The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Saturday, June 06, 2015

I'm lucky in that I run the server, so I can do this type of debugging

As I was unable to reproduce the issue on the development system, but it was all too easy to reproduce on server, I had no choice but to debug on the server. It's not an ideal situation but in this case, as I run the server in question and the appropriate software tools are installed, I figured I might as well debug on the server.

The approach I took is one I've been using for a long time—insert this bit of code:

volatile int g_debug = 1;

while(g_debug)
  ;

into the proper code path (in this case, the program was crashing in cmd_cgi_post_show(), which is the function called when I want to preview a post from the web interface). Then I can submit the data, the web server will spawn mod_blog, and the infinite loop will give me a chance to attach the debugger to it (setting up a fake web environment to debug CGI programs is a pain). And from there, it was pretty straightforward to isolate the issue by setting breakpoints and examining memory.

The issue ended up being multiple calls to free(), and I suppose the difference in behavior between the development system and server can be attributed to the different versions of the standard C library. Yes, in an ideal world, there would be no difference between the two, but this isn't an ideal world and I have to make do with what I have.

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