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    <p>So my solution is to load a shared object containing my debugging routines at run time, using dlopen. Turns out it is even simpler than I thought when you get all the compile flags right.</p> <p>On OS X this means you compile your application and debugging object like this:</p> <pre><code>all : application.x debug_helper.so application.x : application.cpp g++ -g application.cpp -o application.x -fPIC debug_helper.so : debug_helper.o g++ -dynamiclib -o debug_helper.so debug_helper.o debug_helper.o : debug_helper.cpp g++ -Wall -g -fPIC -c debug_helper.cpp </code></pre> <p>The -fPIC on the application is critical, as is the -dynamiclib (rather than trying the linux -shared flag )</p> <p>An example debug_helper.cpp might look like this</p> <pre><code>#include &lt;map&gt; #include &lt;stdio.h&gt; extern "C" void printMap( const std::map&lt;int,int&gt; &amp;m ) { printf("Map of size %d\n", int(m.size()) ); for( std::map&lt;int,int&gt;::const_iterator it = m.begin(); it!=m.end(); ++it ) { printf("%d : %d \n", it-&gt;first, it-&gt;second ); } fflush(stdout); } </code></pre> <p>Don't know why I chose to use stdio rather than iostream stuff... I guess you can use either. (just don't forget to flush the streams...)</p> <p>Now my application file looks like this:</p> <pre><code>#include &lt;map&gt; int main() { std::map&lt;int,int&gt; m; m[1]=2; m[2]=5; m[3]=10; m[4]=17; } </code></pre> <p>And heres an example debugging session (some output removed)</p> <p>Start the application and break at an interesting point</p> <pre><code>(gdb) break main (gdb) run Reading symbols for shared libraries +++. done Breakpoint 1, main () at test.cpp:5 5 std::map&lt;int,int&gt; m; </code></pre> <p>Load in the debug helper library</p> <pre><code>(gdb) print (void*) dlopen("debug_helper.so",2) Reading symbols for shared libraries . done $1 = (void *) 0x100270 (gdb) n 6 m[1]=2; </code></pre> <p>GDB is smart and catches all the new symbols for us so we dont need to use dlsym etc. We can just call the functions directly.</p> <pre><code>(gdb) call printMap(m) Map of size 0 (gdb) n (gdb) n (gdb) n 9 m[4]=17; (gdb) call printMap(m) Map of size 3 1 : 2 2 : 5 3 : 10 </code></pre> <p>Lets add some more info to printMap. First unload the library.</p> <pre><code>(gdb) print (int) dlclose($1) $2 = 0 </code></pre> <p>Edit the source to add in the sum of the entries. Recompile and then load the new library back into gdb (without restarting the executable or gdb )</p> <pre><code>(gdb) print (void*) dlopen("debug_helper.so",2) Reading symbols for shared libraries . done </code></pre> <p>Use the modified function</p> <pre><code>$3 = (void *) 0x100270 (gdb) call printMap(m) Map of size 3 1 : 2 2 : 5 3 : 10 SUM = 17 </code></pre> <p>I think this does everything that I need.</p>
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