1.\" $NetBSD: intro.2,v 1.6 1995/02/27 12:33:41 cgd 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.Nd introduction to system calls and error numbers 42.Sh SYNOPSIS 43.Fd #include <sys/errno.h> 44.Sh DESCRIPTION 45This section provides an overview of the system calls, 46their error returns, and other common definitions and concepts. 47.\".Pp 48.\".Sy System call restart 49.\".Pp 50.\"<more later...> 51.Sh DIAGNOSTICS 52Nearly all of the system calls provide an error number in the external 53variable 54.Va errno , 55which is defined as: 56.Pp 57.Dl extern int errno 58.Pp 59When a system call detects an error, 60it returns an integer value 61indicating failure (usually -1) 62and sets the variable 63.Va errno 64accordingly. 65<This allows interpretation of the failure on receiving 66a -1 and to take action accordingly.> 67Successful calls never set 68.Va errno ; 69once set, it remains until another error occurs. 70It should only be examined after an error. 71Note that a number of system calls overload the meanings of these 72error numbers, and that the meanings must be interpreted according 73to the type and circumstances of the call. 74.Pp 75The following is a complete list of the errors and their 76names as given in 77.Aq Pa sys/errno.h . 78.Bl -hang -width Ds 79.It Er 0 Em "Error 0" . 80Not used. 81.It Er 1 EPERM Em "Operation not permitted" . 82An attempt was made to perform an operation limited to processes 83with appropriate privileges or to the owner of a file or other 84resources. 85.It Er 2 ENOENT Em "No such file or directory" . 86A component of a specified pathname did not exist, or the 87pathname was an empty string. 88.It Er 3 ESRCH Em "No such process" . 89No process could be found corresponding to that specified by the given 90process ID. 91.It Er 4 EINTR Em "Interrupted function call" . 92An asynchronous signal (such as 93.Dv SIGINT 94or 95.Dv SIGQUIT ) 96was caught by the process during the execution of an interruptible 97function. If the signal handler performs a normal return, the 98interrupted function call will seem to have returned the error condition. 99.It Er 5 EIO Em "Input/output error" . 100Some physical input or output error occurred. 101This error will not be reported until a subsequent operation on the same file 102descriptor and may be lost (over written) by any subsequent errors. 103.It Er 6 ENXIO Em "\&No such device or address" . 104Input or output on a special file referred to a device that did not 105exist, or 106made a request beyond the limits of the device. 107This error may also occur when, for example, 108a tape drive is not online or no disk pack is 109loaded on a drive. 110.It Er 7 E2BIG Em "Arg list too long" . 111The number of bytes used for the argument and environment 112list of the new process exceeded the current limit 113of 20480 bytes 114.Pf ( Dv NCARGS 115in 116.Aq Pa sys/param.h ) . 117.It Er 8 ENOEXEC Em "Exec format error" . 118A request was made to execute a file 119that, although it has the appropriate permissions, 120was not in the format required for an 121executable file. 122.It Er 9 EBADF Em "Bad file descriptor" . 123A file descriptor argument was out of range, referred to no open file, 124or a read (write) request was made to a file that was only open for 125writing (reading). 126.It Er 10 ECHILD Em "\&No child processes" . 127A 128.Xr wait 129or 130.Xr waitpid 131function was executed by a process that had no existing or unwaited-for 132child processes. 133.It Er 11 EDEADLK Em "Resource deadlock avoided" . 134An attempt was made to lock a system resource that 135would have resulted in a deadlock situation. 136.It Er 12 ENOMEM Em "Cannot allocate memory" . 137The new process image required more memory than was allowed by the hardware 138or by system-imposed memory management constraints. 139A lack of swap space is normally temporary; however, 140a lack of core is not. 141Soft limits may be increased to their corresponding hard limits. 142.It Er 13 EACCES Em "Permission denied" . 143An attempt was made to access a file in a way forbidden 144by its file access permissions. 145.It Er 14 EFAULT Em "Bad address" . 146The system detected an invalid address in attempting to 147use an argument of a call. 148.It Er 15 ENOTBLK Em "Not a block device" . 149A block device operation was attempted on a non-block device or file. 150.It Er 16 EBUSY Em "Resource busy" . 151An attempt to use a system resource which was in use at the time 152in a manner which would have conflicted with the request. 153.It Er 17 EEXIST Em "File exists" . 154An existing file was mentioned in an inappropriate context, 155for instance, as the new link name in a 156.Xr link 157function. 158.It Er 18 EXDEV Em "Improper link" . 159A hard link to a file on another file system 160was attempted. 161.It Er 19 ENODEV Em "Operation not supported by device" . 162An attempt was made to apply an inappropriate 163function to a device, 164for example, 165trying to read a write-only device such as a printer. 166.It Er 20 ENOTDIR Em "Not a directory" . 167A component of the specified pathname existed, but it was 168not a directory, when a directory was expected. 169.It Er 21 EISDIR Em "Is a directory" . 170An attempt was made to open a directory with write mode specified. 171.It Er 22 EINVAL Em "Invalid argument" . 172Some invalid argument was supplied. (For example, 173specifying an undefined signal to a 174.Xr signal 175or 176.Xr kill 177function). 178.It Er 23 ENFILE Em "Too many open files in system" . 179Maximum number of file descriptors allowable on the system 180has been reached and a requests for an open cannot be satisfied 181until at least one has been closed. 182.It Er 24 EMFILE Em "Too many open files" . 183<As released, the limit on the number of 184open files per process is 64.> 185.Xr Getdtablesize 2 186will obtain the current limit. 187.It Er 25 ENOTTY Em "Inappropriate ioctl for device" . 188A control function (see 189.Xr ioctl 2 ) 190was attempted for a file or 191special device for which the operation was inappropriate. 192.It Er 26 ETXTBSY Em "Text file busy" . 193The new process was a pure procedure (shared text) file 194which was open for writing by another process, or 195while the pure procedure file was being executed an 196.Xr open 197call requested write access. 198.It Er 27 EFBIG Em "File too large" . 199The size of a file exceeded the maximum (about 200.if t 2\u\s-231\s+2\d 201.if n 2.1E9 202bytes). 203.It Er 28 ENOSPC Em "Device out of space" . 204A 205.Xr write 206to an ordinary file, the creation of a 207directory or symbolic link, or the creation of a directory 208entry failed because no more disk blocks were available 209on the file system, or the allocation of an inode for a newly 210created file failed because no more inodes were available 211on the file system. 212.It Er 29 ESPIPE Em "Illegal seek" . 213An 214.Xr lseek 215function was issued on a socket, pipe or 216.Tn FIFO . 217.It Er 30 EROFS Em "Read-only file system" . 218An attempt was made to modify a file or directory 219was made 220on a file system that was read-only at the time. 221.It Er 31 EMLINK Em "Too many links" . 222Maximum allowable hard links to a single file has been exceeded (limit 223of 32767 hard links per file). 224.It Er 32 EPIPE Em "Broken pipe" . 225A write on a pipe, socket or 226.Tn FIFO 227for which there is no process 228to read the data. 229.It Er 33 EDOM Em "Numerical argument out of domain" . 230A numerical input argument was outside the defined domain of the mathematical 231function. 232.It Er 34 ERANGE Em "Numerical result out of range" . 233A numerical result of the function was too large to fit in the 234available space (perhaps exceeded precision). 235.It Er 35 EAGAIN Em "Resource temporarily unavailable" . 236This is a temporary condition and later calls to the 237same routine may complete normally. 238.It Er 36 EINPROGRESS Em "Operation now in progress" . 239An operation that takes a long time to complete (such as 240a 241.Xr connect 2 ) 242was attempted on a non-blocking object (see 243.Xr fcntl 2 ) . 244.It Er 37 EALREADY Em "Operation already in progress" . 245An operation was attempted on a non-blocking object that already 246had an operation in progress. 247.It Er 38 ENOTSOCK Em "Socket operation on non-socket" . 248Self-explanatory. 249.It Er 39 EDESTADDRREQ Em "Destination address required" . 250A required address was omitted from an operation on a socket. 251.It Er 40 EMSGSIZE Em "Message too long" . 252A message sent on a socket was larger than the internal message buffer 253or some other network limit. 254.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" . 255A protocol was specified that does not support the semantics of the 256socket type requested. For example, you cannot use the 257.Tn ARPA 258Internet 259.Tn UDP 260protocol with type 261.Dv SOCK_STREAM . 262.It Er 42 ENOPROTOOPT Em "Protocol not available" . 263A bad option or level was specified in a 264.Xr getsockopt 2 265or 266.Xr setsockopt 2 267call. 268.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" . 269The protocol has not been configured into the 270system or no implementation for it exists. 271.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" . 272The support for the socket type has not been configured into the 273system or no implementation for it exists. 274.It Er 45 EOPNOTSUPP Em "Operation not supported" . 275The attempted operation is not supported for the type of object referenced. 276Usually this occurs when a file descriptor refers to a file or socket 277that cannot support this operation, 278for example, trying to 279.Em accept 280a connection on a datagram socket. 281.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" . 282The protocol family has not been configured into the 283system or no implementation for it exists. 284.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" . 285An address incompatible with the requested protocol was used. 286For example, you shouldn't necessarily expect to be able to use 287.Tn NS 288addresses with 289.Tn ARPA 290Internet protocols. 291.It Er 48 EADDRINUSE Em "Address already in use" . 292Only one usage of each address is normally permitted. 293.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" . 294Normally results from an attempt to create a socket with an 295address not on this machine. 296.It Er 50 ENETDOWN Em "Network is down" . 297A socket operation encountered a dead network. 298.It Er 51 ENETUNREACH Em "Network is unreachable" . 299A socket operation was attempted to an unreachable network. 300.It Er 52 ENETRESET Em "Network dropped connection on reset" . 301The host you were connected to crashed and rebooted. 302.It Er 53 ECONNABORTED Em "Software caused connection abort" . 303A connection abort was caused internal to your host machine. 304.It Er 54 ECONNRESET Em "Connection reset by peer" . 305A connection was forcibly closed by a peer. This normally 306results from a loss of the connection on the remote socket 307due to a timeout or a reboot. 308.It Er 55 ENOBUFS Em "\&No buffer space available" . 309An operation on a socket or pipe was not performed because 310the system lacked sufficient buffer space or because a queue was full. 311.It Er 56 EISCONN Em "Socket is already connected" . 312A 313.Xr connect 314request was made on an already connected socket; or, 315a 316.Xr sendto 317or 318.Xr sendmsg 319request on a connected socket specified a destination 320when already connected. 321.It Er 57 ENOTCONN Em "Socket is not connected" . 322An request to send or receive data was disallowed because 323the socket was not connected and (when sending on a datagram socket) 324no address was supplied. 325.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" . 326A request to send data was disallowed because the socket 327had already been shut down with a previous 328.Xr shutdown 2 329call. 330.It Er 60 ETIMEDOUT Em "Operation timed out" . 331A 332.Xr connect 333or 334.Xr send 335request failed because the connected party did not 336properly respond after a period of time. (The timeout 337period is dependent on the communication protocol.) 338.It Er 61 ECONNREFUSED Em "Connection refused" . 339No connection could be made because the target machine actively 340refused it. This usually results from trying to connect 341to a service that is inactive on the foreign host. 342.It Er 62 ELOOP Em "Too many levels of symbolic links" . 343A path name lookup involved more than 8 symbolic links. 344.It Er 63 ENAMETOOLONG Em "File name too long" . 345A component of a path name exceeded 255 346.Pq Dv MAXNAMELEN 347characters, or an entire 348path name exceeded 1023 349.Pq Dv MAXPATHLEN Ns -1 350characters. 351.It Er 64 EHOSTDOWN Em "Host is down" . 352A socket operation failed because the destination host was down. 353.It Er 65 EHOSTUNREACH Em "No route to host" . 354A socket operation was attempted to an unreachable host. 355.It Er 66 ENOTEMPTY Em "Directory not empty" . 356A directory with entries other than 357.Ql \&. 358and 359.Ql \&.. 360was supplied to a remove directory or rename call. 361.It Er 67 EPROCLIM Em "Too many processes" . 362.It Er 68 EUSERS Em "Too many users" . 363The quota system ran out of table entries. 364.It Er 69 EDQUOT Em "Disc quota exceeded" . 365A 366.Xr write 367to an ordinary file, the creation of a 368directory or symbolic link, or the creation of a directory 369entry failed because the user's quota of disk blocks was 370exhausted, or the allocation of an inode for a newly 371created file failed because the user's quota of inodes 372was exhausted. 373.It Er 70 ESTALE Em "Stale NFS file handle" . 374An attempt was made to access an open file (on an 375.Tn NFS 376filesystem) 377which is now unavailable as referenced by the file descriptor. 378This may indicate the file was deleted on the 379.Tn NFS 380server or some 381other catastrophic event occurred. 382.It Er 72 EBADRPC Em "RPC struct is bad" . 383Exchange of 384.Tn RPC 385information was unsuccessful. 386.It Er 73 ERPCMISMATCH Em "RPC version wrong" . 387The version of 388.Tn RPC 389on the remote peer is not compatible with 390the local version. 391.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" . 392The requested program is not registered on the remote host. 393.It Er 75 EPROGMISMATCH Em "Program version wrong" . 394The requested version of the program is not available 395on the remote host 396.Pq Tn RPC . 397.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" . 398An 399.Tn RPC 400call was attempted for a procedure which doesn't exist 401in the remote program. 402.It Er 77 ENOLCK Em "No locks available" . 403A system-imposed limit on the number of simultaneous file 404locks was reached. 405.It Er 78 ENOSYS Em "Function not implemented" . 406Attempted a system call that is not available on this 407system. 408.Sh DEFINITIONS 409.Bl -tag -width Ds 410.It Process ID . 411Each active process in the system is uniquely identified by a non-negative 412integer called a process ID. The range of this ID is from 0 to 30000. 413.It Parent process ID 414A new process is created by a currently active process; (see 415.Xr fork 2 ) . 416The parent process ID of a process is initially the process ID of its creator. 417If the creating process exits, 418the parent process ID of each child is set to the ID of a system process, 419.Xr init . 420.It Process Group 421Each active process is a member of a process group that is identified by 422a non-negative integer called the process group ID. This is the process 423ID of the group leader. This grouping permits the signaling of related 424processes (see 425.Xr termios 4 ) 426and the job control mechanisms of 427.Xr csh 1 . 428.It Session 429A session is a set of one or more process groups. 430A session is created by a successful call to 431.Xr setsid 2 , 432which causes the caller to become the only member of the only process 433group in the new session. 434.It Session leader 435A process that has created a new session by a successful call to 436.Xr setsid 2 , 437is known as a session leader. 438Only a session leader may acquire a terminal as its controlling terminal (see 439.Xr termios 4 ) . 440.It Controlling process 441A session leader with a controlling terminal is a controlling process. 442.It Controlling terminal 443A terminal that is associated with a session is known as the controlling 444terminal for that session and its members. 445.It "Terminal Process Group ID" 446A terminal may be acquired by a session leader as its controlling terminal. 447Once a terminal is associated with a session, any of the process groups 448within the session may be placed into the foreground by setting 449the terminal process group ID to the ID of the process group. 450This facility is used 451to arbitrate between multiple jobs contending for the same terminal; 452(see 453.Xr csh 1 454and 455.Xr tty 4 ) . 456.It "Orphaned Process Group" 457A process group is considered to be 458.Em orphaned 459if it is not under the control of a job control shell. 460More precisely, a process group is orphaned 461when none of its members has a parent process that is in the same session 462as the group, 463but is in a different process group. 464Note that when a process exits, the parent process for its children 465is changed to be 466.Xr init , 467which is in a separate session. 468Not all members of an orphaned process group are necessarily orphaned 469processes (those whose creating process has exited). 470The process group of a session leader is orphaned by definition. 471.It "Real User ID and Real Group ID" 472Each user on the system is identified by a positive integer 473termed the real user ID. 474.Pp 475Each user is also a member of one or more groups. 476One of these groups is distinguished from others and 477used in implementing accounting facilities. The positive 478integer corresponding to this distinguished group is termed 479the real group ID. 480.Pp 481All processes have a real user ID and real group ID. 482These are initialized from the equivalent attributes 483of the process that created it. 484.It "Effective User Id, Effective Group Id, and Group Access List" 485Access to system resources is governed by two values: 486the effective user ID, and the group access list. 487The first member of the group access list is also known as the 488effective group ID. 489(In POSIX.1, the group access list is known as the set of supplementary 490group IDs, and it is unspecified whether the effective group ID is 491a member of the list.) 492.Pp 493The effective user ID and effective group ID are initially the 494process's real user ID and real group ID respectively. Either 495may be modified through execution of a set-user-ID or set-group-ID 496file (possibly by one its ancestors) (see 497.Xr execve 2 ) . 498By convention, the effective group ID (the first member of the group access 499list) is duplicated, so that the execution of a set-group-ID program 500does not result in the loss of the original (real) group ID. 501.Pp 502The group access list is a set of group IDs 503used only in determining resource accessibility. Access checks 504are performed as described below in ``File Access Permissions''. 505.It "Saved Set User ID and Saved Set Group ID" 506When a process executes a new file, the effective user ID is set 507to the owner of the file if the file is set-user-ID, and the effective 508group ID (first element of the group access list) is set to the group 509of the file if the file is set-group-ID. 510The effective user ID of the process is then recorded as the saved set-user-ID, 511and the effective group ID of the process is recorded as the saved set-group-ID. 512These values may be used to regain those values as the effective user 513or group ID after reverting to the real ID (see 514.Xr setuid 2 ) . 515(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, 516and are used in setuid and setgid, but this does not work as desired 517for the super-user.) 518.It Super-user 519A process is recognized as a 520.Em super-user 521process and is granted special privileges if its effective user ID is 0. 522.It Special Processes 523The processes with process IDs of 0, 1, and 2 are special. 524Process 0 is the scheduler. Process 1 is the initialization process 525.Xr init , 526and is the ancestor of every other process in the system. 527It is used to control the process structure. 528Process 2 is the paging daemon. 529.It Descriptor 530An integer assigned by the system when a file is referenced 531by 532.Xr open 2 533or 534.Xr dup 2 , 535or when a socket is created by 536.Xr pipe 2 , 537.Xr socket 2 538or 539.Xr socketpair 2 , 540which uniquely identifies an access path to that file or socket from 541a given process or any of its children. 542.It File Name 543Names consisting of up to 255 544.Pq Dv MAXNAMELEN 545characters may be used to name 546an ordinary file, special file, or directory. 547.Pp 548These characters may be selected from the set of all 549.Tn ASCII 550character 551excluding 0 (NUL) and the 552.Tn ASCII 553code for 554.Ql \&/ 555(slash). (The parity bit, 556bit 7, must be 0.) 557.Pp 558Note that it is generally unwise to use 559.Ql \&* , 560.Ql \&? , 561.Ql \&[ 562or 563.Ql \&] 564as part of 565file names because of the special meaning attached to these characters 566by the shell. 567.It Path Name 568A path name is a 569.Tn NUL Ns -terminated 570character string starting with an 571optional slash 572.Ql \&/ , 573followed by zero or more directory names separated 574by slashes, optionally followed by a file name. 575The total length of a path name must be less than 1024 576.Pq Dv MAXPATHLEN 577characters. 578.Pp 579If a path name begins with a slash, the path search begins at the 580.Em root 581directory. 582Otherwise, the search begins from the current working directory. 583A slash by itself names the root directory. An empty 584pathname refers to the current directory. 585.It Directory 586A directory is a special type of file that contains entries 587that are references to other files. 588Directory entries are called links. By convention, a directory 589contains at least two links, 590.Ql \&. 591and 592.Ql \&.. , 593referred to as 594.Em dot 595and 596.Em dot-dot 597respectively. Dot refers to the directory itself and 598dot-dot refers to its parent directory. 599.It "Root Directory and Current Working Directory" 600Each process has associated with it a concept of a root directory 601and a current working directory for the purpose of resolving path 602name searches. A process's root directory need not be the root 603directory of the root file system. 604.It File Access Permissions 605Every file in the file system has a set of access permissions. 606These permissions are used in determining whether a process 607may perform a requested operation on the file (such as opening 608a file for writing). Access permissions are established at the 609time a file is created. They may be changed at some later time 610through the 611.Xr chmod 2 612call. 613.Pp 614File access is broken down according to whether a file may be: read, 615written, or executed. Directory files use the execute 616permission to control if the directory may be searched. 617.Pp 618File access permissions are interpreted by the system as 619they apply to three different classes of users: the owner 620of the file, those users in the file's group, anyone else. 621Every file has an independent set of access permissions for 622each of these classes. When an access check is made, the system 623decides if permission should be granted by checking the access 624information applicable to the caller. 625.Pp 626Read, write, and execute/search permissions on 627a file are granted to a process if: 628.Pp 629The process's effective user ID is that of the super-user. (Note: 630even the super-user cannot execute a non-executable file.) 631.Pp 632The process's effective user ID matches the user ID of the owner 633of the file and the owner permissions allow the access. 634.Pp 635The process's effective user ID does not match the user ID of the 636owner of the file, and either the process's effective 637group ID matches the group ID 638of the file, or the group ID of the file is in 639the process's group access list, 640and the group permissions allow the access. 641.Pp 642Neither the effective user ID nor effective group ID 643and group access list of the process 644match the corresponding user ID and group ID of the file, 645but the permissions for ``other users'' allow access. 646.Pp 647Otherwise, permission is denied. 648.It Sockets and Address Families 649.Pp 650A socket is an endpoint for communication between processes. 651Each socket has queues for sending and receiving data. 652.Pp 653Sockets are typed according to their communications properties. 654These properties include whether messages sent and received 655at a socket require the name of the partner, whether communication 656is reliable, the format used in naming message recipients, etc. 657.Pp 658Each instance of the system supports some 659collection of socket types; consult 660.Xr socket 2 661for more information about the types available and 662their properties. 663.Pp 664Each instance of the system supports some number of sets of 665communications protocols. Each protocol set supports addresses 666of a certain format. An Address Family is the set of addresses 667for a specific group of protocols. Each socket has an address 668chosen from the address family in which the socket was created. 669.Sh SEE ALSO 670.Xr intro 3 , 671.Xr perror 3 672.Sh HISTORY 673An 674.Nm intro 675manual page appeared in 676.At v6 . 677