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.\"     @(#)intro.2	8.5 (Berkeley) 2/27/95
.\"
.Dd July 23, 2009
.Dt INTRO 2
.Os
.Sh NAME
.Nm intro ,
.Nm errno
.Nd introduction to system calls and error numbers
.Sh SYNOPSIS
.In errno.h
.Sh DESCRIPTION
This section provides an overview of the system calls,
their error returns, and other common definitions and concepts.
.\".Pp
.\".Sy System call restart
.\".Pp
.\"<more later...>
.Sh DIAGNOSTICS
Nearly all of the system calls provide an error number in the external
variable
.Va errno .
.Va errno
is implemented as a macro which expands to a modifiable lvalue of type
.Fa int .
.Pp
When a system call detects an error,
it returns an integer value
indicating failure (usually \-1)
and sets the variable
.Va errno
accordingly.
(This allows interpretation of the failure on receiving
a \-1 and to take action accordingly.)
Successful calls never set
.Va errno ;
once set, it remains until another error occurs.
It should only be examined after an error has been reported, because
otherwise a leftover value from some previous error may be found
instead.
.Po
Many library functions that are not system calls also set
.Va errno
on return, in the same fashion.
In these cases a nonzero value may be left in
.Va errno
even upon successful return if some internal action failed.
.Pc
.Pp
The manual page for each system call will list some of the common
errno codes that system call can return, but that should not be
considered an exhaustive list, i.e.
a properly written program should be able to gracefully recover from
any error that a system call might return.
Documenting all the error codes that a system call can return in
a more specification-like manner would take more resources than
this project has available.
.Pp
Note also that a number of system calls overload the meanings of these
error numbers, and that in these cases the meanings must be
interpreted according to the type and circumstances of the call.
.Pp
The following is a complete list of the errors and their
names as given in
.In errno.h .
.Bl -hang -width Ds
.It Er 0 Em "Error 0" .
Not used.
.It Er 1 EPERM Em "Operation not permitted" .
An attempt was made to perform an operation limited to processes
with appropriate privileges or to the owner of a file or other
resources.
.It Er 2 ENOENT Em "No such file or directory" .
A component of a specified pathname did not exist, or the
pathname was an empty string.
.It Er 3 ESRCH Em "No such process" .
No process could be found corresponding to that specified by the given
process ID.
.It Er 4 EINTR Em "Interrupted function call" .
An asynchronous signal (such as
.Dv SIGINT
or
.Dv SIGQUIT )
was caught by the process during the execution of an interruptible
function.
If the signal handler performs a normal return, the
interrupted function call will seem to have returned the error condition.
.It Er 5 EIO Em "Input/output error" .
Some physical input or output error occurred.
This error will not be reported until a subsequent operation on the same file
descriptor and may be lost (over written) by any subsequent errors.
.It Er 6 ENXIO Em "Device not configured" .
Input or output on a special file referred to a device that did not
exist, or
made a request beyond the limits of the device.
This error may also occur when, for example,
a tape drive is not online or no disk pack is
loaded on a drive.
.It Er 7 E2BIG Em "Arg list too long" .
The number of bytes used for the argument and environment
list of the new process exceeded the current limit of
.if t 2\u\s-218\s+2\d
.if n 2**18
bytes
.Pf ( Dv ARG_MAX
in
.In sys/syslimits.h ) .
.It Er 8 ENOEXEC Em "Exec format error" .
A request was made to execute a file
that, although it has the appropriate permissions,
was not in the format required for an
executable file.
.It Er 9 EBADF Em "Bad file descriptor" .
A file descriptor argument was out of range, referred to no open file,
had been revoked by
.Xr revoke 2 ,
or a
.Xr read 2
(or
.Xr write 2 )
request was made to a file that was
only open for writing (or reading).
.It Er 10 ECHILD Em "\&No child processes" .
A
.Xr wait 2
or
.Xr waitpid 2
function was executed by a process that had no existing or unwaited-for
child processes.
.It Er 11 EDEADLK Em "Resource deadlock avoided" .
An attempt was made to lock a system resource that
would have resulted in a deadlock situation.
.It Er 12 ENOMEM Em "Cannot allocate memory" .
The new process image required more memory than was allowed by the hardware
or by system-imposed memory management constraints.
A lack of swap space is normally temporary; however,
a lack of core is not.
Soft limits may be increased to their corresponding hard limits.
.It Er 13 EACCES Em "Permission denied" .
An attempt was made to access a file in a way forbidden
by its file access permissions.
.It Er 14 EFAULT Em "Bad address" .
The system detected an invalid address in attempting to
use an argument of a call.
The reliable detection of this error cannot be guaranteed and when not detected
may result in the generation of a signal, indicating an address violation,
which is sent to the process.
.It Er 15 ENOTBLK Em "Block device required" .
A block device operation was attempted on a non-block device or file.
.It Er 16 EBUSY Em "Resource busy" .
An attempt to use a system resource which was in use at the time
in a manner which would have conflicted with the request.
.It Er 17 EEXIST Em "File exists" .
An existing file was mentioned in an inappropriate context,
for instance, as the new link name in a
.Xr link 2
function.
.It Er 18 EXDEV Em "Improper link" .
A hard link to a file on another file system
was attempted.
.It Er 19 ENODEV Em "Operation not supported by device" .
An attempt was made to apply an inappropriate
function to a device,
for example,
trying to read a write-only device such as a printer.
.It Er 20 ENOTDIR Em "Not a directory" .
A component of the specified pathname existed, but it was
not a directory, when a directory was expected.
.It Er 21 EISDIR Em "Is a directory" .
An attempt was made to open a directory with write mode specified.
.It Er 22 EINVAL Em "Invalid argument" .
Some invalid argument was supplied.
(For example, specifying an undefined signal to a
.Xr signal 3
or
.Xr kill 2
function).
.It Er 23 ENFILE Em "Too many open files in system" .
Maximum number of file descriptors allowable on the system
has been reached and a requests for an open cannot be satisfied
until at least one has been closed.
.It Er 24 EMFILE Em "Too many open files" .
\*[Lt]As released, the limit on the number of
open files per process is 64.\*[Gt]
The
.Xr getrlimit 2
call with the
.Ar RLIMIT_NOFILE
resource will obtain the current limit.
.It Er 25 ENOTTY Em "Inappropriate ioctl for device" .
A control function (see
.Xr ioctl 2 )
was attempted for a file or
special device for which the operation was inappropriate.
.It Er 26 ETXTBSY Em "Text file busy" .
The new process was a pure procedure (shared text) file
which was open for writing by another process, or
while the pure procedure file was being executed an
.Xr open 2
call requested write access.
.It Er 27 EFBIG Em "File too large" .
The size of a file exceeded the maximum.
(The system-wide maximum file size is
.if t 2\u\s-263\s+2\d
.if n 2**63
bytes.
Each file system may impose a lower limit for files contained within it).
.It Er 28 ENOSPC Em "Device out of space" .
A
.Xr write 2
to an ordinary file, the creation of a
directory or symbolic link, or the creation of a directory
entry failed because no more disk blocks were available
on the file system, or the allocation of an inode for a newly
created file failed because no more inodes were available
on the file system.
.It Er 29 ESPIPE Em "Illegal seek" .
An
.Xr lseek 2
function was issued on a socket, pipe or
.Tn FIFO .
.It Er 30 EROFS Em "Read-only file system" .
An attempt was made to modify a file or directory
was made
on a file system that was read-only at the time.
.It Er 31 EMLINK Em "Too many links" .
The number of hard links to a single file has exceeded the maximum.
(The system-wide maximum number of hard links is 32767.
Each file system may impose a lower limit for files contained within it).
.It Er 32 EPIPE Em "Broken pipe" .
A write on a pipe, socket or
.Tn FIFO
for which there is no process
to read the data.
.It Er 33 EDOM Em "Numerical argument out of domain" .
A numerical input argument was outside the defined domain of the mathematical
function.
.It Er 34 ERANGE Em "Result too large or too small" .
The result of the function is too large or too small to be represented
in the available space.
.It Er 35 EAGAIN Em "Resource temporarily unavailable" .
This is a temporary condition and later calls to the
same routine may complete normally.
.It Er 36 EINPROGRESS Em "Operation now in progress" .
An operation that takes a long time to complete (such as
a
.Xr connect 2 )
was attempted on a non-blocking object (see
.Xr fcntl 2 ) .
.It Er 37 EALREADY Em "Operation already in progress" .
An operation was attempted on a non-blocking object that already
had an operation in progress.
.It Er 38 ENOTSOCK Em "Socket operation on non-socket" .
Self-explanatory.
.It Er 39 EDESTADDRREQ Em "Destination address required" .
A required address was omitted from an operation on a socket.
.It Er 40 EMSGSIZE Em "Message too long" .
A message sent on a socket was larger than the internal message buffer
or some other network limit.
.It Er 41 EPROTOTYPE Em "Protocol wrong type for socket" .
A protocol was specified that does not support the semantics of the
socket type requested.
For example, you cannot use the
.Tn ARPA
Internet
.Tn UDP
protocol with type
.Dv SOCK_STREAM .
.It Er 42 ENOPROTOOPT Em "Protocol option not available" .
A bad option or level was specified in a
.Xr getsockopt 2
or
.Xr setsockopt 2
call.
.It Er 43 EPROTONOSUPPORT Em "Protocol not supported" .
The protocol has not been configured into the
system or no implementation for it exists.
.It Er 44 ESOCKTNOSUPPORT Em "Socket type not supported" .
The support for the socket type has not been configured into the
system or no implementation for it exists.
.It Er 45 EOPNOTSUPP Em "Operation not supported" .
The attempted operation is not supported for the type of object referenced.
Usually this occurs when a file descriptor refers to a file or socket
that cannot support this operation,
for example, trying to
.Em accept
a connection on a datagram socket.
.It Er 46 EPFNOSUPPORT Em "Protocol family not supported" .
The protocol family has not been configured into the
system or no implementation for it exists.
.It Er 47 EAFNOSUPPORT Em "Address family not supported by protocol family" .
An address incompatible with the requested protocol was used.
For example, you shouldn't necessarily expect to be able to use
.Tn NS
addresses with
.Tn ARPA
Internet protocols.
.It Er 48 EADDRINUSE Em "Address already in use" .
Only one usage of each address is normally permitted.
.It Er 49 EADDRNOTAVAIL Em "Cannot assign requested address" .
Normally results from an attempt to create a socket with an
address not on this machine.
.It Er 50 ENETDOWN Em "Network is down" .
A socket operation encountered a dead network.
.It Er 51 ENETUNREACH Em "Network is unreachable" .
A socket operation was attempted to an unreachable network.
.It Er 52 ENETRESET Em "Network dropped connection on reset" .
The host you were connected to crashed and rebooted.
.It Er 53 ECONNABORTED Em "Software caused connection abort" .
A connection abort was caused internal to your host machine.
.It Er 54 ECONNRESET Em "Connection reset by peer" .
A connection was forcibly closed by a peer.
This normally results from a loss of the connection on the remote
socket due to a timeout or a reboot.
.It Er 55 ENOBUFS Em "\&No buffer space available" .
An operation on a socket or pipe was not performed because
the system lacked sufficient buffer space or because a queue was full.
.It Er 56 EISCONN Em "Socket is already connected" .
A
.Xr connect 2
request was made on an already connected socket; or,
a
.Xr sendto 2
or
.Xr sendmsg 2
request on a connected socket specified a destination
when already connected.
.It Er 57 ENOTCONN Em "Socket is not connected" .
An request to send or receive data was disallowed because
the socket was not connected and (when sending on a datagram socket)
no address was supplied.
.It Er 58 ESHUTDOWN Em "Cannot send after socket shutdown" .
A request to send data was disallowed because the socket
had already been shut down with a previous
.Xr shutdown 2
call.
.It Er 59 ETOOMANYREFS Em "Too many references: can't splice" .
The resource is used up to capacity.
.It Er 60 ETIMEDOUT Em "Operation timed out" .
A
.Xr connect 2
or
.Xr send 2
request failed because the connected party did not
properly respond after a period of time.
(The timeout period is dependent on the communication protocol).
.It Er 61 ECONNREFUSED Em "Connection refused" .
No connection could be made because the target machine actively
refused it.
This usually results from trying to connect
to a service that is inactive on the foreign host.
.It Er 62 ELOOP Em "Too many levels of symbolic links" .
A path name lookup involved more than 32
.Pq Dv MAXSYMLINKS
symbolic links.
.It Er 63 ENAMETOOLONG Em "File name too long" .
A component of a path name exceeded 255
.Pq Dv MAXNAMELEN
characters, or an entire
path name exceeded 1023
.Pq Dv MAXPATHLEN Ns \-1
characters.
.It Er 64 EHOSTDOWN Em "Host is down" .
A socket operation failed because the destination host was down.
.It Er 65 EHOSTUNREACH Em "No route to host" .
A socket operation was attempted to an unreachable host.
.It Er 66 ENOTEMPTY Em "Directory not empty" .
A directory with entries other than
.Ql \&.
and
.Ql \&..
was supplied to a remove directory or rename call.
.It Er 67 EPROCLIM Em "Too many processes" .
.It Er 68 EUSERS Em "Too many users" .
The quota system ran out of table entries.
.It Er 69 EDQUOT Em "Disc quota exceeded" .
A
.Xr write 2
to an ordinary file, the creation of a
directory or symbolic link, or the creation of a directory
entry failed because the user's quota of disk blocks was
exhausted, or the allocation of an inode for a newly
created file failed because the user's quota of inodes
was exhausted.
.It Er 70 ESTALE Em "Stale NFS file handle" .
An attempt was made to access an open file (on an
.Tn NFS
filesystem)
which is now unavailable as referenced by the file descriptor.
This may indicate the file was deleted on the
.Tn NFS
server or some other catastrophic event occurred.
.It Er 71 EREMOTE Em "Too many levels of remote in path" .
.Tn NFS
version 3 RPC return code 71.
.It Er 72 EBADRPC Em "RPC struct is bad" .
Exchange of
.Tn RPC
information was unsuccessful.
.It Er 73 ERPCMISMATCH Em "RPC version wrong" .
The version of
.Tn RPC
on the remote peer is not compatible with
the local version.
.It Er 74 EPROGUNAVAIL Em "RPC prog. not avail" .
The requested program is not registered on the remote host.
.It Er 75 EPROGMISMATCH Em "Program version wrong" .
The requested version of the program is not available
on the remote host
.Pq Tn RPC .
.It Er 76 EPROCUNAVAIL Em "Bad procedure for program" .
An
.Tn RPC
call was attempted for a procedure which doesn't exist
in the remote program.
.It Er 77 ENOLCK Em "No locks available" .
A system-imposed limit on the number of simultaneous file
locks was reached.
.It Er 78 ENOSYS Em "Function not implemented" .
Attempted a system call that is not available on this
system.
.It Er 79 EFTYPE Em "Inappropriate file type or format" .
Attempted a file operation on a file of a type for which it was invalid.
.It Er 80 EAUTH Em "Authentication error" .
Attempted to use an invalid authentication ticket to mount an
.Tn NFS
filesystem.
.It Er 81 ENEEDAUTH Em "Need authenticator" .
An authentication ticket must be obtained before the given
.Tn NFS
filesystem may be mounted.
.It Er 82 EIDRM Em "Identifier removed" .
An IPC identifier was removed while the current process was waiting on it.
.It Er 83 ENOMSG Em "No message of the desired type" .
An IPC message queue does not contain a message of the desired type,
or a message catalog does not contain the requested message.
.It Er 84 EOVERFLOW Em "Value too large to be stored in data type" .
A numerical result of the function was too large to be stored in the
caller-provided space.
.It Er 85 EILSEQ Em "Illegal byte sequence" .
A wide character/multibyte character encoding error occurred.
.It Er 86 ENOTSUP Em "Not supported" .
An attempt was made to set or change a parameter to an unsupported
value.
.It Er 87 ECANCELED Em "Operation canceled" .
The requested operation was canceled.
.It Er 88 EBADMSG Em "Bad or corrupt message" .
A message in the specified message catalog did not satisfy implementation
defined criteria, or a STREAMS operation encountered an invalid message or
a file descriptor at the STREAM head.
.It Er 89 ENODATA Em "No message available" .
No message is available on the STREAM head read queue
.It Er 90 ENOSR Em "No STREAM resources" .
Buffers could not be allocated due to insufficient STREAMs memory resources.
.It Er 91 ENOSTR Em "Not a STREAM" .
A STREAM is not associated with the specified file descriptor.
.It Er 92 ETIME Em "STREAM ioctl timeout" .
The timer set for a STREAMS
.Xr ioctl 2
operation has expired.
.It Er 93 ENOATTR Em "Attribute not found" .
The specified extended attribute does not exist.
.It Er 94 EMULTIHOP Em "Multihop attempted" .
Components of path require hopping to multiple remote machines and the
file system does not allow it.
It occurs when users try to access remote resources which are not directly
accessible.
.It Er 95 ENOLINK Em "Link has been severed" .
Occurs when the link (virtual circuit) connecting to a remote machine is gone.
.It Er 96 EPROTO Em "Protocol error" .
Some protocol error occurred.
This error is device-specific, but is generally not related to a hardware
failure.
.El
.Sh DEFINITIONS
.Bl -tag -width Ds
.It Process ID
Each active process in the system is uniquely identified by a non-negative
integer called a process ID.
The range of this ID is from 0 to 30000.
.It Parent process ID
A new process is created by a currently active process; (see
.Xr fork 2 ) .
The parent process ID of a process is initially the process ID of its creator.
If the creating process exits,
the parent process ID of each child is set to the ID of a system process,
.Xr init 8 .
.It Process Group
Each active process is a member of a process group that is identified by
a non-negative integer called the process group ID.
This is the process ID of the group leader.
This grouping permits the signaling of related processes (see
.Xr termios 4 )
and the job control mechanisms of
.Xr csh 1 .
.It Session
A session is a set of one or more process groups.
A session is created by a successful call to
.Xr setsid 2 ,
which causes the caller to become the only member of the only process
group in the new session.
.It Session leader
A process that has created a new session by a successful call to
.Xr setsid 2 ,
is known as a session leader.
Only a session leader may acquire a terminal as its controlling terminal (see
.Xr termios 4 ) .
.It Controlling process
A session leader with a controlling terminal is a controlling process.
.It Controlling terminal
A terminal that is associated with a session is known as the controlling
terminal for that session and its members.
.It "Terminal Process Group ID"
A terminal may be acquired by a session leader as its controlling terminal.
Once a terminal is associated with a session, any of the process groups
within the session may be placed into the foreground by setting
the terminal process group ID to the ID of the process group.
This facility is used
to arbitrate between multiple jobs contending for the same terminal.
(See
.Xr csh 1
and
.Xr tty 4
for more information on job control.)
.It "Orphaned Process Group"
A process group is considered to be
.Em orphaned
if it is not under the control of a job control shell.
More precisely, a process group is orphaned
when none of its members has a parent process that is in the same session
as the group,
but is in a different process group.
Note that when a process exits, the parent process for its children
is changed to be
.Xr init 8 ,
which is in a separate session.
Not all members of an orphaned process group are necessarily orphaned
processes (those whose creating process has exited).
The process group of a session leader is orphaned by definition.
.It "Real User ID and Real Group ID"
Each user on the system is identified by a positive integer
termed the real user ID.
.Pp
Each user is also a member of one or more groups.
One of these groups is distinguished from others and
used in implementing accounting facilities.
The positive integer corresponding to this distinguished group is
termed the real group ID.
.Pp
All processes have a real user ID and real group ID.
These are initialized from the equivalent attributes
of the process that created it.
.It "Effective User Id, Effective Group Id, and Group Access List"
Access to system resources is governed by two values:
the effective user ID, and the group access list.
The first member of the group access list is also known as the
effective group ID.
(In POSIX.1, the group access list is known as the set of supplementary
group IDs, and it is unspecified whether the effective group ID is
a member of the list.)
.Pp
The effective user ID and effective group ID are initially the
process's real user ID and real group ID respectively.
Either may be modified through execution of a set-user-ID or
set-group-ID file (possibly by one of its ancestors) (see
.Xr execve 2 ) .
By convention, the effective group ID (the first member of the group access
list) is duplicated, so that the execution of a set-group-ID program
does not result in the loss of the original (real) group ID.
.Pp
The group access list is a set of group IDs
used only in determining resource accessibility.
Access checks are performed as described below in
.Qq File Access Permissions .
.It "Saved Set User ID and Saved Set Group ID"
When a process executes a new file, the effective user ID is set
to the owner of the file if the file is set-user-ID, and the effective
group ID (first element of the group access list) is set to the group
of the file if the file is set-group-ID.
The effective user ID of the process is then recorded as the saved set-user-ID,
and the effective group ID of the process is recorded as the saved set-group-ID.
These values may be used to regain those values as the effective user
or group ID after reverting to the real ID (see
.Xr setuid 2 ) .
(In POSIX.1, the saved set-user-ID and saved set-group-ID are optional,
and are used in setuid and setgid, but this does not work as desired
for the super-user.)
.It Super-user
A process is recognized as a
.Em super-user
process and is granted special privileges if its effective user ID is 0.
.It Special Processes
The processes with process IDs of 0 and 1 are special.
Process 0 is the scheduler.
Process 1 is the initialization process
.Xr init 8 ,
and is the ancestor (parent) of every other process in the system.
It is used to control the process structure.
The kernel will allocate other kernel threads to handle certain
periodic tasks or device related tasks, such as:
.Bl -tag -width pagedaemon
.It Sy acctwatch
System accounting disk watcher, see
.Xr acct 2 ,
.Xr acct 5 .
.It Sy aiodoned
Asynchronous I/O done handler, see
.Xr uvm 9 .
.It Sy atabusX
ATA bus handler, see
.Xr ata 4 .
.It Sy cardslotX
CardBus slot watcher thread, see
.Xr cardslot 4 .
.It Sy cryptoret
The software crypto daemon.
.It Sy fssbsX
File system snapshot thread, see
.Xr fss 4 .
.It Sy ioflush
The in-kernel periodic flush the buffer cache to disk task,
which replaces the old
.Sy update
program.
.It Sy nfsio , nfskqpoll
NFS handing daemons.
.It Sy lfs_writer
Log filesystem writer.
.It Sy pagedaemon
The page daemon.
.It Sy raidX , raidioX , raid_parity , raid_recon , raid_reconip , raid_copyback
Raid framework related threads, see
.Xr raid 4 .
.It Sy scsibusX
SCSI bus handler, see
.Xr scsi 4 .
.It Sy smbiodX , smbkq
SMBFS handling daemon, see
.Xr netsmb 4 .
.It Sy swdmover
The software data mover I/O thread, see
.Xr dmoverio 4 .
.It Sy sysmon
The systems monitoring framework daemon.
.It Sy usbX , usbtask
USB bus handler, see
.Xr usb 4 .
.El
.Pp
There are more machine-dependent kernel threads allocated by
different drivers.
See the specific driver manual pages for more information.
.It Descriptor
An integer assigned by the system when a file is referenced
by
.Xr open 2
or
.Xr dup 2 ,
or when a socket is created by
.Xr pipe 2 ,
.Xr socket 2 ,
or
.Xr socketpair 2 ,
which uniquely identifies an access path to that file or socket from
a given process or any of its children.
.It File Name
Names consisting of up to 255
.Pq Dv MAXNAMELEN
characters may be used to name
an ordinary file, special file, or directory.
.Pp
These characters may be selected from the set of all
.Tn ASCII
character
excluding 0 (NUL) and the
.Tn ASCII
code for
.Ql \&/
(slash).
(The parity bit, bit 7, must be 0).
.Pp
Note that it is generally unwise to use
.Ql \&* ,
.Ql \&? ,
.Ql \&[
or
.Ql \&]
as part of
file names because of the special meaning attached to these characters
by the shell.
.It Pathname
A path name is a
.Tn NUL Ns -terminated
character string starting with an
optional slash
.Ql \&/ ,
followed by zero or more directory names separated
by slashes, optionally followed by a file name.
The total length of a path name must be less than 1024
.Pq Dv MAXPATHLEN
characters.
.Pp
If a path name begins with a slash, the path search begins at the
.Em root
directory.
Otherwise, the search begins from the current working directory.
A slash by itself names the root directory.
An empty string is not a valid pathname.
.It Directory
A directory is a special type of file that contains entries
that are references to other files.
Directory entries are called links.
By convention, a directory contains at least two links,
.Ql \&.
and
.Ql \&.. ,
referred to as
.Em dot
and
.Em dot-dot
respectively.
Dot refers to the directory itself and dot-dot refers to its parent directory.
.It "Root Directory and Current Working Directory"
Each process has associated with it a concept of a root directory
and a current working directory for the purpose of resolving path
name searches.
A process's root directory need not be the root
directory of the root file system.
.It File Access Permissions
Every file in the file system has a set of access permissions.
These permissions are used in determining whether a process
may perform a requested operation on the file (such as opening
a file for writing).
Access permissions are established at the time a file is created.
They may be changed at some later time through the
.Xr chmod 2
call.
.Pp
File access is broken down according to whether a file may be: read,
written, or executed.
Directory files use the execute permission to control if the
directory may be searched.
.Pp
File access permissions are interpreted by the system as
they apply to three different classes of users: the owner
of the file, those users in the file's group, anyone else.
Every file has an independent set of access permissions for
each of these classes.
When an access check is made, the system decides if permission should be
granted by checking the access information applicable to the caller.
.Pp
Read, write, and execute/search permissions on
a file are granted to a process if:
.Pp
The process's effective user ID is that of the super-user.
(Note: even the super-user cannot execute a non-executable file).
.Pp
The process's effective user ID matches the user ID of the owner
of the file and the owner permissions allow the access.
.Pp
The process's effective user ID does not match the user ID of the
owner of the file, and either the process's effective
group ID matches the group ID
of the file, or the group ID of the file is in
the process's group access list,
and the group permissions allow the access.
.Pp
Neither the effective user ID nor effective group ID
and group access list of the process
match the corresponding user ID and group ID of the file,
but the permissions for ``other users'' allow access.
.Pp
Otherwise, permission is denied.
.It Sockets and Address Families
A socket is an endpoint for communication between processes.
Each socket has queues for sending and receiving data.
.Pp
Sockets are typed according to their communications properties.
These properties include whether messages sent and received
at a socket require the name of the partner, whether communication
is reliable, the format used in naming message recipients, etc.
.Pp
Each instance of the system supports some
collection of socket types; consult
.Xr socket 2
for more information about the types available and
their properties.
.Pp
Each instance of the system supports some number of sets of
communications protocols.
Each protocol set supports addresses of a certain format.
An Address Family is the set of addresses for a specific group of protocols.
Each socket has an address
chosen from the address family in which the socket was created.
.El
.Sh SEE ALSO
.Xr intro 3 ,
.Xr perror 3
.Sh HISTORY
An
.Nm intro
manual page appeared in
.At v6 .