169 lines
5.6 KiB
HTML
169 lines
5.6 KiB
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>Thread creation using C++ methods as target does not work</title>
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<!--File Edited November 2001 -->
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</head>
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<body>
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<script language="Javascript" src="../rzahg/synch.js" type="text/javascript">
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</script>
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<a name="Top_Of_Page"></a>
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<h2>Thread creation using C++ methods as target does not work</h2>
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<p>Often, as a C++ programmer, you may want to abstract the concept of a thread
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into a C++ class. To do this, you must realize that the Pthread APIs are C
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language APIs. The Pthread APIs use functions that have C linkage and calling
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conventions. For your application to successfully use the pthread functions,
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you must provide helper functions of the appropriate type and linkage for the
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Pthread APIs that take function pointers as parameters.</p>
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<p>When sharing objects between threads, always be aware of which thread is
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manipulating the object, which thread is responsible for freeing the object,
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and what thread safety issues are created by sharing objects between
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threads.</p>
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<p>The following example shows how to successfully create a program that
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abstracts a thread into a C++ class. It can be easily extended to provide a
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mechanism by which the thread creation and manipulation itself is also
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encapsulated into the class.</p>
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<br>
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<h3>Example</h3>
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<p>See <a href="../apiref/aboutapis.htm#codedisclaimer">Code disclaimer information</a>
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for information pertaining to code examples.</p>
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<pre>
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/* This C++ example must be compiled with VisualAge C++ for OS/400 */
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#define _MULTI_THREADED
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#include <stdio.h>
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#include <stdlib.h>
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#include <stddef.h>
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#include <pthread.h>
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class ThreadClass {
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public:
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ThreadClass(char *s) {
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data1 = 42; data2 = strlen(s);
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strncpy(str, s, sizeof(str)-1);
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str[49]=0;
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}
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void *run(void);
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private:
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int data1;
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int data2;
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char str[50];
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};
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extern "C" void *ThreadStartup(void *);
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int main(int argc, char **argv)
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{
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ThreadClass *t=NULL;
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pthread_t thread;
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int rc;
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// Use printf instead of cout.
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// At the time this test was written, the C++ standard class library
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// was not thread safe.
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printf("Entered test %s\n", argv[0]);
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printf("Create a ThreadClass object\n");
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t = new ThreadClass("Testing C++ object/thread creation\n");
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printf("Start a real thread to process the ThreadClass object\n");
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// #define COMPILE_ERROR
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#ifdef COMPILE_ERROR
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// This is an ERROR. You cannot create a thread by using a pointer
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// to a member function. Thread creation requires a C linkage function.
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// If you remove the comments from the line `#define COMPILE_ERROR'
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// the compiler will give a message similar to this:
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// "ATESTCPP0.C", line 46.53: 1540-055: (S) "void*(ThreadClass::*)()"
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// cannot be converted to "extern "C" void*(*)(void*)".
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rc = pthread_create(&thread, NULL, ThreadClass::run, NULL);
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#else
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// Instead, this is the correct way to start a thread on a C++ object
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rc = pthread_create(&thread, NULL, ThreadStartup, t);
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#endif
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if (rc) {
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printf("Failed to create a thread\n");
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exit(EXIT_FAILURE);
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}
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printf("Waiting for thread to complete\n");
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rc = pthread_join(thread, NULL);
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if (rc) {
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printf("Failed to join to the thread, rc=%d\n");
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exit(EXIT_FAILURE);
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}
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printf("Testcase complete\n");
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exit(EXIT_SUCCESS);
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}
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// This function is a helper function. It has normal C linkage, and is
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// as the base for newly created ThreadClass objects. It runs the
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// run method on the ThreadClass object passed to it (as a void *).
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// After the ThreadClass method completes normally (i.e returns),
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// we delete the object.
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void *ThreadStartup(void *_tgtObject) {
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ThreadClass *tgtObject = (ThreadClass *)_tgtObject;
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printf("Running thread object in a new thread\n");
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void *threadResult = tgtObject->run();
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printf("Deleting object\n");
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delete tgtObject;
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return threadResult;
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}
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void *ThreadClass::run(void)
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{
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printf("Entered the thread for object %.8x %.8x %.8x %.8x\n", this);
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printf("Object identity: %d, %d: %s\n", data1, data2, str);
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return NULL;
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}
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</pre>
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<p><strong>Output</strong></p>
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<pre>
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Entered test QP0WTEST/ACPPOBJ
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Create a ThreadClass object
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Start a real thread to process the ThreadClass object
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Waiting for thread to complete
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Running thread object in a new thread
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Entered the thread for object 80000000 00000000 d017dad2 57001f60
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Object identity: 42, 35: Testing C++ object/thread creation
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Deleting object
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Testcase complete
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</pre>
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<hr>
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<center>
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<table cellpadding="2" cellspacing="2">
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<tr align="center">
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<td valign="middle" align="center">
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<a href="#Top_Of_Page">Top</a> |
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<a href="rzah4mst.htm">Pthread APIs</a> |
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<a href="aplist.htm">APIs by category</a></td>
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</tr>
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</table>
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</center>
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</body>
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</html>
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