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Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)intro.2 8.5 (Berkeley) 2/27/95 33.\" $FreeBSD: src/lib/libc/sys/intro.2,v 1.21.2.7 2003/02/24 01:01:48 trhodes Exp $ 34.\" $DragonFly: src/lib/libc/sys/intro.2,v 1.4 2006/05/26 19:39:37 swildner Exp $ 35.\" 36.Dd February 27, 1995 37.Dt INTRO 2 38.Os 39.Sh NAME 40.Nm intro 41.Nd introduction to system calls and error numbers 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In errno.h 46.Sh DESCRIPTION 47This section provides an overview of the system calls, 48their error returns, and other common definitions and concepts. 49.\".Pp 50.\".Sy System call restart 51.\".Pp 52.\"<more later...> 53.Sh RETURN VALUES 54Nearly all of the system calls provide an error number referenced via 55the external identifier 56.Va errno. 57This identifier is defined in 58.In errno.h 59as 60.Pp 61.Dl extern __thread int errno; 62.Dl static __inline int * __error(void); 63.Dl #define errno (* __error()) 64.Pp 65This means there exists a thread-local 66.Va errno 67variable, though it is shadowed by the inline 68.Va __error() 69function to allow compilation of source code which 70erroneously itself declares 71.Va errno 72as 73.Va extern int errno; 74which collides with the thread-local declaration. 75The 76.Va __error() 77function returns a pointer the thread specific 78.Va errno 79variable. As it is defined 80.Va inline , 81it will compile to a no-op, effectively producing 82the same code as if the define wouldn't exist. 83.Pp 84When a system call detects an error, 85it returns an integer value 86indicating failure (usually -1) 87and sets the variable 88.Va errno 89accordingly. 90<This allows interpretation of the failure on receiving 91a -1 and to take action accordingly.> 92Successful calls never set 93.Va errno ; 94once set, it remains until another error occurs. 95It should only be examined after an error. 96Note that a number of system calls overload the meanings of these 97error numbers, and that the meanings must be interpreted according 98to the type and circumstances of the call. 99.Pp 100The following is a complete list of the errors and their 101names as given in 102.In sys/errno.h . 103.Bl -hang -width Ds 104.It Er 0 Em "Undefined error: 0" . 105Not used. 106.It Er 1 EPERM Em "Operation not permitted" . 107An attempt was made to perform an operation limited to processes 108with appropriate privileges or to the owner of a file or other 109resources. 110.It Er 2 ENOENT Em "No such file or directory" . 111A component of a specified pathname did not exist, or the 112pathname was an empty string. 113.It Er 3 ESRCH Em "No such process" . 114No process could be found corresponding to that specified by the given 115process ID. 116.It Er 4 EINTR Em "Interrupted system call" . 117An asynchronous signal (such as 118.Dv SIGINT 119or 120.Dv SIGQUIT ) 121was caught by the process during the execution of an interruptible 122function. 123If the signal handler performs a normal return, the 124interrupted function call will seem to have returned the error condition. 125.It Er 5 EIO Em "Input/output error" . 126Some physical input or output error occurred. 127This error will not be reported until a subsequent operation on the same file 128descriptor and may be lost (over written) by any subsequent errors. 129.It Er 6 ENXIO Em "Device not configured" . 130Input or output on a special file referred to a device that did not 131exist, or 132made a request beyond the limits of the device. 133This error may also occur when, for example, 134a tape drive is not online or no disk pack is 135loaded on a drive. 136.It Er 7 E2BIG Em "Argument list too long" . 137The number of bytes used for the argument and environment 138list of the new process exceeded the current limit 139of 65536 bytes 140.Pf ( Dv NCARGS 141in 142.In sys/param.h ) . 143.It Er 8 ENOEXEC Em "Exec format error" . 144A request was made to execute a file 145that, although it has the appropriate permissions, 146was not in the format required for an 147executable file. 148.It Er 9 EBADF Em "Bad file descriptor" . 149A file descriptor argument was out of range, referred to no open file, 150or a read (write) request was made to a file that was only open for 151writing (reading). 152.Pp 153.It Er 10 ECHILD Em "\&No child processes" . 154A 155.Xr wait 2 156or 157.Xr waitpid 2 158function was executed by a process that had no existing or unwaited-for 159child processes. 160.It Er 11 EDEADLK Em "Resource deadlock avoided" . 161An attempt was made to lock a system resource that 162would have resulted in a deadlock situation. 163.It Er 12 ENOMEM Em "Cannot allocate memory" . 164The new process image required more memory than was allowed by the hardware 165or by system-imposed memory management constraints. 166A lack of swap space is normally temporary; however, 167a lack of core is not. 168Soft limits may be increased to their corresponding hard limits. 169.It Er 13 EACCES Em "Permission denied" . 170An attempt was made to access a file in a way forbidden 171by its file access permissions. 172.It Er 14 EFAULT Em "Bad address" . 173The system detected an invalid address in attempting to 174use an argument of a call. 175.It Er 15 ENOTBLK Em "Block device required" . 176A block device operation was attempted on a non-block device or file. 177.It Er 16 EBUSY Em "Device busy" . 178An attempt to use a system resource which was in use at the time 179in a manner which would have conflicted with the request. 180.It Er 17 EEXIST Em "File exists" . 181An existing file was mentioned in an inappropriate context, 182for instance, as the new link name in a 183.Xr link 2 184function. 185.It Er 18 EXDEV Em "Cross-device link" . 186A hard link to a file on another file system 187was attempted. 188.It Er 19 ENODEV Em "Operation not supported by device" . 189An attempt was made to apply an inappropriate 190function to a device, 191for example, 192trying to read a write-only device such as a printer. 193.It Er 20 ENOTDIR Em "Not a directory" . 194A component of the specified pathname existed, but it was 195not a directory, when a directory was expected. 196.It Er 21 EISDIR Em "Is a directory" . 197An attempt was made to open a directory with write mode specified. 198.It Er 22 EINVAL Em "Invalid argument" . 199Some invalid argument was supplied. 200(For example, 201specifying an undefined signal to a 202.Xr signal 3 203or 204.Xr kill 2 205function). 206.It Er 23 ENFILE Em "Too many open files in system" . 207Maximum number of file descriptors allowable on the system 208has been reached and a requests for an open cannot be satisfied 209until at least one has been closed. 210.It Er 24 EMFILE Em "Too many open files" . 211<As released, the limit on the number of 212open files per process is 64.> 213The 214.Xr getdtablesize 2 215function will obtain the current limit. 216.It Er 25 ENOTTY Em "Inappropriate ioctl for device" . 217A control function (see 218.Xr ioctl 2 ) 219was attempted for a file or 220special device for which the operation was inappropriate. 221.It Er 26 ETXTBSY Em "Text file busy" . 222The new process was a pure procedure (shared text) file 223which was open for writing by another process, or 224while the pure procedure file was being executed an 225.Xr open 2 226call requested write access. 227.It Er 27 EFBIG Em "File too large" . 228The size of a file exceeded the maximum (about 229.if t 2\u\s-231\s+2\d 230.if n 2.1E9 231bytes). 232.It Er 28 ENOSPC Em "No space left on device" . 233A 234.Xr write 2 235to an ordinary file, the creation of a 236directory or symbolic link, or the creation of a directory 237entry failed because no more disk blocks were available 238on the file system, or the allocation of an inode for a newly 239created file failed because no more inodes were available 240on the file system. 241.It Er 29 ESPIPE Em "Illegal seek" . 242An 243.Xr lseek 2 244function was issued on a socket, pipe or 245.Tn FIFO . 246.It Er 30 EROFS Em "Read-only file system" . 247An attempt was made to modify a file or directory 248on a file system that was read-only at the time. 249.It Er 31 EMLINK Em "Too many links" . 250Maximum allowable hard links to a single file has been exceeded (limit 251of 32767 hard links per file). 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 "Result too large" . 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.Pp 323.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" . 324Normally results from an attempt to create a socket with an 325address not on this machine. 326.It Er 50 ENETDOWN Em "Network is down" . 327A socket operation encountered a dead network. 328.It Er 51 ENETUNREACH Em "Network is unreachable" . 329A socket operation was attempted to an unreachable network. 330.It Er 52 ENETRESET Em "Network dropped connection on reset" . 331The host you were connected to crashed and rebooted. 332.It Er 53 ECONNABORTED Em "Software caused connection abort" . 333A connection abort was caused internal to your host machine. 334.It Er 54 ECONNRESET Em "Connection reset by peer" . 335A connection was forcibly closed by a peer. This normally 336results from a loss of the connection on the remote socket 337due to a timeout or a reboot. 338.It Er 55 ENOBUFS Em "\&No buffer space available" . 339An operation on a socket or pipe was not performed because 340the system lacked sufficient buffer space or because a queue was full. 341.It Er 56 EISCONN Em "Socket is already connected" . 342A 343.Xr connect 2 344request was made on an already connected socket; or, 345a 346.Xr sendto 2 347or 348.Xr sendmsg 2 349request on a connected socket specified a destination 350when already connected. 351.It Er 57 ENOTCONN Em "Socket is not connected" . 352An request to send or receive data was disallowed because 353the socket was not connected and (when sending on a datagram socket) 354no address was supplied. 355.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" . 356A request to send data was disallowed because the socket 357had already been shut down with a previous 358.Xr shutdown 2 359call. 360.It Er 60 ETIMEDOUT Em "Operation timed out" . 361A 362.Xr connect 2 363or 364.Xr send 2 365request failed because the connected party did not 366properly respond after a period of time. (The timeout 367period is dependent on the communication protocol.) 368.It Er 61 ECONNREFUSED Em "Connection refused" . 369No connection could be made because the target machine actively 370refused it. This 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 32 374.Pq Dv MAXSYMLINKS 375symbolic links. 376.It Er 63 ENAMETOOLONG Em "File name too long" . 377A component of a path name exceeded 255 378.Pq Dv MAXNAMELEN 379characters, or an entire 380path name exceeded 1023 381.Pq Dv MAXPATHLEN Ns -1 382characters. 383.It Er 64 EHOSTDOWN Em "Host is down" . 384A socket operation failed because the destination host was down. 385.It Er 65 EHOSTUNREACH Em "No route to host" . 386A socket operation was attempted to an unreachable host. 387.It Er 66 ENOTEMPTY Em "Directory not empty" . 388A directory with entries other than 389.Ql .\& 390and 391.Ql ..\& 392was supplied to a remove directory or rename call. 393.It Er 67 EPROCLIM Em "Too many processes" . 394.It Er 68 EUSERS Em "Too many users" . 395The quota system ran out of table entries. 396.It Er 69 EDQUOT Em "Disc quota exceeded" . 397A 398.Xr write 2 399to an ordinary file, the creation of a 400directory or symbolic link, or the creation of a directory 401entry failed because the user's quota of disk blocks was 402exhausted, or the allocation of an inode for a newly 403created file failed because the user's quota of inodes 404was exhausted. 405.It Er 70 ESTALE Em "Stale NFS file handle" . 406An attempt was made to access an open file (on an 407.Tn NFS 408filesystem) 409which is now unavailable as referenced by the file descriptor. 410This may indicate the file was deleted on the 411.Tn NFS 412server or some 413other catastrophic event occurred. 414.It Er 72 EBADRPC Em "RPC struct is bad" . 415Exchange of 416.Tn RPC 417information was unsuccessful. 418.It Er 73 ERPCMISMATCH Em "RPC version wrong" . 419The version of 420.Tn RPC 421on the remote peer is not compatible with 422the local version. 423.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" . 424The requested program is not registered on the remote host. 425.It Er 75 EPROGMISMATCH Em "Program version wrong" . 426The requested version of the program is not available 427on the remote host 428.Pq Tn RPC . 429.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" . 430An 431.Tn RPC 432call was attempted for a procedure which doesn't exist 433in the remote program. 434.It Er 77 ENOLCK Em "No locks available" . 435A system-imposed limit on the number of simultaneous file 436locks was reached. 437.It Er 78 ENOSYS Em "Function not implemented" . 438Attempted a system call that is not available on this 439system. 440.It Er 79 EFTYPE Em "Inappropriate file type or format" . 441The file was the wrong type for the operation, or a data file had 442the wrong format. 443.It Er 80 EAUTH Em "Authentication error" . 444Attempted to use an invalid authentication ticket to mount a 445.Tn NFS 446filesystem. 447.It Er 81 ENEEDAUTH Em "Need authenticator" . 448An authentication ticket must be obtained before the given 449.Tn NFS 450filesystem may be mounted. 451.It Er 82 EIDRM Em "Identifier removed" . 452An IPC identifier was removed while the current process was waiting on it. 453.It Er 83 ENOMSG Em "No message of desired type" . 454An IPC message queue does not contain a message of the desired type, or a 455message catalog does not contain the requested message. 456.It Er 84 EOVERFLOW Em "Value too large to be stored in data type" . 457A numerical result of the function was too large to be stored in the caller 458provided space. 459.It Er 85 ECANCELED Em "Operation canceled" . 460The scheduled operation was canceled. 461.It Er 86 EILSEQ Em "Illegal byte sequence" . 462While decoding a multibyte character the function came along an 463invalid or an incomplete sequence of bytes or the given wide 464character is invalid. 465.El 466.Sh DEFINITIONS 467.Bl -tag -width Ds 468.It Process ID . 469Each active process in the system is uniquely identified by a non-negative 470integer called a process ID. The range of this ID is from 0 to 99999. 471.It Parent process ID 472A new process is created by a currently active process; (see 473.Xr fork 2 ) . 474The parent process ID of a process is initially the process ID of its creator. 475If the creating process exits, 476the parent process ID of each child is set to the ID of a system process, 477.Xr init 8 . 478.It Process Group 479Each active process is a member of a process group that is identified by 480a non-negative integer called the process group ID. This is the process 481ID of the group leader. This grouping permits the signaling of related 482processes (see 483.Xr termios 4 ) 484and the job control mechanisms of 485.Xr csh 1 . 486.It Session 487A session is a set of one or more process groups. 488A session is created by a successful call to 489.Xr setsid 2 , 490which causes the caller to become the only member of the only process 491group in the new session. 492.It Session leader 493A process that has created a new session by a successful call to 494.Xr setsid 2 , 495is known as a session leader. 496Only a session leader may acquire a terminal as its controlling terminal (see 497.Xr termios 4 ) . 498.It Controlling process 499A session leader with a controlling terminal is a controlling process. 500.It Controlling terminal 501A terminal that is associated with a session is known as the controlling 502terminal for that session and its members. 503.It "Terminal Process Group ID" 504A terminal may be acquired by a session leader as its controlling terminal. 505Once a terminal is associated with a session, any of the process groups 506within the session may be placed into the foreground by setting 507the terminal process group ID to the ID of the process group. 508This facility is used 509to arbitrate between multiple jobs contending for the same terminal; 510(see 511.Xr csh 1 512and 513.Xr tty 4 ) . 514.It "Orphaned Process Group" 515A process group is considered to be 516.Em orphaned 517if it is not under the control of a job control shell. 518More precisely, a process group is orphaned 519when none of its members has a parent process that is in the same session 520as the group, 521but is in a different process group. 522Note that when a process exits, the parent process for its children 523is changed to be 524.Xr init 8 , 525which is in a separate session. 526Not all members of an orphaned process group are necessarily orphaned 527processes (those whose creating process has exited). 528The process group of a session leader is orphaned by definition. 529.It "Real User ID and Real Group ID" 530Each user on the system is identified by a positive integer 531termed the real user ID. 532.Pp 533Each user is also a member of one or more groups. 534One of these groups is distinguished from others and 535used in implementing accounting facilities. The positive 536integer corresponding to this distinguished group is termed 537the real group ID. 538.Pp 539All processes have a real user ID and real group ID. 540These are initialized from the equivalent attributes 541of the process that created it. 542.It "Effective User Id, Effective Group Id, and Group Access List" 543Access to system resources is governed by two values: 544the effective user ID, and the group access list. 545The first member of the group access list is also known as the 546effective group ID. 547(In POSIX.1, the group access list is known as the set of supplementary 548group IDs, and it is unspecified whether the effective group ID is 549a member of the list.) 550.Pp 551The effective user ID and effective group ID are initially the 552process's real user ID and real group ID respectively. Either 553may be modified through execution of a set-user-ID or set-group-ID 554file (possibly by one its ancestors) (see 555.Xr execve 2 ) . 556By convention, the effective group ID (the first member of the group access 557list) is duplicated, so that the execution of a set-group-ID program 558does not result in the loss of the original (real) group ID. 559.Pp 560The group access list is a set of group IDs 561used only in determining resource accessibility. Access checks 562are performed as described below in ``File Access Permissions''. 563.It "Saved Set User ID and Saved Set Group ID" 564When a process executes a new file, the effective user ID is set 565to the owner of the file if the file is set-user-ID, and the effective 566group ID (first element of the group access list) is set to the group 567of the file if the file is set-group-ID. 568The effective user ID of the process is then recorded as the saved set-user-ID, 569and the effective group ID of the process is recorded as the saved set-group-ID. 570These values may be used to regain those values as the effective user 571or group ID after reverting to the real ID (see 572.Xr setuid 2 ) . 573(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, 574and are used in setuid and setgid, but this does not work as desired 575for the super-user.) 576.It Super-user 577A process is recognized as a 578.Em super-user 579process and is granted special privileges if its effective user ID is 0. 580.It Special Processes 581The processes with process IDs of 0, 1, and 2 are special. 582Process 0 is the scheduler. Process 1 is the initialization process 583.Xr init 8 , 584and is the ancestor of every other process in the system. 585It is used to control the process structure. 586Process 2 is the paging daemon. 587.It Descriptor 588An integer assigned by the system when a file is referenced 589by 590.Xr open 2 591or 592.Xr dup 2 , 593or when a socket is created by 594.Xr pipe 2 , 595.Xr socket 2 596or 597.Xr socketpair 2 , 598which uniquely identifies an access path to that file or socket from 599a given process or any of its children. 600.It File Name 601Names consisting of up to 255 602.Pq Dv MAXNAMELEN 603characters may be used to name 604an ordinary file, special file, or directory. 605.Pp 606These characters may be selected from the set of all 607.Tn ASCII 608character 609excluding 0 (NUL) and the 610.Tn ASCII 611code for 612.Ql \&/ 613(slash). 614.Pp 615Note that it is generally unwise to use 616.Ql \&* , 617.Ql \&? , 618.Ql \&[ 619or 620.Ql \&] 621as part of 622file names because of the special meaning attached to these characters 623by the shell. 624.It Path Name 625A path name is a 626.Tn NUL Ns -terminated 627character string starting with an 628optional slash 629.Ql \&/ , 630followed by zero or more directory names separated 631by slashes, optionally followed by a file name. 632The total length of a path name must be less than 1024 633.Pq Dv MAXPATHLEN 634characters. 635.Pp 636If a path name begins with a slash, the path search begins at the 637.Em root 638directory. 639Otherwise, the search begins from the current working directory. 640A slash by itself names the root directory. An empty 641pathname refers to the current directory. 642.It Directory 643A directory is a special type of file that contains entries 644that are references to other files. 645Directory entries are called links. By convention, a directory 646contains at least two links, 647.Ql .\& 648and 649.Ql \&.. , 650referred to as 651.Em dot 652and 653.Em dot-dot 654respectively. Dot refers to the directory itself and 655dot-dot refers to its parent directory. 656.It "Root Directory and Current Working Directory" 657Each process has associated with it a concept of a root directory 658and a current working directory for the purpose of resolving path 659name searches. A process's root directory need not be the root 660directory of the root file system. 661.It File Access Permissions 662Every file in the file system has a set of access permissions. 663These permissions are used in determining whether a process 664may perform a requested operation on the file (such as opening 665a file for writing). Access permissions are established at the 666time a file is created. They may be changed at some later time 667through the 668.Xr chmod 2 669call. 670.Pp 671File access is broken down according to whether a file may be: read, 672written, or executed. Directory files use the execute 673permission to control if the directory may be searched. 674.Pp 675File access permissions are interpreted by the system as 676they apply to three different classes of users: the owner 677of the file, those users in the file's group, anyone else. 678Every file has an independent set of access permissions for 679each of these classes. When an access check is made, the system 680decides if permission should be granted by checking the access 681information applicable to the caller. 682.Pp 683Read, write, and execute/search permissions on 684a file are granted to a process if: 685.Pp 686The process's effective user ID is that of the super-user. 687(Note: 688even the super-user cannot execute a non-executable file.) 689.Pp 690The process's effective user ID matches the user ID of the owner 691of the file and the owner permissions allow the access. 692.Pp 693The process's effective user ID does not match the user ID of the 694owner of the file, and either the process's effective 695group ID matches the group ID 696of the file, or the group ID of the file is in 697the process's group access list, 698and the group permissions allow the access. 699.Pp 700Neither the effective user ID nor effective group ID 701and group access list of the process 702match the corresponding user ID and group ID of the file, 703but the permissions for ``other users'' allow access. 704.Pp 705Otherwise, permission is denied. 706.It Sockets and Address Families 707A socket is an endpoint for communication between processes. 708Each socket has queues for sending and receiving data. 709.Pp 710Sockets are typed according to their communications properties. 711These properties include whether messages sent and received 712at a socket require the name of the partner, whether communication 713is reliable, the format used in naming message recipients, etc. 714.Pp 715Each instance of the system supports some 716collection of socket types; consult 717.Xr socket 2 718for more information about the types available and 719their properties. 720.Pp 721Each instance of the system supports some number of sets of 722communications protocols. Each protocol set supports addresses 723of a certain format. An Address Family is the set of addresses 724for a specific group of protocols. Each socket has an address 725chosen from the address family in which the socket was created. 726.El 727.Sh SEE ALSO 728.Xr intro 3 , 729.Xr perror 3 730