1.\" Copyright (c) 1980, 1983, 1986, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. 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 1.2 2003/06/17 04:26:47 dillon 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 errno. 56This identifier is defined in 57.Aq Pa sys/errno.h 58as 59.Pp 60.Dl extern int * __error(); 61.Dl #define errno (* __error()) 62.Pp 63The 64.Va __error() 65function returns a pointer to a field in the thread specific structure for 66threads other than the initial thread. 67For the initial thread and 68non-threaded processes, 69.Va __error() 70returns a pointer to a global 71.Va errno 72variable that is compatible with the previous definition. 73.Pp 74When a system call detects an error, 75it returns an integer value 76indicating failure (usually -1) 77and sets the variable 78.Va errno 79accordingly. 80<This allows interpretation of the failure on receiving 81a -1 and to take action accordingly.> 82Successful calls never set 83.Va errno ; 84once set, it remains until another error occurs. 85It should only be examined after an error. 86Note that a number of system calls overload the meanings of these 87error numbers, and that the meanings must be interpreted according 88to the type and circumstances of the call. 89.Pp 90The following is a complete list of the errors and their 91names as given in 92.Aq Pa sys/errno.h . 93.Bl -hang -width Ds 94.It Er 0 Em "Undefined error: 0" . 95Not used. 96.It Er 1 EPERM Em "Operation not permitted" . 97An attempt was made to perform an operation limited to processes 98with appropriate privileges or to the owner of a file or other 99resources. 100.It Er 2 ENOENT Em "No such file or directory" . 101A component of a specified pathname did not exist, or the 102pathname was an empty string. 103.It Er 3 ESRCH Em "No such process" . 104No process could be found corresponding to that specified by the given 105process ID. 106.It Er 4 EINTR Em "Interrupted system call" . 107An asynchronous signal (such as 108.Dv SIGINT 109or 110.Dv SIGQUIT ) 111was caught by the process during the execution of an interruptible 112function. 113If the signal handler performs a normal return, the 114interrupted function call will seem to have returned the error condition. 115.It Er 5 EIO Em "Input/output error" . 116Some physical input or output error occurred. 117This error will not be reported until a subsequent operation on the same file 118descriptor and may be lost (over written) by any subsequent errors. 119.It Er 6 ENXIO Em "Device not configured" . 120Input or output on a special file referred to a device that did not 121exist, or 122made a request beyond the limits of the device. 123This error may also occur when, for example, 124a tape drive is not online or no disk pack is 125loaded on a drive. 126.It Er 7 E2BIG Em "Argument list too long" . 127The number of bytes used for the argument and environment 128list of the new process exceeded the current limit 129of 65536 bytes 130.Pf ( Dv NCARGS 131in 132.Aq Pa sys/param.h ) . 133.It Er 8 ENOEXEC Em "Exec format error" . 134A request was made to execute a file 135that, although it has the appropriate permissions, 136was not in the format required for an 137executable file. 138.It Er 9 EBADF Em "Bad file descriptor" . 139A file descriptor argument was out of range, referred to no open file, 140or a read (write) request was made to a file that was only open for 141writing (reading). 142.Pp 143.It Er 10 ECHILD Em "\&No child processes" . 144A 145.Xr wait 2 146or 147.Xr waitpid 2 148function was executed by a process that had no existing or unwaited-for 149child processes. 150.It Er 11 EDEADLK Em "Resource deadlock avoided" . 151An attempt was made to lock a system resource that 152would have resulted in a deadlock situation. 153.It Er 12 ENOMEM Em "Cannot allocate memory" . 154The new process image required more memory than was allowed by the hardware 155or by system-imposed memory management constraints. 156A lack of swap space is normally temporary; however, 157a lack of core is not. 158Soft limits may be increased to their corresponding hard limits. 159.It Er 13 EACCES Em "Permission denied" . 160An attempt was made to access a file in a way forbidden 161by its file access permissions. 162.It Er 14 EFAULT Em "Bad address" . 163The system detected an invalid address in attempting to 164use an argument of a call. 165.It Er 15 ENOTBLK Em "Block device required" . 166A block device operation was attempted on a non-block device or file. 167.It Er 16 EBUSY Em "Device busy" . 168An attempt to use a system resource which was in use at the time 169in a manner which would have conflicted with the request. 170.It Er 17 EEXIST Em "File exists" . 171An existing file was mentioned in an inappropriate context, 172for instance, as the new link name in a 173.Xr link 2 174function. 175.It Er 18 EXDEV Em "Cross-device link" . 176A hard link to a file on another file system 177was attempted. 178.It Er 19 ENODEV Em "Operation not supported by device" . 179An attempt was made to apply an inappropriate 180function to a device, 181for example, 182trying to read a write-only device such as a printer. 183.It Er 20 ENOTDIR Em "Not a directory" . 184A component of the specified pathname existed, but it was 185not a directory, when a directory was expected. 186.It Er 21 EISDIR Em "Is a directory" . 187An attempt was made to open a directory with write mode specified. 188.It Er 22 EINVAL Em "Invalid argument" . 189Some invalid argument was supplied. 190(For example, 191specifying an undefined signal to a 192.Xr signal 3 193or 194.Xr kill 2 195function). 196.It Er 23 ENFILE Em "Too many open files in system" . 197Maximum number of file descriptors allowable on the system 198has been reached and a requests for an open cannot be satisfied 199until at least one has been closed. 200.It Er 24 EMFILE Em "Too many open files" . 201<As released, the limit on the number of 202open files per process is 64.> 203The 204.Xr getdtablesize 2 205function will obtain the current limit. 206.It Er 25 ENOTTY Em "Inappropriate ioctl for device" . 207A control function (see 208.Xr ioctl 2 ) 209was attempted for a file or 210special device for which the operation was inappropriate. 211.It Er 26 ETXTBSY Em "Text file busy" . 212The new process was a pure procedure (shared text) file 213which was open for writing by another process, or 214while the pure procedure file was being executed an 215.Xr open 2 216call requested write access. 217.It Er 27 EFBIG Em "File too large" . 218The size of a file exceeded the maximum (about 219.if t 2\u\s-231\s+2\d 220.if n 2.1E9 221bytes). 222.It Er 28 ENOSPC Em "No space left on device" . 223A 224.Xr write 2 225to an ordinary file, the creation of a 226directory or symbolic link, or the creation of a directory 227entry failed because no more disk blocks were available 228on the file system, or the allocation of an inode for a newly 229created file failed because no more inodes were available 230on the file system. 231.It Er 29 ESPIPE Em "Illegal seek" . 232An 233.Xr lseek 2 234function was issued on a socket, pipe or 235.Tn FIFO . 236.It Er 30 EROFS Em "Read-only file system" . 237An attempt was made to modify a file or directory 238on a file system that was read-only at the time. 239.It Er 31 EMLINK Em "Too many links" . 240Maximum allowable hard links to a single file has been exceeded (limit 241of 32767 hard links per file). 242.It Er 32 EPIPE Em "Broken pipe" . 243A write on a pipe, socket or 244.Tn FIFO 245for which there is no process 246to read the data. 247.It Er 33 EDOM Em "Numerical argument out of domain" . 248A numerical input argument was outside the defined domain of the mathematical 249function. 250.It Er 34 ERANGE Em "Result too large" . 251A numerical result of the function was too large to fit in the 252available space (perhaps exceeded precision). 253.It Er 35 EAGAIN Em "Resource temporarily unavailable" . 254This is a temporary condition and later calls to the 255same routine may complete normally. 256.It Er 36 EINPROGRESS Em "Operation now in progress" . 257An operation that takes a long time to complete (such as 258a 259.Xr connect 2 ) 260was attempted on a non-blocking object (see 261.Xr fcntl 2 ) . 262.It Er 37 EALREADY Em "Operation already in progress" . 263An operation was attempted on a non-blocking object that already 264had an operation in progress. 265.It Er 38 ENOTSOCK Em "Socket operation on non-socket" . 266Self-explanatory. 267.It Er 39 EDESTADDRREQ Em "Destination address required" . 268A required address was omitted from an operation on a socket. 269.It Er 40 EMSGSIZE Em "Message too long" . 270A message sent on a socket was larger than the internal message buffer 271or some other network limit. 272.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" . 273A protocol was specified that does not support the semantics of the 274socket type requested. 275For example, you cannot use the 276.Tn ARPA 277Internet 278.Tn UDP 279protocol with type 280.Dv SOCK_STREAM . 281.It Er 42 ENOPROTOOPT Em "Protocol not available" . 282A bad option or level was specified in a 283.Xr getsockopt 2 284or 285.Xr setsockopt 2 286call. 287.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" . 288The protocol has not been configured into the 289system or no implementation for it exists. 290.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" . 291The support for the socket type has not been configured into the 292system or no implementation for it exists. 293.It Er 45 EOPNOTSUPP Em "Operation not supported" . 294The attempted operation is not supported for the type of object referenced. 295Usually this occurs when a file descriptor refers to a file or socket 296that cannot support this operation, 297for example, trying to 298.Em accept 299a connection on a datagram socket. 300.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" . 301The protocol family has not been configured into the 302system or no implementation for it exists. 303.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" . 304An address incompatible with the requested protocol was used. 305For example, you shouldn't necessarily expect to be able to use 306.Tn NS 307addresses with 308.Tn ARPA 309Internet protocols. 310.It Er 48 EADDRINUSE Em "Address already in use" . 311Only one usage of each address is normally permitted. 312.Pp 313.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" . 314Normally results from an attempt to create a socket with an 315address not on this machine. 316.It Er 50 ENETDOWN Em "Network is down" . 317A socket operation encountered a dead network. 318.It Er 51 ENETUNREACH Em "Network is unreachable" . 319A socket operation was attempted to an unreachable network. 320.It Er 52 ENETRESET Em "Network dropped connection on reset" . 321The host you were connected to crashed and rebooted. 322.It Er 53 ECONNABORTED Em "Software caused connection abort" . 323A connection abort was caused internal to your host machine. 324.It Er 54 ECONNRESET Em "Connection reset by peer" . 325A connection was forcibly closed by a peer. This normally 326results from a loss of the connection on the remote socket 327due to a timeout or a reboot. 328.It Er 55 ENOBUFS Em "\&No buffer space available" . 329An operation on a socket or pipe was not performed because 330the system lacked sufficient buffer space or because a queue was full. 331.It Er 56 EISCONN Em "Socket is already connected" . 332A 333.Xr connect 2 334request was made on an already connected socket; or, 335a 336.Xr sendto 2 337or 338.Xr sendmsg 2 339request on a connected socket specified a destination 340when already connected. 341.It Er 57 ENOTCONN Em "Socket is not connected" . 342An request to send or receive data was disallowed because 343the socket was not connected and (when sending on a datagram socket) 344no address was supplied. 345.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" . 346A request to send data was disallowed because the socket 347had already been shut down with a previous 348.Xr shutdown 2 349call. 350.It Er 60 ETIMEDOUT Em "Operation timed out" . 351A 352.Xr connect 2 353or 354.Xr send 2 355request failed because the connected party did not 356properly respond after a period of time. (The timeout 357period is dependent on the communication protocol.) 358.It Er 61 ECONNREFUSED Em "Connection refused" . 359No connection could be made because the target machine actively 360refused it. This usually results from trying to connect 361to a service that is inactive on the foreign host. 362.It Er 62 ELOOP Em "Too many levels of symbolic links" . 363A path name lookup involved more than 32 364.Pq Dv MAXSYMLINKS 365symbolic links. 366.It Er 63 ENAMETOOLONG Em "File name too long" . 367A component of a path name exceeded 255 368.Pq Dv MAXNAMELEN 369characters, or an entire 370path name exceeded 1023 371.Pq Dv MAXPATHLEN Ns -1 372characters. 373.It Er 64 EHOSTDOWN Em "Host is down" . 374A socket operation failed because the destination host was down. 375.It Er 65 EHOSTUNREACH Em "No route to host" . 376A socket operation was attempted to an unreachable host. 377.It Er 66 ENOTEMPTY Em "Directory not empty" . 378A directory with entries other than 379.Ql .\& 380and 381.Ql ..\& 382was supplied to a remove directory or rename call. 383.It Er 67 EPROCLIM Em "Too many processes" . 384.It Er 68 EUSERS Em "Too many users" . 385The quota system ran out of table entries. 386.It Er 69 EDQUOT Em "Disc quota exceeded" . 387A 388.Xr write 2 389to an ordinary file, the creation of a 390directory or symbolic link, or the creation of a directory 391entry failed because the user's quota of disk blocks was 392exhausted, or the allocation of an inode for a newly 393created file failed because the user's quota of inodes 394was exhausted. 395.It Er 70 ESTALE Em "Stale NFS file handle" . 396An attempt was made to access an open file (on an 397.Tn NFS 398filesystem) 399which is now unavailable as referenced by the file descriptor. 400This may indicate the file was deleted on the 401.Tn NFS 402server or some 403other catastrophic event occurred. 404.It Er 72 EBADRPC Em "RPC struct is bad" . 405Exchange of 406.Tn RPC 407information was unsuccessful. 408.It Er 73 ERPCMISMATCH Em "RPC version wrong" . 409The version of 410.Tn RPC 411on the remote peer is not compatible with 412the local version. 413.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" . 414The requested program is not registered on the remote host. 415.It Er 75 EPROGMISMATCH Em "Program version wrong" . 416The requested version of the program is not available 417on the remote host 418.Pq Tn RPC . 419.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" . 420An 421.Tn RPC 422call was attempted for a procedure which doesn't exist 423in the remote program. 424.It Er 77 ENOLCK Em "No locks available" . 425A system-imposed limit on the number of simultaneous file 426locks was reached. 427.It Er 78 ENOSYS Em "Function not implemented" . 428Attempted a system call that is not available on this 429system. 430.It Er 79 EFTYPE Em "Inappropriate file type or format" . 431The file was the wrong type for the operation, or a data file had 432the wrong format. 433.It Er 80 EAUTH Em "Authentication error" . 434Attempted to use an invalid authentication ticket to mount a 435.Tn NFS 436filesystem. 437.It Er 81 ENEEDAUTH Em "Need authenticator" . 438An authentication ticket must be obtained before the given 439.Tn NFS 440filesystem may be mounted. 441.It Er 82 EIDRM Em "Identifier removed" . 442An IPC identifier was removed while the current process was waiting on it. 443.It Er 83 ENOMSG Em "No message of desired type" . 444An IPC message queue does not contain a message of the desired type, or a 445message catalog does not contain the requested message. 446.It Er 84 EOVERFLOW Em "Value too large to be stored in data type" . 447A numerical result of the function was too large to be stored in the caller 448provided space. 449.It Er 85 ECANCELED Em "Operation canceled" . 450The scheduled operation was canceled. 451.It Er 86 EILSEQ Em "Illegal byte sequence" . 452While decoding a multibyte character the function came along an 453invalid or an incomplete sequence of bytes or the given wide 454character is invalid. 455.El 456.Sh DEFINITIONS 457.Bl -tag -width Ds 458.It Process ID . 459Each active process in the system is uniquely identified by a non-negative 460integer called a process ID. The range of this ID is from 0 to 99999. 461.It Parent process ID 462A new process is created by a currently active process; (see 463.Xr fork 2 ) . 464The parent process ID of a process is initially the process ID of its creator. 465If the creating process exits, 466the parent process ID of each child is set to the ID of a system process, 467.Xr init 8 . 468.It Process Group 469Each active process is a member of a process group that is identified by 470a non-negative integer called the process group ID. This is the process 471ID of the group leader. This grouping permits the signaling of related 472processes (see 473.Xr termios 4 ) 474and the job control mechanisms of 475.Xr csh 1 . 476.It Session 477A session is a set of one or more process groups. 478A session is created by a successful call to 479.Xr setsid 2 , 480which causes the caller to become the only member of the only process 481group in the new session. 482.It Session leader 483A process that has created a new session by a successful call to 484.Xr setsid 2 , 485is known as a session leader. 486Only a session leader may acquire a terminal as its controlling terminal (see 487.Xr termios 4 ) . 488.It Controlling process 489A session leader with a controlling terminal is a controlling process. 490.It Controlling terminal 491A terminal that is associated with a session is known as the controlling 492terminal for that session and its members. 493.It "Terminal Process Group ID" 494A terminal may be acquired by a session leader as its controlling terminal. 495Once a terminal is associated with a session, any of the process groups 496within the session may be placed into the foreground by setting 497the terminal process group ID to the ID of the process group. 498This facility is used 499to arbitrate between multiple jobs contending for the same terminal; 500(see 501.Xr csh 1 502and 503.Xr tty 4 ) . 504.It "Orphaned Process Group" 505A process group is considered to be 506.Em orphaned 507if it is not under the control of a job control shell. 508More precisely, a process group is orphaned 509when none of its members has a parent process that is in the same session 510as the group, 511but is in a different process group. 512Note that when a process exits, the parent process for its children 513is changed to be 514.Xr init 8 , 515which is in a separate session. 516Not all members of an orphaned process group are necessarily orphaned 517processes (those whose creating process has exited). 518The process group of a session leader is orphaned by definition. 519.It "Real User ID and Real Group ID" 520Each user on the system is identified by a positive integer 521termed the real user ID. 522.Pp 523Each user is also a member of one or more groups. 524One of these groups is distinguished from others and 525used in implementing accounting facilities. The positive 526integer corresponding to this distinguished group is termed 527the real group ID. 528.Pp 529All processes have a real user ID and real group ID. 530These are initialized from the equivalent attributes 531of the process that created it. 532.It "Effective User Id, Effective Group Id, and Group Access List" 533Access to system resources is governed by two values: 534the effective user ID, and the group access list. 535The first member of the group access list is also known as the 536effective group ID. 537(In POSIX.1, the group access list is known as the set of supplementary 538group IDs, and it is unspecified whether the effective group ID is 539a member of the list.) 540.Pp 541The effective user ID and effective group ID are initially the 542process's real user ID and real group ID respectively. Either 543may be modified through execution of a set-user-ID or set-group-ID 544file (possibly by one its ancestors) (see 545.Xr execve 2 ) . 546By convention, the effective group ID (the first member of the group access 547list) is duplicated, so that the execution of a set-group-ID program 548does not result in the loss of the original (real) group ID. 549.Pp 550The group access list is a set of group IDs 551used only in determining resource accessibility. Access checks 552are performed as described below in ``File Access Permissions''. 553.It "Saved Set User ID and Saved Set Group ID" 554When a process executes a new file, the effective user ID is set 555to the owner of the file if the file is set-user-ID, and the effective 556group ID (first element of the group access list) is set to the group 557of the file if the file is set-group-ID. 558The effective user ID of the process is then recorded as the saved set-user-ID, 559and the effective group ID of the process is recorded as the saved set-group-ID. 560These values may be used to regain those values as the effective user 561or group ID after reverting to the real ID (see 562.Xr setuid 2 ) . 563(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, 564and are used in setuid and setgid, but this does not work as desired 565for the super-user.) 566.It Super-user 567A process is recognized as a 568.Em super-user 569process and is granted special privileges if its effective user ID is 0. 570.It Special Processes 571The processes with process IDs of 0, 1, and 2 are special. 572Process 0 is the scheduler. Process 1 is the initialization process 573.Xr init 8 , 574and is the ancestor of every other process in the system. 575It is used to control the process structure. 576Process 2 is the paging daemon. 577.It Descriptor 578An integer assigned by the system when a file is referenced 579by 580.Xr open 2 581or 582.Xr dup 2 , 583or when a socket is created by 584.Xr pipe 2 , 585.Xr socket 2 586or 587.Xr socketpair 2 , 588which uniquely identifies an access path to that file or socket from 589a given process or any of its children. 590.It File Name 591Names consisting of up to 255 592.Pq Dv MAXNAMELEN 593characters may be used to name 594an ordinary file, special file, or directory. 595.Pp 596These characters may be selected from the set of all 597.Tn ASCII 598character 599excluding 0 (NUL) and the 600.Tn ASCII 601code for 602.Ql \&/ 603(slash). 604.Pp 605Note that it is generally unwise to use 606.Ql \&* , 607.Ql \&? , 608.Ql \&[ 609or 610.Ql \&] 611as part of 612file names because of the special meaning attached to these characters 613by the shell. 614.It Path Name 615A path name is a 616.Tn NUL Ns -terminated 617character string starting with an 618optional slash 619.Ql \&/ , 620followed by zero or more directory names separated 621by slashes, optionally followed by a file name. 622The total length of a path name must be less than 1024 623.Pq Dv MAXPATHLEN 624characters. 625.Pp 626If a path name begins with a slash, the path search begins at the 627.Em root 628directory. 629Otherwise, the search begins from the current working directory. 630A slash by itself names the root directory. An empty 631pathname refers to the current directory. 632.It Directory 633A directory is a special type of file that contains entries 634that are references to other files. 635Directory entries are called links. By convention, a directory 636contains at least two links, 637.Ql .\& 638and 639.Ql \&.. , 640referred to as 641.Em dot 642and 643.Em dot-dot 644respectively. Dot refers to the directory itself and 645dot-dot refers to its parent directory. 646.It "Root Directory and Current Working Directory" 647Each process has associated with it a concept of a root directory 648and a current working directory for the purpose of resolving path 649name searches. A process's root directory need not be the root 650directory of the root file system. 651.It File Access Permissions 652Every file in the file system has a set of access permissions. 653These permissions are used in determining whether a process 654may perform a requested operation on the file (such as opening 655a file for writing). Access permissions are established at the 656time a file is created. They may be changed at some later time 657through the 658.Xr chmod 2 659call. 660.Pp 661File access is broken down according to whether a file may be: read, 662written, or executed. Directory files use the execute 663permission to control if the directory may be searched. 664.Pp 665File access permissions are interpreted by the system as 666they apply to three different classes of users: the owner 667of the file, those users in the file's group, anyone else. 668Every file has an independent set of access permissions for 669each of these classes. When an access check is made, the system 670decides if permission should be granted by checking the access 671information applicable to the caller. 672.Pp 673Read, write, and execute/search permissions on 674a file are granted to a process if: 675.Pp 676The process's effective user ID is that of the super-user. 677(Note: 678even the super-user cannot execute a non-executable file.) 679.Pp 680The process's effective user ID matches the user ID of the owner 681of the file and the owner permissions allow the access. 682.Pp 683The process's effective user ID does not match the user ID of the 684owner of the file, and either the process's effective 685group ID matches the group ID 686of the file, or the group ID of the file is in 687the process's group access list, 688and the group permissions allow the access. 689.Pp 690Neither the effective user ID nor effective group ID 691and group access list of the process 692match the corresponding user ID and group ID of the file, 693but the permissions for ``other users'' allow access. 694.Pp 695Otherwise, permission is denied. 696.It Sockets and Address Families 697A socket is an endpoint for communication between processes. 698Each socket has queues for sending and receiving data. 699.Pp 700Sockets are typed according to their communications properties. 701These properties include whether messages sent and received 702at a socket require the name of the partner, whether communication 703is reliable, the format used in naming message recipients, etc. 704.Pp 705Each instance of the system supports some 706collection of socket types; consult 707.Xr socket 2 708for more information about the types available and 709their properties. 710.Pp 711Each instance of the system supports some number of sets of 712communications protocols. Each protocol set supports addresses 713of a certain format. An Address Family is the set of addresses 714for a specific group of protocols. Each socket has an address 715chosen from the address family in which the socket was created. 716.El 717.Sh SEE ALSO 718.Xr intro 3 , 719.Xr perror 3 720