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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>dlsym</title></head><body bgcolor=white><center><font size=2>The Single UNIX ® Specification, Version 2<br>Copyright © 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_003_151"> </a>NAME</h4><blockquote>dlsym - obtain the address of a symbol from a dlopen() object</blockquote><h4><a name = "tag_000_003_152"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <<a href="dlfcn.h.html">dlfcn.h</a>>void *dlsym(void *<i>handle</i>, const char *<i>name</i>);</code></pre></blockquote><h4><a name = "tag_000_003_153"> </a>DESCRIPTION</h4><blockquote><i>dlsym()</i>allows a process to obtain the address of a symbol definedwithin an object made accessible through a<i><a href="dlopen.html">dlopen()</a></i>call.<i>handle</i>is the value returned from a call to<i><a href="dlopen.html">dlopen()</a></i>(and which has not since been released via a call to<i><a href="dlclose.html">dlclose()</a></i>),<i>name</i>is the symbol's name as a character string.<p><i>dlsym()</i>will search for the named symbol in all objects loaded automatically as a result of loading theobject referenced by <i>handle</i> (see<i><a href="dlopen.html">dlopen()</a></i>).<i>Load</i> ordering is used in <i>dlsym()</i>operations upon the global symbol object.The symbol resolution algorithm used will be<i>dependency</i> order as described in <i><a href="dlopen.html">dlopen()</a></i>.</blockquote><h4><a name = "tag_000_003_154"> </a>RETURN VALUE</h4><blockquote>If <i>handle</i>does not refer to a valid object opened by<i><a href="dlopen.html">dlopen()</a></i>,or if the named symbol cannot be found within any of theobjects associated with<i>handle</i>,<i>dlsym()</i>will return NULL.More detailed diagnostic information will beavailable through<i><a href="dlerror.html">dlerror()</a></i>.</blockquote><h4><a name = "tag_000_003_155"> </a>ERRORS</h4><blockquote>No errors are defined.</blockquote><h4><a name = "tag_000_003_156"> </a>EXAMPLES</h4><blockquote>The following example shows how one can use <i><a href="dlopen.html">dlopen()</a></i>and<i>dlsym()</i>to access either function or data objects. For simplicity,error checking has been omitted.<pre><code>void *handle;int *iptr, (*fptr)(int);/* open the needed object */handle = dlopen("/usr/home/me/libfoo.so.1", RTLD_LAZY);/* find the address of function and data objects */fptr = (int (*)(int))dlsym(handle, "my_function");iptr = (int *)dlsym(handle, "my_object");/* invoke function, passing value of integer as a parameter */(*fptr)(*iptr);</code></pre></blockquote><h4><a name = "tag_000_003_157"> </a>APPLICATION USAGE</h4><blockquote>Special purpose values for<i>handle</i>are reserved for future use. These values and their meanings are:<dl compact><dt>RTLD_NEXT<dd>Specifies the next object after this one that defines <i>name</i>.<i>This one</i> refers to the object containing the invocation of<i>dlsym()</i>.The <i>next</i> object is the one found upon the application of a <i>load</i>order symbol resolution algorithm (see<i><a href="dlopen.html">dlopen()</a></i>).The next object is either one of global scope (because it wasintroduced as part of the original process image or because itwas added with a<i><a href="dlopen.html">dlopen()</a></i>operation including the RTLD_GLOBALflag), or is an object that was included in the same<i><a href="dlopen.html">dlopen()</a></i>operation that loaded this one.The RTLD_NEXT flag is useful tonavigate an intentionally created hierarchy of multiply defined symbolscreated through <i>interposition</i>. For example, if a program wished tocreate an implementation of<i><a href="malloc.html">malloc()</a></i>that embedded some statisticsgathering about memory allocations, such an implementation could use thereal<i><a href="malloc.html">malloc()</a></i>definition to perform the memory allocation - anditself only embed the necessary logic to implement the statistics gatheringfunction.</dl></blockquote><h4><a name = "tag_000_003_158"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_003_159"> </a>SEE ALSO</h4><blockquote><i><a href="dlclose.html">dlclose()</a></i>,<i><a href="dlerror.html">dlerror()</a></i>,<i><a href="dlopen.html">dlopen()</a></i>.</blockquote><hr size=2 noshade><center><font size=2>UNIX ® is a registered Trademark of The Open Group.<br>Copyright © 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>⌨️ 快捷键说明
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