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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)intro.2 8.5 (Berkeley) 2/27/95 29.\" $FreeBSD: src/lib/libc/sys/intro.2,v 1.48 2007/01/09 00:28:14 imp Exp $ 30.\" $DragonFly: src/lib/libc/sys/intro.2,v 1.7 2007/04/11 09:12:08 swildner Exp $ 31.\" 32.Dd February 27, 1995 33.Dt INTRO 2 34.Os 35.Sh NAME 36.Nm intro 37.Nd introduction to system calls and error numbers 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In errno.h 42.Sh DESCRIPTION 43This section provides an overview of the system calls, 44their error returns, and other common definitions and concepts. 45.\".Pp 46.\".Sy System call restart 47.\".Pp 48.\"(more later...) 49.Sh RETURN VALUES 50Nearly all of the system calls provide an error number referenced via 51the external identifier 52.Va errno . 53This identifier is defined in 54.In errno.h 55as 56.Pp 57.Dl extern __thread int errno; 58.Dl static __inline int * __error(void); 59.Dl #define errno (* __error()) 60.Pp 61This means there exists a thread-local 62.Va errno 63variable, though it is shadowed by the inline 64.Fn __error 65function to allow compilation of source code which 66erroneously itself declares 67.Va errno 68as 69.Vt extern int errno; 70which collides with the thread-local declaration. 71The 72.Fn __error 73function returns a pointer the thread specific 74.Va errno 75variable. 76As it is defined 77.Vt inline , 78it will compile to a no-op, effectively producing 79the same code as if the define wouldn't exist. 80.Pp 81When a system call detects an error, 82it returns an integer value 83indicating failure (usually -1) 84and sets the variable 85.Va errno 86accordingly. 87(This allows interpretation of the failure on receiving 88a -1 and to take action accordingly.) 89Successful calls never set 90.Va errno ; 91once set, it remains until another error occurs. 92It should only be examined after an error. 93Note that a number of system calls overload the meanings of these 94error numbers, and that the meanings must be interpreted according 95to the type and circumstances of the call. 96.Pp 97The following is a complete list of the errors and their 98names as given in 99.In sys/errno.h . 100.Bl -hang -width Ds 101.It Er 0 Em "Undefined error: 0" . 102Not used. 103.It Er 1 EPERM Em "Operation not permitted" . 104An attempt was made to perform an operation limited to processes 105with appropriate privileges or to the owner of a file or other 106resources. 107.It Er 2 ENOENT Em "No such file or directory" . 108A component of a specified pathname did not exist, or the 109pathname was an empty string. 110.It Er 3 ESRCH Em "No such process" . 111No process could be found corresponding to that specified by the given 112process ID. 113.It Er 4 EINTR Em "Interrupted system call" . 114An asynchronous signal (such as 115.Dv SIGINT 116or 117.Dv SIGQUIT ) 118was caught by the process during the execution of an interruptible 119function. 120If the signal handler performs a normal return, the 121interrupted system call will seem to have returned the error condition. 122.It Er 5 EIO Em "Input/output error" . 123Some physical input or output error occurred. 124This error will not be reported until a subsequent operation on the same file 125descriptor and may be lost (over written) by any subsequent errors. 126.It Er 6 ENXIO Em "Device not configured" . 127Input or output on a special file referred to a device that did not 128exist, or 129made a request beyond the limits of the device. 130This error may also occur when, for example, 131a tape drive is not online or no disk pack is 132loaded on a drive. 133.It Er 7 E2BIG Em "Argument list too long" . 134The number of bytes used for the argument and environment 135list of the new process exceeded the current limit 136.Dv ( NCARGS 137in 138.In sys/param.h ) . 139.It Er 8 ENOEXEC Em "Exec format error" . 140A request was made to execute a file 141that, although it has the appropriate permissions, 142was not in the format required for an 143executable file. 144.It Er 9 EBADF Em "Bad file descriptor" . 145A file descriptor argument was out of range, referred to no open file, 146or a read (write) request was made to a file that was only open for 147writing (reading). 148.Pp 149.It Er 10 ECHILD Em "\&No child processes" . 150A 151.Xr wait 2 152or 153.Xr waitpid 2 154function was executed by a process that had no existing or unwaited-for 155child processes. 156.It Er 11 EDEADLK Em "Resource deadlock avoided" . 157An attempt was made to lock a system resource that 158would have resulted in a deadlock situation. 159.It Er 12 ENOMEM Em "Cannot allocate memory" . 160The new process image required more memory than was allowed by the hardware 161or by system-imposed memory management constraints. 162A lack of swap space is normally temporary; however, 163a lack of core is not. 164Soft limits may be increased to their corresponding hard limits. 165.It Er 13 EACCES Em "Permission denied" . 166An attempt was made to access a file in a way forbidden 167by its file access permissions. 168.It Er 14 EFAULT Em "Bad address" . 169The system detected an invalid address in attempting to 170use an argument of a call. 171.It Er 15 ENOTBLK Em "Block device required" . 172A block device operation was attempted on a non-block device or file. 173.It Er 16 EBUSY Em "Device busy" . 174An attempt to use a system resource which was in use at the time 175in a manner which would have conflicted with the request. 176.It Er 17 EEXIST Em "File exists" . 177An existing file was mentioned in an inappropriate context, 178for instance, as the new link name in a 179.Xr link 2 180system call. 181.It Er 18 EXDEV Em "Cross-device link" . 182A hard link to a file on another file system 183was attempted. 184.It Er 19 ENODEV Em "Operation not supported by device" . 185An attempt was made to apply an inappropriate 186function to a device, 187for example, 188trying to read a write-only device such as a printer. 189.It Er 20 ENOTDIR Em "Not a directory" . 190A component of the specified pathname existed, but it was 191not a directory, when a directory was expected. 192.It Er 21 EISDIR Em "Is a directory" . 193An attempt was made to open a directory with write mode specified. 194.It Er 22 EINVAL Em "Invalid argument" . 195Some invalid argument was supplied. 196(For example, 197specifying an undefined signal to a 198.Xr signal 3 199function 200or a 201.Xr kill 2 202system call). 203.It Er 23 ENFILE Em "Too many open files in system" . 204Maximum number of file descriptors allowable on the system 205has been reached and a requests for an open cannot be satisfied 206until at least one has been closed. 207.It Er 24 EMFILE Em "Too many open files" . 208(As released, the limit on the number of 209open files per process is 64.) 210The 211.Xr getdtablesize 2 212system call will obtain the current limit. 213.It Er 25 ENOTTY Em "Inappropriate ioctl for device" . 214A control function (see 215.Xr ioctl 2 ) 216was attempted for a file or 217special device for which the operation was inappropriate. 218.It Er 26 ETXTBSY Em "Text file busy" . 219The new process was a pure procedure (shared text) file 220which was open for writing by another process, or 221while the pure procedure file was being executed an 222.Xr open 2 223call requested write access. 224.It Er 27 EFBIG Em "File too large" . 225The size of a file exceeded the maximum. 226.It Er 28 ENOSPC Em "No space left on device" . 227A 228.Xr write 2 229to an ordinary file, the creation of a 230directory or symbolic link, or the creation of a directory 231entry failed because no more disk blocks were available 232on the file system, or the allocation of an inode for a newly 233created file failed because no more inodes were available 234on the file system. 235.It Er 29 ESPIPE Em "Illegal seek" . 236An 237.Xr lseek 2 238system call was issued on a socket, pipe or 239.Tn FIFO . 240.It Er 30 EROFS Em "Read-only file system" . 241An attempt was made to modify a file or directory 242on a file system that was read-only at the time. 243.It Er 31 EMLINK Em "Too many links" . 244Maximum allowable hard links to a single file has been exceeded (limit 245of 32767 hard links per file). 246.It Er 32 EPIPE Em "Broken pipe" . 247A write on a pipe, socket or 248.Tn FIFO 249for which there is no process 250to read the data. 251.It Er 33 EDOM Em "Numerical argument out of domain" . 252A numerical input argument was outside the defined domain of the mathematical 253function. 254.It Er 34 ERANGE Em "Result too large" . 255A numerical result of the function was too large to fit in the 256available space (perhaps exceeded precision). 257.It Er 35 EAGAIN Em "Resource temporarily unavailable" . 258This is a temporary condition and later calls to the 259same routine may complete normally. 260.It Er 36 EINPROGRESS Em "Operation now in progress" . 261An operation that takes a long time to complete (such as 262a 263.Xr connect 2 ) 264was attempted on a non-blocking object (see 265.Xr fcntl 2 ) . 266.It Er 37 EALREADY Em "Operation already in progress" . 267An operation was attempted on a non-blocking object that already 268had an operation in progress. 269.It Er 38 ENOTSOCK Em "Socket operation on non-socket" . 270Self-explanatory. 271.It Er 39 EDESTADDRREQ Em "Destination address required" . 272A required address was omitted from an operation on a socket. 273.It Er 40 EMSGSIZE Em "Message too long" . 274A message sent on a socket was larger than the internal message buffer 275or some other network limit. 276.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" . 277A protocol was specified that does not support the semantics of the 278socket type requested. 279For example, you cannot use the 280.Tn ARPA 281Internet 282.Tn UDP 283protocol with type 284.Dv SOCK_STREAM . 285.It Er 42 ENOPROTOOPT Em "Protocol not available" . 286A bad option or level was specified in a 287.Xr getsockopt 2 288or 289.Xr setsockopt 2 290call. 291.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" . 292The protocol has not been configured into the 293system or no implementation for it exists. 294.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" . 295The support for the socket type has not been configured into the 296system or no implementation for it exists. 297.It Er 45 EOPNOTSUPP Em "Operation not supported" . 298The attempted operation is not supported for the type of object referenced. 299Usually this occurs when a file descriptor refers to a file or socket 300that cannot support this operation, 301for example, trying to 302.Em accept 303a connection on a datagram socket. 304.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" . 305The protocol family has not been configured into the 306system or no implementation for it exists. 307.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" . 308An address incompatible with the requested protocol was used. 309For example, you should not necessarily expect to be able to use 310.Tn NS 311addresses with 312.Tn ARPA 313Internet protocols. 314.It Er 48 EADDRINUSE Em "Address already in use" . 315Only one usage of each address is normally permitted. 316.Pp 317.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" . 318Normally results from an attempt to create a socket with an 319address not on this machine. 320.It Er 50 ENETDOWN Em "Network is down" . 321A socket operation encountered a dead network. 322.It Er 51 ENETUNREACH Em "Network is unreachable" . 323A socket operation was attempted to an unreachable network. 324.It Er 52 ENETRESET Em "Network dropped connection on reset" . 325The host you were connected to crashed and rebooted. 326.It Er 53 ECONNABORTED Em "Software caused connection abort" . 327A connection abort was caused internal to your host machine. 328.It Er 54 ECONNRESET Em "Connection reset by peer" . 329A connection was forcibly closed by a peer. 330This normally 331results from a loss of the connection on the remote socket 332due to a timeout or a reboot. 333.It Er 55 ENOBUFS Em "\&No buffer space available" . 334An operation on a socket or pipe was not performed because 335the system lacked sufficient buffer space or because a queue was full. 336.It Er 56 EISCONN Em "Socket is already connected" . 337A 338.Xr connect 2 339request was made on an already connected socket; or, 340a 341.Xr sendto 2 342or 343.Xr sendmsg 2 344request on a connected socket specified a destination 345when already connected. 346.It Er 57 ENOTCONN Em "Socket is not connected" . 347An request to send or receive data was disallowed because 348the socket was not connected and (when sending on a datagram socket) 349no address was supplied. 350.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" . 351A request to send data was disallowed because the socket 352had already been shut down with a previous 353.Xr shutdown 2 354call. 355.It Er 60 ETIMEDOUT Em "Operation timed out" . 356A 357.Xr connect 2 358or 359.Xr send 2 360request failed because the connected party did not 361properly respond after a period of time. 362(The timeout 363period is dependent on the communication protocol.) 364.It Er 61 ECONNREFUSED Em "Connection refused" . 365No connection could be made because the target machine actively 366refused it. 367This usually results from trying to connect 368to a service that is inactive on the foreign host. 369.It Er 62 ELOOP Em "Too many levels of symbolic links" . 370A path name lookup involved more than 32 371.Pq Dv MAXSYMLINKS 372symbolic links. 373.It Er 63 ENAMETOOLONG Em "File name too long" . 374A component of a path name exceeded 375.Brq Dv NAME_MAX 376characters, or an entire 377path name exceeded 378.Brq Dv PATH_MAX 379characters. 380(See also the description of 381.Dv _PC_NO_TRUNC 382in 383.Xr pathconf 2 . ) 384.It Er 64 EHOSTDOWN Em "Host is down" . 385A socket operation failed because the destination host was down. 386.It Er 65 EHOSTUNREACH Em "No route to host" . 387A socket operation was attempted to an unreachable host. 388.It Er 66 ENOTEMPTY Em "Directory not empty" . 389A directory with entries other than 390.Ql .\& 391and 392.Ql ..\& 393was supplied to a remove directory or rename call. 394.It Er 67 EPROCLIM Em "Too many processes" . 395.It Er 68 EUSERS Em "Too many users" . 396The quota system ran out of table entries. 397.It Er 69 EDQUOT Em "Disc quota exceeded" . 398A 399.Xr write 2 400to an ordinary file, the creation of a 401directory or symbolic link, or the creation of a directory 402entry failed because the user's quota of disk blocks was 403exhausted, or the allocation of an inode for a newly 404created file failed because the user's quota of inodes 405was exhausted. 406.It Er 70 ESTALE Em "Stale NFS file handle" . 407An attempt was made to access an open file (on an 408.Tn NFS 409file system) 410which is now unavailable as referenced by the file descriptor. 411This may indicate the file was deleted on the 412.Tn NFS 413server or some 414other catastrophic event occurred. 415.It Er 72 EBADRPC Em "RPC struct is bad" . 416Exchange of 417.Tn RPC 418information was unsuccessful. 419.It Er 73 ERPCMISMATCH Em "RPC version wrong" . 420The version of 421.Tn RPC 422on the remote peer is not compatible with 423the local version. 424.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" . 425The requested program is not registered on the remote host. 426.It Er 75 EPROGMISMATCH Em "Program version wrong" . 427The requested version of the program is not available 428on the remote host 429.Pq Tn RPC . 430.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" . 431An 432.Tn RPC 433call was attempted for a procedure which does not exist 434in the remote program. 435.It Er 77 ENOLCK Em "No locks available" . 436A system-imposed limit on the number of simultaneous file 437locks was reached. 438.It Er 78 ENOSYS Em "Function not implemented" . 439Attempted a system call that is not available on this 440system. 441.It Er 79 EFTYPE Em "Inappropriate file type or format" . 442The file was the wrong type for the operation, or a data file had 443the wrong format. 444.It Er 80 EAUTH Em "Authentication error" . 445Attempted to use an invalid authentication ticket to mount a 446.Tn NFS 447file system. 448.It Er 81 ENEEDAUTH Em "Need authenticator" . 449An authentication ticket must be obtained before the given 450.Tn NFS 451file system may be mounted. 452.It Er 82 EIDRM Em "Identifier removed" . 453An IPC identifier was removed while the current process was waiting on it. 454.It Er 83 ENOMSG Em "No message of desired type" . 455An IPC message queue does not contain a message of the desired type, or a 456message catalog does not contain the requested message. 457.It Er 84 EOVERFLOW Em "Value too large to be stored in data type" . 458A numerical result of the function was too large to be stored in the caller 459provided space. 460.It Er 85 ECANCELED Em "Operation canceled" . 461The scheduled operation was canceled. 462.It Er 86 EILSEQ Em "Illegal byte sequence" . 463While decoding a multibyte character the function came along an 464invalid or an incomplete sequence of bytes or the given wide 465character is invalid. 466.It Er 87 ENOATTR Em "Attribute not found" . 467The specified extended attribute does not exist. 468.It Er 88 EDOOFUS Em "Programming error" . 469A function or API is being abused in a way which could only be detected 470at run-time. 471.El 472.Sh DEFINITIONS 473.Bl -tag -width Ds 474.It Process ID . 475Each active process in the system is uniquely identified by a non-negative 476integer called a process ID. 477The range of this ID is from 0 to 99999. 478.It Parent process ID 479A new process is created by a currently active process (see 480.Xr fork 2 ) . 481The parent process ID of a process is initially the process ID of its creator. 482If the creating process exits, 483the parent process ID of each child is set to the ID of a system process, 484.Xr init 8 . 485.It Process Group 486Each active process is a member of a process group that is identified by 487a non-negative integer called the process group ID. 488This is the process 489ID of the group leader. 490This grouping permits the signaling of related 491processes (see 492.Xr termios 4 ) 493and the job control mechanisms of 494.Xr csh 1 . 495.It Session 496A session is a set of one or more process groups. 497A session is created by a successful call to 498.Xr setsid 2 , 499which causes the caller to become the only member of the only process 500group in the new session. 501.It Session leader 502A process that has created a new session by a successful call to 503.Xr setsid 2 , 504is known as a session leader. 505Only a session leader may acquire a terminal as its controlling terminal (see 506.Xr termios 4 ) . 507.It Controlling process 508A session leader with a controlling terminal is a controlling process. 509.It Controlling terminal 510A terminal that is associated with a session is known as the controlling 511terminal for that session and its members. 512.It "Terminal Process Group ID" 513A terminal may be acquired by a session leader as its controlling terminal. 514Once a terminal is associated with a session, any of the process groups 515within the session may be placed into the foreground by setting 516the terminal process group ID to the ID of the process group. 517This facility is used 518to arbitrate between multiple jobs contending for the same terminal; 519(see 520.Xr csh 1 521and 522.Xr tty 4 ) . 523.It "Orphaned Process Group" 524A process group is considered to be 525.Em orphaned 526if it is not under the control of a job control shell. 527More precisely, a process group is orphaned 528when none of its members has a parent process that is in the same session 529as the group, 530but is in a different process group. 531Note that when a process exits, the parent process for its children 532is changed to be 533.Xr init 8 , 534which is in a separate session. 535Not all members of an orphaned process group are necessarily orphaned 536processes (those whose creating process has exited). 537The process group of a session leader is orphaned by definition. 538.It "Real User ID and Real Group ID" 539Each user on the system is identified by a positive integer 540termed the real user ID. 541.Pp 542Each user is also a member of one or more groups. 543One of these groups is distinguished from others and 544used in implementing accounting facilities. 545The positive 546integer corresponding to this distinguished group is termed 547the real group ID. 548.Pp 549All processes have a real user ID and real group ID. 550These are initialized from the equivalent attributes 551of the process that created it. 552.It "Effective User Id, Effective Group Id, and Group Access List" 553Access to system resources is governed by two values: 554the effective user ID, and the group access list. 555The first member of the group access list is also known as the 556effective group ID. 557(In POSIX.1, the group access list is known as the set of supplementary 558group IDs, and it is unspecified whether the effective group ID is 559a member of the list.) 560.Pp 561The effective user ID and effective group ID are initially the 562process's real user ID and real group ID respectively. 563Either 564may be modified through execution of a set-user-ID or set-group-ID 565file (possibly by one its ancestors) (see 566.Xr execve 2 ) . 567By convention, the effective group ID (the first member of the group access 568list) is duplicated, so that the execution of a set-group-ID program 569does not result in the loss of the original (real) group ID. 570.Pp 571The group access list is a set of group IDs 572used only in determining resource accessibility. 573Access checks 574are performed as described below in ``File Access Permissions''. 575.It "Saved Set User ID and Saved Set Group ID" 576When a process executes a new file, the effective user ID is set 577to the owner of the file if the file is set-user-ID, and the effective 578group ID (first element of the group access list) is set to the group 579of the file if the file is set-group-ID. 580The effective user ID of the process is then recorded as the saved set-user-ID, 581and the effective group ID of the process is recorded as the saved set-group-ID. 582These values may be used to regain those values as the effective user 583or group ID after reverting to the real ID (see 584.Xr setuid 2 ) . 585(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, 586and are used in setuid and setgid, but this does not work as desired 587for the super-user.) 588.It Super-user 589A process is recognized as a 590.Em super-user 591process and is granted special privileges if its effective user ID is 0. 592.It Descriptor 593An integer assigned by the system when a file is referenced 594by 595.Xr open 2 596or 597.Xr dup 2 , 598or when a socket is created by 599.Xr pipe 2 , 600.Xr socket 2 601or 602.Xr socketpair 2 , 603which uniquely identifies an access path to that file or socket from 604a given process or any of its children. 605.It File Name 606Names consisting of up to 607.Brq Dv NAME_MAX 608characters may be used to name 609an ordinary file, special file, or directory. 610.Pp 611These characters may be arbitrary eight-bit values, 612excluding 613.Dv NUL 614.Tn ( ASCII 6150) and the 616.Ql \&/ 617character (slash, 618.Tn ASCII 61947). 620.Pp 621Note that it is generally unwise to use 622.Ql \&* , 623.Ql \&? , 624.Ql \&[ 625or 626.Ql \&] 627as part of 628file names because of the special meaning attached to these characters 629by the shell. 630.It Path Name 631A path name is a 632.Dv NUL Ns -terminated 633character string starting with an 634optional slash 635.Ql \&/ , 636followed by zero or more directory names separated 637by slashes, optionally followed by a file name. 638The total length of a path name must be less than 639.Brq Dv PATH_MAX 640characters. 641(On some systems, this limit may be infinite.) 642.Pp 643If a path name begins with a slash, the path search begins at the 644.Em root 645directory. 646Otherwise, the search begins from the current working directory. 647A slash by itself names the root directory. 648An empty 649pathname refers to the current directory. 650.It Directory 651A directory is a special type of file that contains entries 652that are references to other files. 653Directory entries are called links. 654By convention, a directory 655contains at least two links, 656.Ql .\& 657and 658.Ql \&.. , 659referred to as 660.Em dot 661and 662.Em dot-dot 663respectively. 664Dot refers to the directory itself and 665dot-dot refers to its parent directory. 666.It "Root Directory and Current Working Directory" 667Each process has associated with it a concept of a root directory 668and a current working directory for the purpose of resolving path 669name searches. 670A process's root directory need not be the root 671directory of the root file system. 672.It File Access Permissions 673Every file in the file system has a set of access permissions. 674These permissions are used in determining whether a process 675may perform a requested operation on the file (such as opening 676a file for writing). 677Access permissions are established at the 678time a file is created. 679They may be changed at some later time 680through the 681.Xr chmod 2 682call. 683.Pp 684File access is broken down according to whether a file may be: read, 685written, or executed. 686Directory files use the execute 687permission to control if the directory may be searched. 688.Pp 689File access permissions are interpreted by the system as 690they apply to three different classes of users: the owner 691of the file, those users in the file's group, anyone else. 692Every file has an independent set of access permissions for 693each of these classes. 694When an access check is made, the system 695decides if permission should be granted by checking the access 696information applicable to the caller. 697.Pp 698Read, write, and execute/search permissions on 699a file are granted to a process if: 700.Pp 701The process's effective user ID is that of the super-user. 702(Note: 703even the super-user cannot execute a non-executable file.) 704.Pp 705The process's effective user ID matches the user ID of the owner 706of the file and the owner permissions allow the access. 707.Pp 708The process's effective user ID does not match the user ID of the 709owner of the file, and either the process's effective 710group ID matches the group ID 711of the file, or the group ID of the file is in 712the process's group access list, 713and the group permissions allow the access. 714.Pp 715Neither the effective user ID nor effective group ID 716and group access list of the process 717match the corresponding user ID and group ID of the file, 718but the permissions for ``other users'' allow access. 719.Pp 720Otherwise, permission is denied. 721.It Sockets and Address Families 722A socket is an endpoint for communication between processes. 723Each socket has queues for sending and receiving data. 724.Pp 725Sockets are typed according to their communications properties. 726These properties include whether messages sent and received 727at a socket require the name of the partner, whether communication 728is reliable, the format used in naming message recipients, etc. 729.Pp 730Each instance of the system supports some 731collection of socket types; consult 732.Xr socket 2 733for more information about the types available and 734their properties. 735.Pp 736Each instance of the system supports some number of sets of 737communications protocols. 738Each protocol set supports addresses of a certain format. 739An Address Family is the set of addresses for a specific group of protocols. 740Each socket has an address 741chosen from the address family in which the socket was created. 742.El 743.Sh SEE ALSO 744.Xr intro 3 , 745.Xr perror 3 746