xbd_chap04.html

IEEE 1003.1-2003, Single Unix Specification v3

<p>Unless explicitly stated otherwise, if one of the above functions returns an error, it is unspecified whether the invocation
causes memory to be synchronized.</p>

<p>Applications may allow more than one thread of control to read a memory location simultaneously.</p>

<h3><a name="tag_04_11"></a>Pathname Resolution</h3>

<p>Pathname resolution is performed for a process to resolve a pathname to a particular file in a file hierarchy. There may be
multiple pathnames that resolve to the same file.</p>

<p>Each filename in the pathname is located in the directory specified by its predecessor (for example, in the pathname fragment
<b>a/b</b>, file <b>b</b> is located in directory <b>a</b>). Pathname resolution shall fail if this cannot be accomplished. If the
pathname begins with a slash, the predecessor of the first filename in the pathname shall be taken to be the root directory of the
process (such pathnames are referred to as &quot;absolute pathnames&quot;). If the pathname does not begin with a slash, the predecessor of
the first filename of the pathname shall be taken to be the current working directory of the process (such pathnames are referred
to as &quot;relative pathnames&quot;).</p>

<p>The interpretation of a pathname component is dependent on the value of {NAME_MAX} and _POSIX_NO_TRUNC associated with the path
prefix of that component. If any pathname component is longer than {NAME_MAX}, the implementation shall consider this an error.</p>

<p>A pathname that contains at least one non-slash character and that ends with one or more trailing slashes shall be resolved as
if a single dot character ( <tt>'.'</tt> ) were appended to the pathname.</p>

<p>If a symbolic link is encountered during pathname resolution, the behavior shall depend on whether the pathname component is at
the end of the pathname and on the function being performed. If all of the following are true, then pathname resolution is
complete:</p>

<ol>
<li>
<p>This is the last pathname component of the pathname.</p>
</li>

<li>
<p>The pathname has no trailing slash.</p>
</li>

<li>
<p>The function is required to act on the symbolic link itself, or certain arguments direct that the function act on the symbolic
link itself.</p>
</li>
</ol>

<p>In all other cases, the system shall prefix the remaining pathname, if any, with the contents of the symbolic link. If the
combined length exceeds {PATH_MAX}, and the implementation considers this to be an error, <i>errno</i> shall be set to
[ENAMETOOLONG] and an error indication shall be returned. Otherwise, the resolved pathname shall be the resolution of the pathname
just created. If the resulting pathname does not begin with a slash, the predecessor of the first filename of the pathname is taken
to be the directory containing the symbolic link.</p>

<p>If the system detects a loop in the pathname resolution process, it shall set <i>errno</i> to [ELOOP] and return an error
indication. The same may happen if during the resolution process more symbolic links were followed than the implementation allows.
This implementation-defined limit shall not be smaller than {SYMLOOP_MAX}.</p>

<p>The special filename dot shall refer to the directory specified by its predecessor. The special filename dot-dot shall refer to
the parent directory of its predecessor directory. As a special case, in the root directory, dot-dot may refer to the root
directory itself.</p>

<p>A pathname consisting of a single slash shall resolve to the root directory of the process. A null pathname shall not be
successfully resolved. A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner,
although more than two leading slashes shall be treated as a single slash.</p>

<h3><a name="tag_04_12"></a>Process ID Reuse</h3>

<p>A process group ID shall not be reused by the system until the process group lifetime ends.</p>

<p>A process ID shall not be reused by the system until the process lifetime ends. In addition, if there exists a process group
whose process group ID is equal to that process ID, the process ID shall not be reused by the system until the process group
lifetime ends. A process that is not a system process shall not have a process ID of 1.</p>

<h3><a name="tag_04_13"></a>Scheduling Policy</h3>

<p>A scheduling policy affects process or thread ordering:</p>

<ul>
<li>
<p>When a process or thread is a running thread and it becomes a blocked thread</p>
</li>

<li>
<p>When a process or thread is a running thread and it becomes a preempted thread</p>
</li>

<li>
<p>When a process or thread is a blocked thread and it becomes a runnable thread</p>
</li>

<li>
<p>When a running thread calls a function that can change the priority or scheduling policy of a process or thread</p>
</li>

<li>
<p>In other scheduling policy-defined circumstances</p>
</li>
</ul>

<p>Conforming implementations shall define the manner in which each of the scheduling policies may modify the priorities or
otherwise affect the ordering of processes or threads at each of the occurrences listed above. Additionally, conforming
implementations shall define in what other circumstances and in what manner each scheduling policy may modify the priorities or
affect the ordering of processes or threads.</p>

<h3><a name="tag_04_14"></a>Seconds Since the Epoch</h3>

<p>A value that approximates the number of seconds that have elapsed since the Epoch. A Coordinated Universal Time name (specified
in terms of seconds (<i>tm_sec</i>), minutes (<i>tm_min</i>), hours (<i>tm_hour</i>), days since January 1 of the year
(<i>tm_yday</i>), and calendar year minus 1900 (<i>tm_year</i>)) is related to a time represented as seconds since the Epoch,
according to the expression below.</p>

<p>If the year is &lt;1970 or the value is negative, the relationship is undefined. If the year is &gt;=1970 and the value is
non-negative, the value is related to a Coordinated Universal Time name according to the C-language expression, where
<i>tm_sec</i>, <i>tm_min</i>, <i>tm_hour</i>, <i>tm_yday</i>, and <i>tm_year</i> are all integer types:</p>

<blockquote>
<pre>
<i>tm_sec</i> <tt>+</tt> <i>tm_min</i><tt>*60 +</tt> <i>tm_hour</i><tt>*3600 +</tt> <i>tm_yday</i><tt>*86400 +
    (</tt><i>tm_year</i><tt>-70)*31536000 + ((</tt><i>tm_year</i><tt>-69)/4)*86400 -
    ((</tt><i>tm_year</i><tt>-1)/100)*86400 + ((</tt><i>tm_year</i><tt>+299)/400)*86400
</tt>
</pre>
</blockquote>

<p>The relationship between the actual time of day and the current value for seconds since the Epoch is unspecified.</p>

<p>How any changes to the value of seconds since the Epoch are made to align to a desired relationship with the current actual time
are made is implementation-defined. As represented in seconds since the Epoch, each and every day shall be accounted for by exactly
86400 seconds. <basefont size="2"></p>

<dl>
<dt><b>Note:</b></dt>

<dd>The last three terms of the expression add in a day for each year that follows a leap year starting with the first leap year
since the Epoch. The first term adds a day every 4 years starting in 1973, the second subtracts a day back out every 100 years
starting in 2001, and the third adds a day back in every 400 years starting in 2001. The divisions in the formula are integer
divisions; that is, the remainder is discarded leaving only the integer quotient.</dd>
</dl>

<basefont size="3"> 

<h3><a name="tag_04_15"></a>Semaphore</h3>

<p>A minimum synchronization primitive to serve as a basis for more complex synchronization mechanisms to be defined by the
application program.</p>

<p>For the semaphores associated with the Semaphores option, a semaphore is represented as a shareable resource that has a
non-negative integer value. When the value is zero, there is a (possibly empty) set of threads awaiting the availability of the
semaphore.</p>

<p>For the semaphores associated with the X/Open System Interface Extension (XSI), a semaphore is a positive integer (0 through
32767). The <a href="../functions/semget.html"><i>semget</i>()</a> function can be called to create a set or array of semaphores. A
semaphore set can contain one or more semaphores up to an implementation-defined value.</p>

<h5><a name="tag_04_15_00_01"></a>Semaphore Lock Operation</h5>

<p>An operation that is applied to a semaphore. If, prior to the operation, the value of the semaphore is zero, the semaphore lock
operation shall cause the calling thread to be blocked and added to the set of threads awaiting the semaphore; otherwise, the value
shall be decremented.</p>

<h5><a name="tag_04_15_00_02"></a>Semaphore Unlock Operation</h5>

<p>An operation that is applied to a semaphore. If, prior to the operation, there are any threads in the set of threads awaiting
the semaphore, then some thread from that set shall be removed from the set and becomes unblocked; otherwise, the semaphore value
shall be incremented.</p>

<h3><a name="tag_04_16"></a>Thread-Safety</h3>

<p>Refer to the System Interfaces volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href="../functions/xsh_chap02_09.html">Section 2.9,
Threads</a>.</p>

<h3><a name="tag_04_17"></a>Tracing</h3>

<p>The trace system allows a traced process to have a selection of events created for it. Traces consist of streams of trace event
types.</p>

<p>A trace event type is identified on the one hand by a trace event type name, also referenced as a trace event name, and on the
other hand by a trace event type identifier. A trace event name is a human-readable string. A trace event type identifier is an
opaque identifier used by the trace system. There shall be a one-to-one relationship between trace event type identifiers and trace
event names for a given trace stream and also for a given traced process. The trace event type identifier shall be generated
automatically from a trace event name by the trace system either when a trace controller process invokes <a href=
"../functions/posix_trace_trid_eventid_open.html"><i>posix_trace_trid_eventid_open</i>()</a> or when an instrumented application
process invokes <a href="../functions/posix_trace_eventid_open.html"><i>posix_trace_eventid_open</i>()</a>. Trace event type
identifiers are used to filter trace event types, to allow interpretation of user data, and to identify the kind of trace point
that generated a trace event.</p>

<p>Each trace event shall be of a particular trace event type, and associated with a trace event type identifier. The execution of
a trace point shall generate a trace event if a trace stream has been created and started for the process that executed the trace
point and if the corresponding trace event type identifier is not ignored by filtering.</p>

<p>A generated trace event shall be recorded in a trace stream, and optionally also in a trace log if a trace log is associated
with the trace stream, except that:</p>

<ul>
<li>
<p>For a trace stream, if no resources are available for the event, the event is lost.</p>
</li>

<li>
<p>For a trace log, if no resources are available for the event, or a flush operation does not succeed, the event is lost.</p>
</li>
</ul>

<p>A trace event recorded in an active trace stream may be retrieved by an application having the appropriate privileges.</p>

<p>A trace event recorded in a trace log may be retrieved by an application having the appropriate privileges after opening the
trace log as a pre-recorded trace stream, with the function <a href=
"../functions/posix_trace_open.html"><i>posix_trace_open</i>()</a>.</p>

<p>When a trace event is reported it is possible to retrieve the following:</p>

<ul>
<li>
<p>A trace event type identifier</p>
</li>

<li>
<p>A timestamp</p>
</li>

<li>
<p>The process ID of the traced process, if the trace event is process-dependent</p>
</li>

<li>
<p>Any optional trace event data including its length</p>
</li>

<li>
<p>If the Threads option is supported, the thread ID, if the trace event is process-dependent</p>
</li>

<li>
<p>The program address at which the trace point was invoked</p>
</li>
</ul>

<p>Trace events may be mapped from trace event types to trace event names. One such mapping shall be associated with each trace
stream. An active trace stream is associated with a traced process, and also with its children if the Trace Inherit option is
supported and also the inheritance policy is set to _POSIX_TRACE_INHERIT. Therefore each traced process has a mapping of the trace
event names to trace event type identifiers that have been defined for that process.</p>

<p>Traces can be recorded into either trace streams or trace logs.</p>

<p>The implementation and format of a trace stream are unspecified. A trace stream need not be and generally is not persistent. A
trace stream may be either active or pre-recorded:</p>

<ul>
<li>
<p>An active trace stream is a trace stream that has been created and has not yet been shut down. It can be of one of the two
following classes:</p>

<ol>
<li>
<p>An active trace stream without a trace log that was created with the <a href=
"../functions/posix_trace_create.html"><i>posix_trace_create</i>()</a> function</p>
</li>