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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.\" 35.Dd February 27, 1995 36.Dt INTRO 2 37.Os 38.Sh NAME 39.Nm intro 40.Nd introduction to system calls and error numbers 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In 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 RETURN VALUES 53Nearly all of the system calls provide an error number referenced via 54the external identifier errno. 55This identifier is defined in 56.Aq Pa sys/errno.h 57as 58.Pp 59.Dl extern int * __error(); 60.Dl #define errno (* __error()) 61.Pp 62The 63.Va __error() 64function returns a pointer to a field in the thread specific structure for 65threads other than the initial thread. 66For the initial thread and 67non-threaded processes, 68.Va __error() 69returns a pointer to a global 70.Va errno 71variable that is compatible with the previous definition. 72.Pp 73When a system call detects an error, 74it returns an integer value 75indicating failure (usually -1) 76and sets the variable 77.Va errno 78accordingly. 79<This allows interpretation of the failure on receiving 80a -1 and to take action accordingly.> 81Successful calls never set 82.Va errno ; 83once set, it remains until another error occurs. 84It should only be examined after an error. 85Note that a number of system calls overload the meanings of these 86error numbers, and that the meanings must be interpreted according 87to the type and circumstances of the call. 88.Pp 89The following is a complete list of the errors and their 90names as given in 91.Aq Pa sys/errno.h . 92.Bl -hang -width Ds 93.It Er 0 Em "Undefined error: 0" . 94Not used. 95.It Er 1 EPERM Em "Operation not permitted" . 96An attempt was made to perform an operation limited to processes 97with appropriate privileges or to the owner of a file or other 98resources. 99.It Er 2 ENOENT Em "No such file or directory" . 100A component of a specified pathname did not exist, or the 101pathname was an empty string. 102.It Er 3 ESRCH Em "No such process" . 103No process could be found corresponding to that specified by the given 104process ID. 105.It Er 4 EINTR Em "Interrupted system call" . 106An asynchronous signal (such as 107.Dv SIGINT 108or 109.Dv SIGQUIT ) 110was caught by the process during the execution of an interruptible 111function. 112If the signal handler performs a normal return, the 113interrupted function call will seem to have returned the error condition. 114.It Er 5 EIO Em "Input/output error" . 115Some physical input or output error occurred. 116This error will not be reported until a subsequent operation on the same file 117descriptor and may be lost (over written) by any subsequent errors. 118.It Er 6 ENXIO Em "Device not configured" . 119Input or output on a special file referred to a device that did not 120exist, or 121made a request beyond the limits of the device. 122This error may also occur when, for example, 123a tape drive is not online or no disk pack is 124loaded on a drive. 125.It Er 7 E2BIG Em "Argument list too long" . 126The number of bytes used for the argument and environment 127list of the new process exceeded the current limit 128of 65536 bytes 129.Pf ( Dv NCARGS 130in 131.Aq Pa sys/param.h ) . 132.It Er 8 ENOEXEC Em "Exec format error" . 133A request was made to execute a file 134that, although it has the appropriate permissions, 135was not in the format required for an 136executable file. 137.It Er 9 EBADF Em "Bad file descriptor" . 138A file descriptor argument was out of range, referred to no open file, 139or a read (write) request was made to a file that was only open for 140writing (reading). 141.Pp 142.It Er 10 ECHILD Em "\&No child processes" . 143A 144.Xr wait 2 145or 146.Xr waitpid 2 147function was executed by a process that had no existing or unwaited-for 148child processes. 149.It Er 11 EDEADLK Em "Resource deadlock avoided" . 150An attempt was made to lock a system resource that 151would have resulted in a deadlock situation. 152.It Er 12 ENOMEM Em "Cannot allocate memory" . 153The new process image required more memory than was allowed by the hardware 154or by system-imposed memory management constraints. 155A lack of swap space is normally temporary; however, 156a lack of core is not. 157Soft limits may be increased to their corresponding hard limits. 158.It Er 13 EACCES Em "Permission denied" . 159An attempt was made to access a file in a way forbidden 160by its file access permissions. 161.It Er 14 EFAULT Em "Bad address" . 162The system detected an invalid address in attempting to 163use an argument of a call. 164.It Er 15 ENOTBLK Em "Block device required" . 165A block device operation was attempted on a non-block device or file. 166.It Er 16 EBUSY Em "Device busy" . 167An attempt to use a system resource which was in use at the time 168in a manner which would have conflicted with the request. 169.It Er 17 EEXIST Em "File exists" . 170An existing file was mentioned in an inappropriate context, 171for instance, as the new link name in a 172.Xr link 2 173function. 174.It Er 18 EXDEV Em "Cross-device link" . 175A hard link to a file on another file system 176was attempted. 177.It Er 19 ENODEV Em "Operation not supported by device" . 178An attempt was made to apply an inappropriate 179function to a device, 180for example, 181trying to read a write-only device such as a printer. 182.It Er 20 ENOTDIR Em "Not a directory" . 183A component of the specified pathname existed, but it was 184not a directory, when a directory was expected. 185.It Er 21 EISDIR Em "Is a directory" . 186An attempt was made to open a directory with write mode specified. 187.It Er 22 EINVAL Em "Invalid argument" . 188Some invalid argument was supplied. 189(For example, 190specifying an undefined signal to a 191.Xr signal 3 192or 193.Xr kill 2 194function). 195.It Er 23 ENFILE Em "Too many open files in system" . 196Maximum number of file descriptors allowable on the system 197has been reached and a requests for an open cannot be satisfied 198until at least one has been closed. 199.It Er 24 EMFILE Em "Too many open files" . 200<As released, the limit on the number of 201open files per process is 64.> 202The 203.Xr getdtablesize 2 204function will obtain the current limit. 205.It Er 25 ENOTTY Em "Inappropriate ioctl for device" . 206A control function (see 207.Xr ioctl 2 ) 208was attempted for a file or 209special device for which the operation was inappropriate. 210.It Er 26 ETXTBSY Em "Text file busy" . 211The new process was a pure procedure (shared text) file 212which was open for writing by another process, or 213while the pure procedure file was being executed an 214.Xr open 2 215call requested write access. 216.It Er 27 EFBIG Em "File too large" . 217The size of a file exceeded the maximum (about 218.if t 2\u\s-231\s+2\d 219.if n 2.1E9 220bytes). 221.It Er 28 ENOSPC Em "No space left on device" . 222A 223.Xr write 2 224to an ordinary file, the creation of a 225directory or symbolic link, or the creation of a directory 226entry failed because no more disk blocks were available 227on the file system, or the allocation of an inode for a newly 228created file failed because no more inodes were available 229on the file system. 230.It Er 29 ESPIPE Em "Illegal seek" . 231An 232.Xr lseek 2 233function was issued on a socket, pipe or 234.Tn FIFO . 235.It Er 30 EROFS Em "Read-only file system" . 236An attempt was made to modify a file or directory 237on a file system that was read-only at the time. 238.It Er 31 EMLINK Em "Too many links" . 239Maximum allowable hard links to a single file has been exceeded (limit 240of 32767 hard links per file). 241.It Er 32 EPIPE Em "Broken pipe" . 242A write on a pipe, socket or 243.Tn FIFO 244for which there is no process 245to read the data. 246.It Er 33 EDOM Em "Numerical argument out of domain" . 247A numerical input argument was outside the defined domain of the mathematical 248function. 249.It Er 34 ERANGE Em "Result too large" . 250A numerical result of the function was too large to fit in the 251available space (perhaps exceeded precision). 252.It Er 35 EAGAIN Em "Resource temporarily unavailable" . 253This is a temporary condition and later calls to the 254same routine may complete normally. 255.It Er 36 EINPROGRESS Em "Operation now in progress" . 256An operation that takes a long time to complete (such as 257a 258.Xr connect 2 ) 259was attempted on a non-blocking object (see 260.Xr fcntl 2 ) . 261.It Er 37 EALREADY Em "Operation already in progress" . 262An operation was attempted on a non-blocking object that already 263had an operation in progress. 264.It Er 38 ENOTSOCK Em "Socket operation on non-socket" . 265Self-explanatory. 266.It Er 39 EDESTADDRREQ Em "Destination address required" . 267A required address was omitted from an operation on a socket. 268.It Er 40 EMSGSIZE Em "Message too long" . 269A message sent on a socket was larger than the internal message buffer 270or some other network limit. 271.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" . 272A protocol was specified that does not support the semantics of the 273socket type requested. 274For example, you cannot use the 275.Tn ARPA 276Internet 277.Tn UDP 278protocol with type 279.Dv SOCK_STREAM . 280.It Er 42 ENOPROTOOPT Em "Protocol not available" . 281A bad option or level was specified in a 282.Xr getsockopt 2 283or 284.Xr setsockopt 2 285call. 286.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" . 287The protocol has not been configured into the 288system or no implementation for it exists. 289.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" . 290The support for the socket type has not been configured into the 291system or no implementation for it exists. 292.It Er 45 EOPNOTSUPP Em "Operation not supported" . 293The attempted operation is not supported for the type of object referenced. 294Usually this occurs when a file descriptor refers to a file or socket 295that cannot support this operation, 296for example, trying to 297.Em accept 298a connection on a datagram socket. 299.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" . 300The protocol family has not been configured into the 301system or no implementation for it exists. 302.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" . 303An address incompatible with the requested protocol was used. 304For example, you shouldn't necessarily expect to be able to use 305.Tn NS 306addresses with 307.Tn ARPA 308Internet protocols. 309.It Er 48 EADDRINUSE Em "Address already in use" . 310Only one usage of each address is normally permitted. 311.Pp 312.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" . 313Normally results from an attempt to create a socket with an 314address not on this machine. 315.It Er 50 ENETDOWN Em "Network is down" . 316A socket operation encountered a dead network. 317.It Er 51 ENETUNREACH Em "Network is unreachable" . 318A socket operation was attempted to an unreachable network. 319.It Er 52 ENETRESET Em "Network dropped connection on reset" . 320The host you were connected to crashed and rebooted. 321.It Er 53 ECONNABORTED Em "Software caused connection abort" . 322A connection abort was caused internal to your host machine. 323.It Er 54 ECONNRESET Em "Connection reset by peer" . 324A connection was forcibly closed by a peer. This normally 325results from a loss of the connection on the remote socket 326due to a timeout or a reboot. 327.It Er 55 ENOBUFS Em "\&No buffer space available" . 328An operation on a socket or pipe was not performed because 329the system lacked sufficient buffer space or because a queue was full. 330.It Er 56 EISCONN Em "Socket is already connected" . 331A 332.Xr connect 2 333request was made on an already connected socket; or, 334a 335.Xr sendto 2 336or 337.Xr sendmsg 2 338request on a connected socket specified a destination 339when already connected. 340.It Er 57 ENOTCONN Em "Socket is not connected" . 341An request to send or receive data was disallowed because 342the socket was not connected and (when sending on a datagram socket) 343no address was supplied. 344.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" . 345A request to send data was disallowed because the socket 346had already been shut down with a previous 347.Xr shutdown 2 348call. 349.It Er 60 ETIMEDOUT Em "Operation timed out" . 350A 351.Xr connect 2 352or 353.Xr send 2 354request failed because the connected party did not 355properly respond after a period of time. (The timeout 356period is dependent on the communication protocol.) 357.It Er 61 ECONNREFUSED Em "Connection refused" . 358No connection could be made because the target machine actively 359refused it. This usually results from trying to connect 360to a service that is inactive on the foreign host. 361.It Er 62 ELOOP Em "Too many levels of symbolic links" . 362A path name lookup involved more than 32 363.Pq Dv MAXSYMLINKS 364symbolic links. 365.It Er 63 ENAMETOOLONG Em "File name too long" . 366A component of a path name exceeded 255 367.Pq Dv MAXNAMELEN 368characters, or an entire 369path name exceeded 1023 370.Pq Dv MAXPATHLEN Ns -1 371characters. 372.It Er 64 EHOSTDOWN Em "Host is down" . 373A socket operation failed because the destination host was down. 374.It Er 65 EHOSTUNREACH Em "No route to host" . 375A socket operation was attempted to an unreachable host. 376.It Er 66 ENOTEMPTY Em "Directory not empty" . 377A directory with entries other than 378.Ql .\& 379and 380.Ql ..\& 381was supplied to a remove directory or rename call. 382.It Er 67 EPROCLIM Em "Too many processes" . 383.It Er 68 EUSERS Em "Too many users" . 384The quota system ran out of table entries. 385.It Er 69 EDQUOT Em "Disc quota exceeded" . 386A 387.Xr write 2 388to an ordinary file, the creation of a 389directory or symbolic link, or the creation of a directory 390entry failed because the user's quota of disk blocks was 391exhausted, or the allocation of an inode for a newly 392created file failed because the user's quota of inodes 393was exhausted. 394.It Er 70 ESTALE Em "Stale NFS file handle" . 395An attempt was made to access an open file (on an 396.Tn NFS 397filesystem) 398which is now unavailable as referenced by the file descriptor. 399This may indicate the file was deleted on the 400.Tn NFS 401server or some 402other catastrophic event occurred. 403.It Er 72 EBADRPC Em "RPC struct is bad" . 404Exchange of 405.Tn RPC 406information was unsuccessful. 407.It Er 73 ERPCMISMATCH Em "RPC version wrong" . 408The version of 409.Tn RPC 410on the remote peer is not compatible with 411the local version. 412.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" . 413The requested program is not registered on the remote host. 414.It Er 75 EPROGMISMATCH Em "Program version wrong" . 415The requested version of the program is not available 416on the remote host 417.Pq Tn RPC . 418.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" . 419An 420.Tn RPC 421call was attempted for a procedure which doesn't exist 422in the remote program. 423.It Er 77 ENOLCK Em "No locks available" . 424A system-imposed limit on the number of simultaneous file 425locks was reached. 426.It Er 78 ENOSYS Em "Function not implemented" . 427Attempted a system call that is not available on this 428system. 429.It Er 79 EFTYPE Em "Inappropriate file type or format" . 430The file was the wrong type for the operation, or a data file had 431the wrong format. 432.It Er 80 EAUTH Em "Authentication error" . 433Attempted to use an invalid authentication ticket to mount a 434.Tn NFS 435filesystem. 436.It Er 81 ENEEDAUTH Em "Need authenticator" . 437An authentication ticket must be obtained before the given 438.Tn NFS 439filesystem may be mounted. 440.It Er 82 EIDRM Em "Identifier removed" . 441An IPC identifier was removed while the current process was waiting on it. 442.It Er 83 ENOMSG Em "No message of desired type" . 443An IPC message queue does not contain a message of the desired type, or a 444message catalog does not contain the requested message. 445.It Er 84 EOVERFLOW Em "Value too large to be stored in data type" . 446A numerical result of the function was too large to be stored in the caller 447provided space. 448.It Er 85 ECANCELED Em "Operation canceled" . 449The scheduled operation was canceled. 450.It Er 86 EILSEQ Em "Illegal byte sequence" . 451While decoding a multibyte character the function came along an 452invalid or an incomplete sequence of bytes or the given wide 453character is invalid. 454.El 455.Sh DEFINITIONS 456.Bl -tag -width Ds 457.It Process ID . 458Each active process in the system is uniquely identified by a non-negative 459integer called a process ID. The range of this ID is from 0 to 99999. 460.It Parent process ID 461A new process is created by a currently active process; (see 462.Xr fork 2 ) . 463The parent process ID of a process is initially the process ID of its creator. 464If the creating process exits, 465the parent process ID of each child is set to the ID of a system process, 466.Xr init 8 . 467.It Process Group 468Each active process is a member of a process group that is identified by 469a non-negative integer called the process group ID. This is the process 470ID of the group leader. This grouping permits the signaling of related 471processes (see 472.Xr termios 4 ) 473and the job control mechanisms of 474.Xr csh 1 . 475.It Session 476A session is a set of one or more process groups. 477A session is created by a successful call to 478.Xr setsid 2 , 479which causes the caller to become the only member of the only process 480group in the new session. 481.It Session leader 482A process that has created a new session by a successful call to 483.Xr setsid 2 , 484is known as a session leader. 485Only a session leader may acquire a terminal as its controlling terminal (see 486.Xr termios 4 ) . 487.It Controlling process 488A session leader with a controlling terminal is a controlling process. 489.It Controlling terminal 490A terminal that is associated with a session is known as the controlling 491terminal for that session and its members. 492.It "Terminal Process Group ID" 493A terminal may be acquired by a session leader as its controlling terminal. 494Once a terminal is associated with a session, any of the process groups 495within the session may be placed into the foreground by setting 496the terminal process group ID to the ID of the process group. 497This facility is used 498to arbitrate between multiple jobs contending for the same terminal; 499(see 500.Xr csh 1 501and 502.Xr tty 4 ) . 503.It "Orphaned Process Group" 504A process group is considered to be 505.Em orphaned 506if it is not under the control of a job control shell. 507More precisely, a process group is orphaned 508when none of its members has a parent process that is in the same session 509as the group, 510but is in a different process group. 511Note that when a process exits, the parent process for its children 512is changed to be 513.Xr init 8 , 514which is in a separate session. 515Not all members of an orphaned process group are necessarily orphaned 516processes (those whose creating process has exited). 517The process group of a session leader is orphaned by definition. 518.It "Real User ID and Real Group ID" 519Each user on the system is identified by a positive integer 520termed the real user ID. 521.Pp 522Each user is also a member of one or more groups. 523One of these groups is distinguished from others and 524used in implementing accounting facilities. The positive 525integer corresponding to this distinguished group is termed 526the real group ID. 527.Pp 528All processes have a real user ID and real group ID. 529These are initialized from the equivalent attributes 530of the process that created it. 531.It "Effective User Id, Effective Group Id, and Group Access List" 532Access to system resources is governed by two values: 533the effective user ID, and the group access list. 534The first member of the group access list is also known as the 535effective group ID. 536(In POSIX.1, the group access list is known as the set of supplementary 537group IDs, and it is unspecified whether the effective group ID is 538a member of the list.) 539.Pp 540The effective user ID and effective group ID are initially the 541process's real user ID and real group ID respectively. Either 542may be modified through execution of a set-user-ID or set-group-ID 543file (possibly by one its ancestors) (see 544.Xr execve 2 ) . 545By convention, the effective group ID (the first member of the group access 546list) is duplicated, so that the execution of a set-group-ID program 547does not result in the loss of the original (real) group ID. 548.Pp 549The group access list is a set of group IDs 550used only in determining resource accessibility. Access checks 551are performed as described below in ``File Access Permissions''. 552.It "Saved Set User ID and Saved Set Group ID" 553When a process executes a new file, the effective user ID is set 554to the owner of the file if the file is set-user-ID, and the effective 555group ID (first element of the group access list) is set to the group 556of the file if the file is set-group-ID. 557The effective user ID of the process is then recorded as the saved set-user-ID, 558and the effective group ID of the process is recorded as the saved set-group-ID. 559These values may be used to regain those values as the effective user 560or group ID after reverting to the real ID (see 561.Xr setuid 2 ) . 562(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, 563and are used in setuid and setgid, but this does not work as desired 564for the super-user.) 565.It Super-user 566A process is recognized as a 567.Em super-user 568process and is granted special privileges if its effective user ID is 0. 569.It Special Processes 570The processes with process IDs of 0, 1, and 2 are special. 571Process 0 is the scheduler. Process 1 is the initialization process 572.Xr init 8 , 573and is the ancestor of every other process in the system. 574It is used to control the process structure. 575Process 2 is the paging daemon. 576.It Descriptor 577An integer assigned by the system when a file is referenced 578by 579.Xr open 2 580or 581.Xr dup 2 , 582or when a socket is created by 583.Xr pipe 2 , 584.Xr socket 2 585or 586.Xr socketpair 2 , 587which uniquely identifies an access path to that file or socket from 588a given process or any of its children. 589.It File Name 590Names consisting of up to 255 591.Pq Dv MAXNAMELEN 592characters may be used to name 593an ordinary file, special file, or directory. 594.Pp 595These characters may be selected from the set of all 596.Tn ASCII 597character 598excluding 0 (NUL) and the 599.Tn ASCII 600code for 601.Ql \&/ 602(slash). 603.Pp 604Note that it is generally unwise to use 605.Ql \&* , 606.Ql \&? , 607.Ql \&[ 608or 609.Ql \&] 610as part of 611file names because of the special meaning attached to these characters 612by the shell. 613.It Path Name 614A path name is a 615.Tn NUL Ns -terminated 616character string starting with an 617optional slash 618.Ql \&/ , 619followed by zero or more directory names separated 620by slashes, optionally followed by a file name. 621The total length of a path name must be less than 1024 622.Pq Dv MAXPATHLEN 623characters. 624.Pp 625If a path name begins with a slash, the path search begins at the 626.Em root 627directory. 628Otherwise, the search begins from the current working directory. 629A slash by itself names the root directory. An empty 630pathname refers to the current directory. 631.It Directory 632A directory is a special type of file that contains entries 633that are references to other files. 634Directory entries are called links. By convention, a directory 635contains at least two links, 636.Ql .\& 637and 638.Ql \&.. , 639referred to as 640.Em dot 641and 642.Em dot-dot 643respectively. Dot refers to the directory itself and 644dot-dot refers to its parent directory. 645.It "Root Directory and Current Working Directory" 646Each process has associated with it a concept of a root directory 647and a current working directory for the purpose of resolving path 648name searches. A process's root directory need not be the root 649directory of the root file system. 650.It File Access Permissions 651Every file in the file system has a set of access permissions. 652These permissions are used in determining whether a process 653may perform a requested operation on the file (such as opening 654a file for writing). Access permissions are established at the 655time a file is created. They may be changed at some later time 656through the 657.Xr chmod 2 658call. 659.Pp 660File access is broken down according to whether a file may be: read, 661written, or executed. Directory files use the execute 662permission to control if the directory may be searched. 663.Pp 664File access permissions are interpreted by the system as 665they apply to three different classes of users: the owner 666of the file, those users in the file's group, anyone else. 667Every file has an independent set of access permissions for 668each of these classes. When an access check is made, the system 669decides if permission should be granted by checking the access 670information applicable to the caller. 671.Pp 672Read, write, and execute/search permissions on 673a file are granted to a process if: 674.Pp 675The process's effective user ID is that of the super-user. 676(Note: 677even the super-user cannot execute a non-executable file.) 678.Pp 679The process's effective user ID matches the user ID of the owner 680of the file and the owner permissions allow the access. 681.Pp 682The process's effective user ID does not match the user ID of the 683owner of the file, and either the process's effective 684group ID matches the group ID 685of the file, or the group ID of the file is in 686the process's group access list, 687and the group permissions allow the access. 688.Pp 689Neither the effective user ID nor effective group ID 690and group access list of the process 691match the corresponding user ID and group ID of the file, 692but the permissions for ``other users'' allow access. 693.Pp 694Otherwise, permission is denied. 695.It Sockets and Address Families 696A socket is an endpoint for communication between processes. 697Each socket has queues for sending and receiving data. 698.Pp 699Sockets are typed according to their communications properties. 700These properties include whether messages sent and received 701at a socket require the name of the partner, whether communication 702is reliable, the format used in naming message recipients, etc. 703.Pp 704Each instance of the system supports some 705collection of socket types; consult 706.Xr socket 2 707for more information about the types available and 708their properties. 709.Pp 710Each instance of the system supports some number of sets of 711communications protocols. Each protocol set supports addresses 712of a certain format. An Address Family is the set of addresses 713for a specific group of protocols. Each socket has an address 714chosen from the address family in which the socket was created. 715.El 716.Sh SEE ALSO 717.Xr intro 3 , 718.Xr perror 3 719