452.It Er 84 EOVERFLOW Em "Value too large to be stored in data type" . 453A numerical result of the function was too large to be stored in the 454caller-provided space. 455.It Er 85 EILSEQ Em "Illegal byte sequence" . 456A wide character/multibyte character encoding error occurred. 457.El 458.Sh DEFINITIONS 459.Bl -tag -width Ds 460.It Process ID . 461Each active process in the system is uniquely identified by a non-negative 462integer called a process ID. 463The range of this ID is from 0 to 30000. 464.It Parent process ID 465A new process is created by a currently active process; (see 466.Xr fork 2 ) . 467The parent process ID of a process is initially the process ID of its creator. 468If the creating process exits, 469the parent process ID of each child is set to the ID of a system process, 470.Xr init 8 . 471.It Process Group 472Each active process is a member of a process group that is identified by 473a non-negative integer called the process group ID. 474This is the process ID of the group leader. 475This grouping permits the signaling of related processes (see 476.Xr termios 4 ) 477and the job control mechanisms of 478.Xr csh 1 . 479.It Session 480A session is a set of one or more process groups. 481A session is created by a successful call to 482.Xr setsid 2 , 483which causes the caller to become the only member of the only process 484group in the new session. 485.It Session leader 486A process that has created a new session by a successful call to 487.Xr setsid 2 , 488is known as a session leader. 489Only a session leader may acquire a terminal as its controlling terminal (see 490.Xr termios 4 ) . 491.It Controlling process 492A session leader with a controlling terminal is a controlling process. 493.It Controlling terminal 494A terminal that is associated with a session is known as the controlling 495terminal for that session and its members. 496.It "Terminal Process Group ID" 497A terminal may be acquired by a session leader as its controlling terminal. 498Once a terminal is associated with a session, any of the process groups 499within the session may be placed into the foreground by setting 500the terminal process group ID to the ID of the process group. 501This facility is used 502to arbitrate between multiple jobs contending for the same terminal. 503(See 504.Xr csh 1 505and 506.Xr tty 4 507for more information on job control.) 508.It "Orphaned Process Group" 509A process group is considered to be 510.Em orphaned 511if it is not under the control of a job control shell. 512More precisely, a process group is orphaned 513when none of its members has a parent process that is in the same session 514as the group, 515but is in a different process group. 516Note that when a process exits, the parent process for its children 517is changed to be 518.Xr init 8 , 519which is in a separate session. 520Not all members of an orphaned process group are necessarily orphaned 521processes (those whose creating process has exited). 522The process group of a session leader is orphaned by definition. 523.It "Real User ID and Real Group ID" 524Each user on the system is identified by a positive integer 525termed the real user ID. 526.Pp 527Each user is also a member of one or more groups. 528One of these groups is distinguished from others and 529used in implementing accounting facilities. 530The positive integer corresponding to this distinguished group is 531termed the real group ID. 532.Pp 533All processes have a real user ID and real group ID. 534These are initialized from the equivalent attributes 535of the process that created it. 536.It "Effective User Id, Effective Group Id, and Group Access List" 537Access to system resources is governed by two values: 538the effective user ID, and the group access list. 539The first member of the group access list is also known as the 540effective group ID. 541(In POSIX.1, the group access list is known as the set of supplementary 542group IDs, and it is unspecified whether the effective group ID is 543a member of the list.) 544.Pp 545The effective user ID and effective group ID are initially the 546process's real user ID and real group ID respectively. 547Either may be modified through execution of a set-user-ID or 548set-group-ID file (possibly by one its ancestors) (see 549.Xr execve 2 ) . 550By convention, the effective group ID (the first member of the group access 551list) is duplicated, so that the execution of a set-group-ID program 552does not result in the loss of the original (real) group ID. 553.Pp 554The group access list is a set of group IDs 555used only in determining resource accessibility. 556Access checks are performed as described below in 557.Qq File Access Permissions . 558.It "Saved Set User ID and Saved Set Group ID" 559When a process executes a new file, the effective user ID is set 560to the owner of the file if the file is set-user-ID, and the effective 561group ID (first element of the group access list) is set to the group 562of the file if the file is set-group-ID. 563The effective user ID of the process is then recorded as the saved set-user-ID, 564and the effective group ID of the process is recorded as the saved set-group-ID. 565These values may be used to regain those values as the effective user 566or group ID after reverting to the real ID (see 567.Xr setuid 2 ) . 568(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, 569and are used in setuid and setgid, but this does not work as desired 570for the super-user.) 571.It Super-user 572A process is recognized as a 573.Em super-user 574process and is granted special privileges if its effective user ID is 0. 575.It Special Processes 576The processes with process IDs of 0, 1, 2, 3, and 4 are special. 577Process 0 is the scheduler. 578Process 1 is the initialization process 579.Xr init 8 , 580and is the ancestor (parent) of every other process in the system. 581It is used to control the process structure. 582Process 2 is the paging daemon. 583Process 3 is the 584.Sy reaper , 585which handles deallocating resources for exiting processes. 586Process 4 is 587.Sy ioflush , 588which replaces the old 589.Sy update 590program that periodically flushes the disk buffer cache. 591.It Descriptor 592An integer assigned by the system when a file is referenced 593by 594.Xr open 2 595or 596.Xr dup 2 , 597or when a socket is created by 598.Xr pipe 2 , 599.Xr socket 2 , 600or 601.Xr socketpair 2 , 602which uniquely identifies an access path to that file or socket from 603a given process or any of its children. 604.It File Name 605Names consisting of up to 255 606.Pq Dv MAXNAMELEN 607characters may be used to name 608an ordinary file, special file, or directory. 609.Pp 610These characters may be selected from the set of all 611.Tn ASCII 612character 613excluding 0 (NUL) and the 614.Tn ASCII 615code for 616.Ql \&/ 617(slash). 618(The parity bit, bit 7, must be 0). 619.Pp 620Note that it is generally unwise to use 621.Ql \&* , 622.Ql \&? , 623.Ql \&[ 624or 625.Ql \&] 626as part of 627file names because of the special meaning attached to these characters 628by the shell. 629.It Path Name 630A path name is a 631.Tn NUL Ns -terminated 632character string starting with an 633optional slash 634.Ql \&/ , 635followed by zero or more directory names separated 636by slashes, optionally followed by a file name. 637The total length of a path name must be less than 1024 638.Pq Dv MAXPATHLEN 639characters. 640.Pp 641If a path name begins with a slash, the path search begins at the 642.Em root 643directory. 644Otherwise, the search begins from the current working directory. 645A slash by itself names the root directory. 646An empty pathname refers to the current directory. 647.It Directory 648A directory is a special type of file that contains entries 649that are references to other files. 650Directory entries are called links. 651By convention, a directory contains at least two links, 652.Ql \&. 653and 654.Ql \&.. , 655referred to as 656.Em dot 657and 658.Em dot-dot 659respectively. 660Dot refers to the directory itself and dot-dot refers to its parent directory. 661.It "Root Directory and Current Working Directory" 662Each process has associated with it a concept of a root directory 663and a current working directory for the purpose of resolving path 664name searches. 665A process's root directory need not be the root 666directory of the root file system. 667.It File Access Permissions 668Every file in the file system has a set of access permissions. 669These permissions are used in determining whether a process 670may perform a requested operation on the file (such as opening 671a file for writing). 672Access permissions are established at the time a file is created. 673They may be changed at some later time through the 674.Xr chmod 2 675call. 676.Pp 677File access is broken down according to whether a file may be: read, 678written, or executed. 679Directory files use the execute permission to control if the 680directory may be searched. 681.Pp 682File access permissions are interpreted by the system as 683they apply to three different classes of users: the owner 684of the file, those users in the file's group, anyone else. 685Every file has an independent set of access permissions for 686each of these classes. 687When an access check is made, the system decides if permission should be 688granted by checking the access information applicable to the caller. 689.Pp 690Read, write, and execute/search permissions on 691a file are granted to a process if: 692.Pp 693The process's effective user ID is that of the super-user. 694(Note: even the super-user cannot execute a non-executable file). 695.Pp 696The process's effective user ID matches the user ID of the owner 697of the file and the owner permissions allow the access. 698.Pp 699The process's effective user ID does not match the user ID of the 700owner of the file, and either the process's effective 701group ID matches the group ID 702of the file, or the group ID of the file is in 703the process's group access list, 704and the group permissions allow the access. 705.Pp 706Neither the effective user ID nor effective group ID 707and group access list of the process 708match the corresponding user ID and group ID of the file, 709but the permissions for ``other users'' allow access. 710.Pp 711Otherwise, permission is denied. 712.It Sockets and Address Families 713A socket is an endpoint for communication between processes. 714Each socket has queues for sending and receiving data. 715.Pp 716Sockets are typed according to their communications properties. 717These properties include whether messages sent and received 718at a socket require the name of the partner, whether communication 719is reliable, the format used in naming message recipients, etc. 720.Pp 721Each instance of the system supports some 722collection of socket types; consult 723.Xr socket 2 724for more information about the types available and 725their properties. 726.Pp 727Each instance of the system supports some number of sets of 728communications protocols. 729Each protocol set supports addresses of a certain format. 730An Address Family is the set of addresses for a specific group of protocols. 731Each socket has an address 732chosen from the address family in which the socket was created. 733.El 734.Sh SEE ALSO 735.Xr intro 3 , 736.Xr perror 3 737.Sh HISTORY 738An 739.Nm intro 740manual page appeared in 741.At v6 . 742