[prog] Re: [Techtalk] GDB and C question

[Sujita Purushothaman]

bsweeney at physics.ucsb.edu wrote:

> Hey all you C gurus-
>

I'm no C guru, but I like tracing segfaults.  :-)

> I, being a miserable programmer, am stuck debugging a problem with a rather old
> piece of in-house software written in C.  It's segfaulting to be precise, with
> (shocker) no good error messages being generated.  I've run the program through
> gdb,

Compile with Electric Fence ftp://ftp.perens.com/pub/ElectricFence/ to make sure
that gdb stops at the exact line the segfault occurs; Otherwise it may or may not.

> and from what I can see from the stack trace it looks like the segfault's
> happening when it tries to open a file.  Makes sense; the error gdb originally
> reported was a no such file or directory error in some low-level library.  I
> have narrowed the function call down generating the error, and it's a fprintf
> statement with a file and a bunch of strings as characters. My question is, if
> I know the name of the file pointer (in this case parm_fp), is it possible to
> get the name of the file associated with that pointer?

> I know that gdb will
> let me print out the value of variables; I've already had it print out integer
> variables with great success.  But of course when I say:
>
>   gdb> p parm_fp
>
> I get a memory address back.  Using p/a gave the same results.  I'm not
> terribly familiar with gdb, so I'm not sure what else to try.
>

AFAIK, parm_fp is a pointer to the file, not pointer to path/name of file. So
it makes sense that it doesn't print out the file name.

> Any way to get the filename from the pointer or address?

Can't help you with this, sorry. But don't worry, I'm sure someone else here
will be able to :-)

> I realize the best
> way would probably be to go through the source code and find the original fopen
> command for the file in question, but the code is massive and hard to follow.
> Any advice would be appreciated.  Unfortunately, it's part of our financial
> software and (shock again) the finance stuff needs to get generated by the end
> of the week.  *sigh*, why doesn't anyone ever find problems when they DON'T
> matter?

Because then they wouldn't be a problem ;-) !

Best of Luck!
Sujita