glossary.html

unix 下的C开发手册,还用详细的例程。

However, it is assumed that when the writing is directly
to the terminal (it has not been redirected elsewhere),
there is no practical way for a user or test suite to
determine whether a file descriptor is being used or not.
Therefore, the use of a file descriptor is mandated
only for the redirection case and the implementation
is free to use any method when the output is not redirected.
The verb
<i>write</i>
is used almost exclusively, with the very few exceptions of
those utilities where output redirection need not be supported:
<i><a href="../xcu/tabs.html">tabs</a></i>,
<i><a href="../xcu/talk.html">talk</a></i>,
<i><a href="../xcu/tput.html">tput</a></i>
and
<i><a href="../xcu/vi.html">vi</a></i>.
<p>
<h4><a name = "tag_004_000_079">&nbsp;</a>dollar sign</h4><xref type="11" name="dollarsign"></xref>
The character "$".
<p>
<h4><a name = "tag_004_000_080">&nbsp;</a>dot</h4>The filename consisting of a single dot character (.).
See
<xref href=pathres><a href="#tag_004_000_198">
pathname resolution
</a></xref>.
In the context of shell special built-in utilities, see
<i><a href="../xcu/chap2.html#tag_001_014_004">dot</a></i>
in
the <b>XCU</b> specification, <a href="../xcu/chap2.html#tag_001_014"><b>Special Built-in Utilities</b>&nbsp;</a>.
<p>
<h4><a name = "tag_004_000_081">&nbsp;</a>dot-dot</h4>The filename consisting solely of two dot characters (..).
See
<xref href=pathres><a href="#tag_004_000_198">
pathname resolution
</a></xref>.
<p>
<h4><a name = "tag_004_000_082">&nbsp;</a>double-quote</h4>The character """, also known as
<i>quotation-mark</i>.
<p>
<h4><a name = "tag_004_000_083">&nbsp;</a>downshifting</h4>The conversion of an upper-case character to its
lower-case representation.
<p>
<h4><a name = "tag_004_000_084">&nbsp;</a>(clock) drift rate</h4>The rate at which the time measured by a clock
deviates from the actual passage of real time.
<p>
A positive drift rate causes a clock to gain time with respect 
to real time;
a negative drift rate causes a clock to lose time with respect 
to real time.
<p>
<h4><a name = "tag_004_000_085">&nbsp;</a>driver</h4>A module that controls data transferred to and received from peripheral
devices.  Drivers are traditionally written to be a part of the system
implementation, although they are frequently written separately from the
writing of the implementation.  A driver may contain processor-specific code,
and therefore be non-portable.
<p>
<h4><a name = "tag_004_000_086">&nbsp;</a>effective group ID</h4>An attribute of a process that is used in determining
various permissions, including file access permissions, described in
<xref href=fileaccperm><a href="#tag_004_000_107">
file access permissions
</a></xref>.
See
<xref href=groupid><a href="#tag_004_000_132">
group ID
</a></xref>.
This value is subject to change during the process lifetime,
as described in the
<i>exec</i>
family of functions and
<i><a href="../xsh/setgid.html">setgid()</a></i>.
<p>
<h4><a name = "tag_004_000_087">&nbsp;</a>effective user ID</h4>An attribute of a process that is used in determining
various permissions, including file access permissions.
See
<xref href=userid><a href="#tag_004_000_318">
user ID
</a></xref>.
This value is subject to change during the
process lifetime,
as described in
<i>exec</i>
and
<i><a href="../xsh/setuid.html">setuid()</a></i>.
<p>
<h4><a name = "tag_004_000_088">&nbsp;</a>eight-bit transparency</h4>The ability of a software component to process 8-bit
characters without modifying or utilising any part of the
character in a way that is inconsistent with the rules of the
current coded character set.
<p>
<h4><a name = "tag_004_000_089">&nbsp;</a>empty directory</h4>A directory that contains, at most,
directory entries for dot and dot-dot.
<p>
<h4><a name = "tag_004_000_090">&nbsp;</a>empty line</h4><xref type="11" name="emptyline"></xref>
A line consisting of only a
newline
character.
See also
<xref href=blankline><a href="#tag_004_000_032">
blank line
</a></xref>.
<p>
<h4><a name = "tag_004_000_091">&nbsp;</a>empty string</h4><xref type="11" name="emptystr"></xref>
(Or <b>null string</b>.)
A string whose first byte is a null byte.
<p>
<h4><a name = "tag_004_000_092">&nbsp;</a>empty wide-character string</h4>A wide-character string whose first element is a
null wide-character code.
<p>
<h4><a name = "tag_004_000_093">&nbsp;</a>epoch</h4>The time zero hours, zero minutes, zero seconds, on January 1, 1970
Coordinated Universal Time.
See
<xref href=secsepoch><a href="#tag_004_000_255">
seconds since the epoch
</a></xref>.
<p>
<h4><a name = "tag_004_000_094">&nbsp;</a>equivalence class</h4><xref type="11" name="equivclass"></xref>
A set of collating elements with the same
primary collation weight.
<p>
Elements in an equivalence class are typically elements
that naturally group together, such as all accented letters based on
the same base letter.
<p>
The collation order of elements within an equivalence class is determined by
the weights assigned on any subsequent levels after the primary weight.
<p>
<h4><a name = "tag_004_000_095">&nbsp;</a>era</h4>An alternative method for counting and displaying years.
See
<xref href=lc_time><a href="locale.html#tag_005_003_005">
LC_TIME
</a></xref>.
<p>
<h4><a name = "tag_004_000_096">&nbsp;</a>executable file</h4>A regular file acceptable as a new process image file
by the equivalent of the
<i>exec</i>
family of functions, and thus usable as one form of a utility.
The standard utilities described as compilers can produce
executable files, but other unspecified methods
of producing executable files may also be provided.
The internal format of an executable file is unspecified, but a conforming
application cannot assume an executable file is a text file.
<p>
<h4><a name = "tag_004_000_097">&nbsp;</a>execute</h4><xref type="11" name="execute"></xref>
To perform the actions described in
the <b>XCU</b> specification, <a href="../xcu/chap2.html#tag_001_009_001_001"><b>Command Search and Execution</b>&nbsp;</a>.
See also
<xref href=invoke><a href="#tag_004_000_141">
invoke
</a></xref>.
<p>
<h4><a name = "tag_004_000_098">&nbsp;</a>expand</h4>In the shell,
when not qualified, the act of applying all the expansions
described in
the <b>XCU</b> specification, <a href="../xcu/chap2.html#tag_001_006"><b>Word Expansions</b>&nbsp;</a>.
<p>
<h4><a name = "tag_004_000_099">&nbsp;</a>extended regular expression</h4>A pattern constructed according to the rules defined in
<xref href=ere><a href="re.html#tag_007_004">
Extended Regular Expressions
</a></xref>.
<p>
<h4><a name = "tag_004_000_100">&nbsp;</a>extended signed integral type</h4>A signed integral type or an implementation-dependent type with similar
properties.
<p>
<h4><a name = "tag_004_000_101">&nbsp;</a>extended security controls</h4>The access control (see
<xref href=fileaccperm><a href="#tag_004_000_107">
file access permissions
</a></xref>)
and privilege (see
<xref href=appropriv><a href="#tag_004_000_012">
appropriate privileges
</a></xref>)
mechanisms have been defined to allow implementation-dependent
extended security controls.
These permit an implementation to provide
security mechanisms to
support different security policies
from those described in this specification set.
These mechanisms do not alter or override the defined
semantics of any of the functions or utilities in this specification set.
<p>
<h4><a name = "tag_004_000_102">&nbsp;</a>extended unsigned integral type</h4>An unsigned integral type or an implementation-dependent type with
similar properties.
<p>
<h4><a name = "tag_004_000_103">&nbsp;</a>feature test macro</h4>A macro used to determine whether a particular
set of features will be included from a header.
See
the <b>XSH</b> specification, <a href="../xsh/compilation.html"><b>The Compilation Environment</b>&nbsp;</a>.
<p>
<h4><a name = "tag_004_000_104">&nbsp;</a>field</h4>In the shell,
a unit of text that is the result of parameter expansion (see the <b>XCU</b> specification, <a href="../xcu/chap2.html#tag_001_006_002"><b>Parameter Expansion</b>&nbsp;</a>),
arithmetic expansion (see the <b>XCU</b> specification, <a href="../xcu/chap2.html#tag_001_006_004"><b>Arithmetic  Expansion</b>&nbsp;</a>),
command substitution (see the <b>XCU</b> specification, <a href="../xcu/chap2.html#tag_001_006_003"><b>Command  Substitution</b>&nbsp;</a>),
or field splitting (see the <b>XCU</b> specification, <a href="../xcu/chap2.html#tag_001_006_005"><b>Field Splitting</b>&nbsp;</a>).
During command processing (see
the <b>XCU</b> specification, <a href="../xcu/chap2.html#tag_001_009_001"><b>Simple Commands</b>&nbsp;</a>),
the resulting fields are used as the command name and its arguments.
<p>
<h4><a name = "tag_004_000_105">&nbsp;</a>FIFO special file</h4>(Or <b>FIFO</b>.)
A type of file with the property that
data written to such a file
is read on a first-in-first-out basis.
Other characteristics of FIFOs
are described in
<i><a href="../xsh/open.html">open()</a></i>,
<i><a href="../xsh/read.html">read()</a></i>,
<i><a href="../xsh/write.html">write()</a></i>
and
<i><a href="../xsh/lseek.html">lseek()</a></i>.
<p>
<h4><a name = "tag_004_000_106">&nbsp;</a>file</h4><xref type="11" name="filedef"></xref>
An object that can be written to, or read from, or both.
A
file
has certain attributes, including access permissions and type.
File types include
regular file,
character special file,
block special file,
FIFO special file and directory.
Other types of files may be supported by the implementation.
<p>
<h4><a name = "tag_004_000_107">&nbsp;</a>file access permissions</h4><xref type="11" name="fileaccperm"></xref>
The standard file access control mechanism uses the
file permission bits,
as described below.
These bits are set at
the time of file creation by
functions such as
<i><a href="../xsh/open.html">open()</a></i>,
<i><a href="../xsh/creat.html">creat()</a></i>,
<i><a href="../xsh/mkdir.html">mkdir()</a></i>
and
<i><a href="../xsh/mkfifo.html">mkfifo()</a></i>
and are changed by
<i><a href="../xsh/chmod.html">chmod()</a></i>.
These bits are read by
<i><a href="../xsh/stat.html">stat()</a></i>
or
<i><a href="../xsh/fstat.html">fstat()</a></i>.
<p>
Implementations may provide
<i>additional</i>
or
<i>alternate</i>
file access control mechanisms, or both.
An additional access control mechanism will only
further restrict the access permissions defined by the
file permission bits.
An alternate file access control mechanism will:
<ul>
<p>
<li>
specify file permission bits for the
file owner class, file group class, and file other class
of that file, corresponding to the access permissions, to be
returned by
<i><a href="../xsh/stat.html">stat()</a></i>
or
<i><a href="../xsh/fstat.html">fstat()</a></i>
<p>
<li>
be enabled only by explicit user action, on a per-file basis
by the file owner or a user with the appropriate privilege
<p>
<li>
be disabled for a file after the file permission bits
are changed for that file with
<i><a href="../xsh/chmod.html">chmod()</a></i>.
The disabling of the alternate mechanism need not disable any additional
mechanisms supported by an implementation.
<p>
</ul>
<p>
Whenever a process requests
file access permission
for read, write
or execute/search, if no additional mechanism denies access, access is
determined as follows:
<ul>
<p>
<li>
If a process has the
appropriate privilege:
<ul>
<p>
<li>
If read, write or directory search permission is requested,
access is granted.
<p>
<li>
If execute permission is requested, access is granted if
execute permission is granted to at least one user by the
file permission bits
or by an alternate access control
mechanism; otherwise, access is denied.
<p>
</ul>
<p>
<li>
Otherwise:
<ul>
<p>
<li>
The file permission bits of a file
contain read, write and execute/search permissions for the
file owner class, file group class and file other class.
<p>
<li>
Access is granted if an alternate access control mechanism is
not enabled and the requested access permission bit is set for the class
(file owner class, file group class, or file other class)
to which the process belongs,
or if an alternate access
control mechanism is enabled and it allows the requested
access; otherwise, access is denied.
<p>
</ul>
<p>
</ul>
<p>
<h4><a name = "tag_004_000_108">&nbsp;</a>file description</h4>See
<xref href=openfiledes><a href="#tag_004_000_184">
open file description
</a></xref>.
<p>
<h4><a name = "tag_004_000_109">&nbsp;</a>file descriptor</h4>A per-process unique, non-negative integer
used to identify an open file for the purpose of file access.
The value of a file descriptor is from zero to
{OPEN_MAX}.
A process can have no more than
{OPEN_MAX}
file descriptors open simultaneously.
File descriptors may also be
used to implement