So here’s a cool feature of GNU’s implementation of libc: you can get a stack backtrace (as an array of strings) dynamically in your code. This can be really useful when trying to determine the code path taken when an error occurs. Most times, it’s faster to just run the code in a debugger and use it to display a backtrace, but there are instances when doing it programmatically is your best option. For example, you could get a backtrace in your application’s exception handler and use it to augment error log messages.

First, you need to include execinfo.h to your code:

#include <execinfo.h>

Next, call the backtrace() function to get an array of void pointers that represents the current stack (the pointers are the return addresses for each stack frame).

void* tracePtrs[100];
int count = backtrace( tracePtrs, 100 );

The backtrace() function returns the number of entries in the array (read the man pages for more info about the array size).

Finally, you need to resolve the function names associated with the pointers. You have 2 options: backtrace_symbols() and backtrace_symbols_fd(). Both of these methods resolve the pointers to strings, but the difference is that backtrace_symbols() allocates the strings on the heap while backtrace_symbols_fd() writes the strings to a file descriptor that you can read. Just keep in mind that backtrace_symbols() won’t work if the heap has been trashed.

Here’s an example using backtrace_symbols():

char** funcNames = backtrace_symbols( tracePtrs, count );

// Print the stack trace
for( int ii = 0; ii < count; ii++ )
   printf( “%s\n”, funcNames[ii] );

// Free the string pointers
free( funcNames );

NOTE: Make sure you call free() on the array of strings returned from backtrace_symbols().

For more information, here’s a good article from the Linux Journal.

The Windows development environment provided by VisualStudio has some neat tools for detecting memory leaks in code. You simply #define _CRTDBG_MAP_ALLOC before including your headers, and #include <crtdbg.h> as the last header:

#define _CRTDBG_MAP_ALLOC

// Include header files here

#include <crtdbg.h>

Then, you call _CrtDumpMemoryLeaks() before your application exits. If your program exits at many points, you can alternatively call _CrtSetDbgFlag( _CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF ) at the beginning of you application, which will cause the leaks to also be printed when it exits. The results are printed to the Debug Window and look like the following:

Detected memory leaks!
Dumping objects ->
C:\PROGRAM FILES\VISUAL STUDIO\MyProjects\leaktest\leaktest.cpp(20) : {18}
normal block at 0×00780E80, 64 bytes long.
Data: < > CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD
Object dump complete.

Cool, Huh?! However, some libraries don’t play nice with this, as I explain below.

Read the rest…

Debugging C++ templates is difficult. Debugging C++ templates with GDB can be an act of torture for even seasoned GDB users. I like GDB, but there are some tricks you should know when using it to debug templates. In this post, I deal with setting breakpoints.

Breakpoint Basics:

Setting a breakpoint in GDB is supposed to be simple. Here we set a breakpoint at line 50 in file main.cpp:

(gdb) b main.cpp:50
Breakpoint 1 at 0×804937a: file main.cpp, line 50.

We can also use the function name and GDB will attempt to find the correct location for us:

(gdb) b DoSomething
Breakpoint 2 at 0×8049334: file main.cpp, line 150

Simple, right? Just wait…

Breakpoint Gotchas:

GDB’s breakpoint logic is pretty handy for simple projects, but it can break down fast when things get more complicated.

For example, let’s say your application is plugin-driven, with each plugin being a separate library. Now assume each plugin has a Plugin.cpp file under it’s own Source directory. Try to set a breakpoint in the Initialize() method of the Plugin class:

(gdb) b Initialize
Breakpoint 3 at 0×8049717: file main.cpp, line 230

Oops! There is an Initialize() method in main.cpp and GDB thought that’s where we wanted to put it: wrong!

Read the rest…