1.\" $NetBSD: intro.2,v 1.9 1997/11/12 00:47:40 mrg 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 8.3 (Berkeley) 12/11/93 35.\" 36.Dd December 11, 1993 37.Dt INTRO 2 38.Os BSD 4 39.Sh NAME 40.Nm intro , 41.Nm errno 42.Nd introduction to system calls and error numbers 43.Sh SYNOPSIS 44.Fd #include <sys/errno.h> 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 , 56which is defined as: 57.Pp 58.Dl extern int errno 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<This allows interpretation of the failure on receiving 67a -1 and to take action accordingly.> 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 following is a complete list of the errors and their 77names as given in 78.Aq Pa sys/errno.h . 79.Bl -hang -width Ds 80.It Er 0 Em "Error 0" . 81Not used. 82.It Er 1 EPERM Em "Operation not permitted" . 83An attempt was made to perform an operation limited to processes 84with appropriate privileges or to the owner of a file or other 85resources. 86.It Er 2 ENOENT Em "No such file or directory" . 87A component of a specified pathname did not exist, or the 88pathname was an empty string. 89.It Er 3 ESRCH Em "No such process" . 90No process could be found corresponding to that specified by the given 91process ID. 92.It Er 4 EINTR Em "Interrupted function call" . 93An asynchronous signal (such as 94.Dv SIGINT 95or 96.Dv SIGQUIT ) 97was caught by the process during the execution of an interruptible 98function. If the signal handler performs a normal return, the 99interrupted function call will seem to have returned the error condition. 100.It Er 5 EIO Em "Input/output error" . 101Some physical input or output error occurred. 102This error will not be reported until a subsequent operation on the same file 103descriptor and may be lost (over written) by any subsequent errors. 104.It Er 6 ENXIO Em "\&No such device or address" . 105Input or output on a special file referred to a device that did not 106exist, or 107made a request beyond the limits of the device. 108This error may also occur when, for example, 109a tape drive is not online or no disk pack is 110loaded on a drive. 111.It Er 7 E2BIG Em "Arg list too long" . 112The number of bytes used for the argument and environment 113list of the new process exceeded the current limit 114of 20480 bytes 115.Pf ( Dv NCARGS 116in 117.Aq Pa sys/param.h ) . 118.It Er 8 ENOEXEC Em "Exec format error" . 119A request was made to execute a file 120that, although it has the appropriate permissions, 121was not in the format required for an 122executable file. 123.It Er 9 EBADF Em "Bad file descriptor" . 124A file descriptor argument was out of range, referred to no open file, 125or a read (write) request was made to a file that was only open for 126writing (reading). 127.It Er 10 ECHILD Em "\&No child processes" . 128A 129.Xr wait 2 130or 131.Xr waitpid 2 132function was executed by a process that had no existing or unwaited-for 133child processes. 134.It Er 11 EDEADLK Em "Resource deadlock avoided" . 135An attempt was made to lock a system resource that 136would have resulted in a deadlock situation. 137.It Er 12 ENOMEM Em "Cannot allocate memory" . 138The new process image required more memory than was allowed by the hardware 139or by system-imposed memory management constraints. 140A lack of swap space is normally temporary; however, 141a lack of core is not. 142Soft limits may be increased to their corresponding hard limits. 143.It Er 13 EACCES Em "Permission denied" . 144An attempt was made to access a file in a way forbidden 145by its file access permissions. 146.It Er 14 EFAULT Em "Bad address" . 147The system detected an invalid address in attempting to 148use an argument of a call. 149.It Er 15 ENOTBLK Em "Not a block device" . 150A block device operation was attempted on a non-block device or file. 151.It Er 16 EBUSY Em "Resource busy" . 152An attempt to use a system resource which was in use at the time 153in a manner which would have conflicted with the request. 154.It Er 17 EEXIST Em "File exists" . 155An existing file was mentioned in an inappropriate context, 156for instance, as the new link name in a 157.Xr link 2 158function. 159.It Er 18 EXDEV Em "Improper link" . 160A hard link to a file on another file system 161was attempted. 162.It Er 19 ENODEV Em "Operation not supported by device" . 163An attempt was made to apply an inappropriate 164function to a device, 165for example, 166trying to read a write-only device such as a printer. 167.It Er 20 ENOTDIR Em "Not a directory" . 168A component of the specified pathname existed, but it was 169not a directory, when a directory was expected. 170.It Er 21 EISDIR Em "Is a directory" . 171An attempt was made to open a directory with write mode specified. 172.It Er 22 EINVAL Em "Invalid argument" . 173Some invalid argument was supplied. (For example, 174specifying an undefined signal to a 175.Xr signal 3 176or 177.Xr kill 2 178function). 179.It Er 23 ENFILE Em "Too many open files in system" . 180Maximum number of file descriptors allowable on the system 181has been reached and a requests for an open cannot be satisfied 182until at least one has been closed. 183.It Er 24 EMFILE Em "Too many open files" . 184<As released, the limit on the number of 185open files per process is 64.> 186.Xr Getdtablesize 3 187will obtain the current limit. 188.It Er 25 ENOTTY Em "Inappropriate ioctl for device" . 189A control function (see 190.Xr ioctl 2 ) 191was attempted for a file or 192special device for which the operation was inappropriate. 193.It Er 26 ETXTBSY Em "Text file busy" . 194The new process was a pure procedure (shared text) file 195which was open for writing by another process, or 196while the pure procedure file was being executed an 197.Xr open 2 198call requested write access. 199.It Er 27 EFBIG Em "File too large" . 200The size of a file exceeded the maximum (about 201.if t 2\u\s-231\s+2\d 202.if n 2.1E9 203bytes). 204.It Er 28 ENOSPC Em "Device out of space" . 205A 206.Xr write 2 207to an ordinary file, the creation of a 208directory or symbolic link, or the creation of a directory 209entry failed because no more disk blocks were available 210on the file system, or the allocation of an inode for a newly 211created file failed because no more inodes were available 212on the file system. 213.It Er 29 ESPIPE Em "Illegal seek" . 214An 215.Xr lseek 2 216function was issued on a socket, pipe or 217.Tn FIFO . 218.It Er 30 EROFS Em "Read-only file system" . 219An attempt was made to modify a file or directory 220was made 221on a file system that was read-only at the time. 222.It Er 31 EMLINK Em "Too many links" . 223Maximum allowable hard links to a single file has been exceeded (limit 224of 32767 hard links per file). 225.It Er 32 EPIPE Em "Broken pipe" . 226A write on a pipe, socket or 227.Tn FIFO 228for which there is no process 229to read the data. 230.It Er 33 EDOM Em "Numerical argument out of domain" . 231A numerical input argument was outside the defined domain of the mathematical 232function. 233.It Er 34 ERANGE Em "Numerical result out of range" . 234A numerical result of the function was too large to fit in the 235available space (perhaps exceeded precision). 236.It Er 35 EAGAIN Em "Resource temporarily unavailable" . 237This is a temporary condition and later calls to the 238same routine may complete normally. 239.It Er 36 EINPROGRESS Em "Operation now in progress" . 240An operation that takes a long time to complete (such as 241a 242.Xr connect 2 ) 243was attempted on a non-blocking object (see 244.Xr fcntl 2 ) . 245.It Er 37 EALREADY Em "Operation already in progress" . 246An operation was attempted on a non-blocking object that already 247had an operation in progress. 248.It Er 38 ENOTSOCK Em "Socket operation on non-socket" . 249Self-explanatory. 250.It Er 39 EDESTADDRREQ Em "Destination address required" . 251A required address was omitted from an operation on a socket. 252.It Er 40 EMSGSIZE Em "Message too long" . 253A message sent on a socket was larger than the internal message buffer 254or some other network limit. 255.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" . 256A protocol was specified that does not support the semantics of the 257socket type requested. For example, you cannot use the 258.Tn ARPA 259Internet 260.Tn UDP 261protocol with type 262.Dv SOCK_STREAM . 263.It Er 42 ENOPROTOOPT Em "Protocol not available" . 264A bad option or level was specified in a 265.Xr getsockopt 2 266or 267.Xr setsockopt 2 268call. 269.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" . 270The protocol has not been configured into the 271system or no implementation for it exists. 272.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" . 273The support for the socket type has not been configured into the 274system or no implementation for it exists. 275.It Er 45 EOPNOTSUPP Em "Operation not supported" . 276The attempted operation is not supported for the type of object referenced. 277Usually this occurs when a file descriptor refers to a file or socket 278that cannot support this operation, 279for example, trying to 280.Em accept 281a connection on a datagram socket. 282.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" . 283The protocol family has not been configured into the 284system or no implementation for it exists. 285.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" . 286An address incompatible with the requested protocol was used. 287For example, you shouldn't necessarily expect to be able to use 288.Tn NS 289addresses with 290.Tn ARPA 291Internet protocols. 292.It Er 48 EADDRINUSE Em "Address already in use" . 293Only one usage of each address is normally permitted. 294.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" . 295Normally results from an attempt to create a socket with an 296address not on this machine. 297.It Er 50 ENETDOWN Em "Network is down" . 298A socket operation encountered a dead network. 299.It Er 51 ENETUNREACH Em "Network is unreachable" . 300A socket operation was attempted to an unreachable network. 301.It Er 52 ENETRESET Em "Network dropped connection on reset" . 302The host you were connected to crashed and rebooted. 303.It Er 53 ECONNABORTED Em "Software caused connection abort" . 304A connection abort was caused internal to your host machine. 305.It Er 54 ECONNRESET Em "Connection reset by peer" . 306A connection was forcibly closed by a peer. This normally 307results from a loss of the connection on the remote socket 308due to a timeout or a reboot. 309.It Er 55 ENOBUFS Em "\&No buffer space available" . 310An operation on a socket or pipe was not performed because 311the system lacked sufficient buffer space or because a queue was full. 312.It Er 56 EISCONN Em "Socket is already connected" . 313A 314.Xr connect 2 315request was made on an already connected socket; or, 316a 317.Xr sendto 2 318or 319.Xr sendmsg 2 320request on a connected socket specified a destination 321when already connected. 322.It Er 57 ENOTCONN Em "Socket is not connected" . 323An request to send or receive data was disallowed because 324the socket was not connected and (when sending on a datagram socket) 325no address was supplied. 326.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" . 327A request to send data was disallowed because the socket 328had already been shut down with a previous 329.Xr shutdown 2 330call. 331.It Er 60 ETIMEDOUT Em "Operation timed out" . 332A 333.Xr connect 2 334or 335.Xr send 2 336request failed because the connected party did not 337properly respond after a period of time. (The timeout 338period is dependent on the communication protocol.) 339.It Er 61 ECONNREFUSED Em "Connection refused" . 340No connection could be made because the target machine actively 341refused it. This usually results from trying to connect 342to a service that is inactive on the foreign host. 343.It Er 62 ELOOP Em "Too many levels of symbolic links" . 344A path name lookup involved more than 8 symbolic links. 345.It Er 63 ENAMETOOLONG Em "File name too long" . 346A component of a path name exceeded 255 347.Pq Dv MAXNAMELEN 348characters, or an entire 349path name exceeded 1023 350.Pq Dv MAXPATHLEN Ns -1 351characters. 352.It Er 64 EHOSTDOWN Em "Host is down" . 353A socket operation failed because the destination host was down. 354.It Er 65 EHOSTUNREACH Em "No route to host" . 355A socket operation was attempted to an unreachable host. 356.It Er 66 ENOTEMPTY Em "Directory not empty" . 357A directory with entries other than 358.Ql \&. 359and 360.Ql \&.. 361was supplied to a remove directory or rename call. 362.It Er 67 EPROCLIM Em "Too many processes" . 363.It Er 68 EUSERS Em "Too many users" . 364The quota system ran out of table entries. 365.It Er 69 EDQUOT Em "Disc quota exceeded" . 366A 367.Xr write 2 368to an ordinary file, the creation of a 369directory or symbolic link, or the creation of a directory 370entry failed because the user's quota of disk blocks was 371exhausted, or the allocation of an inode for a newly 372created file failed because the user's quota of inodes 373was exhausted. 374.It Er 70 ESTALE Em "Stale NFS file handle" . 375An attempt was made to access an open file (on an 376.Tn NFS 377filesystem) 378which is now unavailable as referenced by the file descriptor. 379This may indicate the file was deleted on the 380.Tn NFS 381server or some 382other catastrophic event occurred. 383.It Er 72 EBADRPC Em "RPC struct is bad" . 384Exchange of 385.Tn RPC 386information was unsuccessful. 387.It Er 73 ERPCMISMATCH Em "RPC version wrong" . 388The version of 389.Tn RPC 390on the remote peer is not compatible with 391the local version. 392.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" . 393The requested program is not registered on the remote host. 394.It Er 75 EPROGMISMATCH Em "Program version wrong" . 395The requested version of the program is not available 396on the remote host 397.Pq Tn RPC . 398.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" . 399An 400.Tn RPC 401call was attempted for a procedure which doesn't exist 402in the remote program. 403.It Er 77 ENOLCK Em "No locks available" . 404A system-imposed limit on the number of simultaneous file 405locks was reached. 406.It Er 78 ENOSYS Em "Function not implemented" . 407Attempted a system call that is not available on this 408system. 409.Sh DEFINITIONS 410.Bl -tag -width Ds 411.It Process ID . 412Each active process in the system is uniquely identified by a non-negative 413integer called a process ID. The range of this ID is from 0 to 30000. 414.It Parent process ID 415A new process is created by a currently active process; (see 416.Xr fork 2 ) . 417The parent process ID of a process is initially the process ID of its creator. 418If the creating process exits, 419the parent process ID of each child is set to the ID of a system process, 420.Xr init 8 . 421.It Process Group 422Each active process is a member of a process group that is identified by 423a non-negative integer called the process group ID. This is the process 424ID of the group leader. This grouping permits the signaling of related 425processes (see 426.Xr termios 4 ) 427and the job control mechanisms of 428.Xr csh 1 . 429.It Session 430A session is a set of one or more process groups. 431A session is created by a successful call to 432.Xr setsid 2 , 433which causes the caller to become the only member of the only process 434group in the new session. 435.It Session leader 436A process that has created a new session by a successful call to 437.Xr setsid 2 , 438is known as a session leader. 439Only a session leader may acquire a terminal as its controlling terminal (see 440.Xr termios 4 ) . 441.It Controlling process 442A session leader with a controlling terminal is a controlling process. 443.It Controlling terminal 444A terminal that is associated with a session is known as the controlling 445terminal for that session and its members. 446.It "Terminal Process Group ID" 447A terminal may be acquired by a session leader as its controlling terminal. 448Once a terminal is associated with a session, any of the process groups 449within the session may be placed into the foreground by setting 450the terminal process group ID to the ID of the process group. 451This facility is used 452to arbitrate between multiple jobs contending for the same terminal; 453(see 454.Xr csh 1 455and 456.Xr tty 4 ) . 457.It "Orphaned Process Group" 458A process group is considered to be 459.Em orphaned 460if it is not under the control of a job control shell. 461More precisely, a process group is orphaned 462when none of its members has a parent process that is in the same session 463as the group, 464but is in a different process group. 465Note that when a process exits, the parent process for its children 466is changed to be 467.Xr init 8 , 468which is in a separate session. 469Not all members of an orphaned process group are necessarily orphaned 470processes (those whose creating process has exited). 471The process group of a session leader is orphaned by definition. 472.It "Real User ID and Real Group ID" 473Each user on the system is identified by a positive integer 474termed the real user ID. 475.Pp 476Each user is also a member of one or more groups. 477One of these groups is distinguished from others and 478used in implementing accounting facilities. The positive 479integer corresponding to this distinguished group is termed 480the real group ID. 481.Pp 482All processes have a real user ID and real group ID. 483These are initialized from the equivalent attributes 484of the process that created it. 485.It "Effective User Id, Effective Group Id, and Group Access List" 486Access to system resources is governed by two values: 487the effective user ID, and the group access list. 488The first member of the group access list is also known as the 489effective group ID. 490(In POSIX.1, the group access list is known as the set of supplementary 491group IDs, and it is unspecified whether the effective group ID is 492a member of the list.) 493.Pp 494The effective user ID and effective group ID are initially the 495process's real user ID and real group ID respectively. Either 496may be modified through execution of a set-user-ID or set-group-ID 497file (possibly by one its ancestors) (see 498.Xr execve 2 ) . 499By convention, the effective group ID (the first member of the group access 500list) is duplicated, so that the execution of a set-group-ID program 501does not result in the loss of the original (real) group ID. 502.Pp 503The group access list is a set of group IDs 504used only in determining resource accessibility. Access checks 505are performed as described below in ``File Access Permissions''. 506.It "Saved Set User ID and Saved Set Group ID" 507When a process executes a new file, the effective user ID is set 508to the owner of the file if the file is set-user-ID, and the effective 509group ID (first element of the group access list) is set to the group 510of the file if the file is set-group-ID. 511The effective user ID of the process is then recorded as the saved set-user-ID, 512and the effective group ID of the process is recorded as the saved set-group-ID. 513These values may be used to regain those values as the effective user 514or group ID after reverting to the real ID (see 515.Xr setuid 2 ) . 516(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, 517and are used in setuid and setgid, but this does not work as desired 518for the super-user.) 519.It Super-user 520A process is recognized as a 521.Em super-user 522process and is granted special privileges if its effective user ID is 0. 523.It Special Processes 524The processes with process IDs of 0, 1, and 2 are special. 525Process 0 is the scheduler. Process 1 is the initialization process 526.Xr init 8 , 527and is the ancestor of every other process in the system. 528It is used to control the process structure. 529Process 2 is the paging daemon. 530.It Descriptor 531An integer assigned by the system when a file is referenced 532by 533.Xr open 2 534or 535.Xr dup 2 , 536or when a socket is created by 537.Xr pipe 2 , 538.Xr socket 2 , 539or 540.Xr socketpair 2 , 541which uniquely identifies an access path to that file or socket from 542a given process or any of its children. 543.It File Name 544Names consisting of up to 255 545.Pq Dv MAXNAMELEN 546characters may be used to name 547an ordinary file, special file, or directory. 548.Pp 549These characters may be selected from the set of all 550.Tn ASCII 551character 552excluding 0 (NUL) and the 553.Tn ASCII 554code for 555.Ql \&/ 556(slash). (The parity bit, 557bit 7, must be 0.) 558.Pp 559Note that it is generally unwise to use 560.Ql \&* , 561.Ql \&? , 562.Ql \&[ 563or 564.Ql \&] 565as part of 566file names because of the special meaning attached to these characters 567by the shell. 568.It Path Name 569A path name is a 570.Tn NUL Ns -terminated 571character string starting with an 572optional slash 573.Ql \&/ , 574followed by zero or more directory names separated 575by slashes, optionally followed by a file name. 576The total length of a path name must be less than 1024 577.Pq Dv MAXPATHLEN 578characters. 579.Pp 580If a path name begins with a slash, the path search begins at the 581.Em root 582directory. 583Otherwise, the search begins from the current working directory. 584A slash by itself names the root directory. An empty 585pathname refers to the current directory. 586.It Directory 587A directory is a special type of file that contains entries 588that are references to other files. 589Directory entries are called links. By convention, a directory 590contains at least two links, 591.Ql \&. 592and 593.Ql \&.. , 594referred to as 595.Em dot 596and 597.Em dot-dot 598respectively. Dot refers to the directory itself and 599dot-dot refers to its parent directory. 600.It "Root Directory and Current Working Directory" 601Each process has associated with it a concept of a root directory 602and a current working directory for the purpose of resolving path 603name searches. A process's root directory need not be the root 604directory of the root file system. 605.It File Access Permissions 606Every file in the file system has a set of access permissions. 607These permissions are used in determining whether a process 608may perform a requested operation on the file (such as opening 609a file for writing). Access permissions are established at the 610time a file is created. They may be changed at some later time 611through the 612.Xr chmod 2 613call. 614.Pp 615File access is broken down according to whether a file may be: read, 616written, or executed. Directory files use the execute 617permission to control if the directory may be searched. 618.Pp 619File access permissions are interpreted by the system as 620they apply to three different classes of users: the owner 621of the file, those users in the file's group, anyone else. 622Every file has an independent set of access permissions for 623each of these classes. When an access check is made, the system 624decides if permission should be granted by checking the access 625information applicable to the caller. 626.Pp 627Read, write, and execute/search permissions on 628a file are granted to a process if: 629.Pp 630The process's effective user ID is that of the super-user. (Note: 631even the super-user cannot execute a non-executable file.) 632.Pp 633The process's effective user ID matches the user ID of the owner 634of the file and the owner permissions allow the access. 635.Pp 636The process's effective user ID does not match the user ID of the 637owner of the file, and either the process's effective 638group ID matches the group ID 639of the file, or the group ID of the file is in 640the process's group access list, 641and the group permissions allow the access. 642.Pp 643Neither the effective user ID nor effective group ID 644and group access list of the process 645match the corresponding user ID and group ID of the file, 646but the permissions for ``other users'' allow access. 647.Pp 648Otherwise, permission is denied. 649.It Sockets and Address Families 650.Pp 651A socket is an endpoint for communication between processes. 652Each socket has queues for sending and receiving data. 653.Pp 654Sockets are typed according to their communications properties. 655These properties include whether messages sent and received 656at a socket require the name of the partner, whether communication 657is reliable, the format used in naming message recipients, etc. 658.Pp 659Each instance of the system supports some 660collection of socket types; consult 661.Xr socket 2 662for more information about the types available and 663their properties. 664.Pp 665Each instance of the system supports some number of sets of 666communications protocols. Each protocol set supports addresses 667of a certain format. An Address Family is the set of addresses 668for a specific group of protocols. Each socket has an address 669chosen from the address family in which the socket was created. 670.Sh SEE ALSO 671.Xr intro 3 , 672.Xr perror 3 673.Sh HISTORY 674An 675.Nm intro 676manual page appeared in 677.At v6 . 678