Re: Debugging variable arguments functions (stdarg)

[Michael Snyder <msnyder@vmware.com>]

G wrote:
> Hello,
> 
> How can I look at the values of "..." in a function which can
> take a variable number of arguments (i.e. uses va_start(),
> va_end() etc.) in gdb?

I don't think you can, other than at low level (regs and stack).

> 
> I have a core dump of a program which I try to analyze in gdb:
> 
> debian:~ $ gdb /usr/local/bin/teamlogs core-teamlogs
> GNU gdb 6.8-debian
> [snip]
> Loaded symbols for /lib/libnss_files.so.2
> Core was generated by `/usr/local/bin/teamlogs'.
> Program terminated with signal 11, Segmentation fault.
> [New process 31754]
> #0  0x00002ac4de25f030 in strlen () from /lib/libc.so.6
> (gdb) bt
> #0  0x00002ac4de25f030 in strlen () from /lib/libc.so.6
> #1  0x00002ac4de22bcb1 in vfprintf () from /lib/libc.so.6
> #2  0x00002ac4de2affbe in __vsyslog_chk () from /lib/libc.so.6
> #3  0x00000000004062cb in WriteLog (facility=128, level=4,
>     fmt=0x406bf0 "MysqlWrapper(): Query failed: \"%s\": %s")
>     at src/teamlogs.cpp:119
> #4  0x00000000004041e2 in MysqlWrapper (
>     query=0x7c8e80 "INSERT INTO results.matches (time, team, won)
> VALUES (1257353711, 'Juniors1', 0)")
>     at src/teamlogs.cpp:563
> [snip]
> (gdb) frame 3
> #3  0x00000000004062cb in WriteLog (facility=128, level=4,
>     fmt=0x406bf0 "MysqlWrapper(): Query failed: \"%s\": %s")
>     at src/teamlogs.cpp:119
> 119     vsyslog(facility, level, fmt, ap);
> (gdb) print facility
> $2 = 128
> (gdb) print level
> $3 = 4
> (gdb) print fmt
> $4 = 0x406bf0 "MysqlWrapper(): Query failed: \"%s\": %s"
> (gdb) print ap
> $5 = {{gp_offset = 40, fp_offset = 48, overflow_arg_area = 0x7fffcd89ce80,
>     reg_save_area = 0x7fffcd89cdb0}}
> (gdb) frame 2
> #2  0x00002ac4de2affbe in __vsyslog_chk () from /lib/libc.so.6
> (gdb) print priority
> No symbol "priority" in current context.
> (gdb) print flag
> No symbol "flag" in current context.
> (gdb) print format
> No symbol "format" in current context.
> (gdb) print ap
> No symbol "ap" in current context.
> 
> I need to look at the values in "..." to see what was being
> passed on in my call to vsyslog(). How do I do that? I haven't
> discovered how to despite googling and reading the gdb manual.
> 
> The source of the function WriteLog looks like this:
> 
> void
> WriteLog(int facility, int level, const char *fmt, ...)
> {
>         va_list ap;
>         va_start(ap, fmt);
>         vsyslog(facility, level, fmt, ap);
>         va_end(ap);
>         return;
> }
> 
> and it is called like this:
>         WriteLog(LOG_LOCAL0, LOG_WARNING, "MysqlWrapper(): Query failed: "
>                 \"%s\": %s", query, mysql_error(sql_handle));
> 
> openlog() has been called earlier, like this:
>         openlog(argv[0], LOG_CONS | LOG_ODELAY | LOG_PID, LOG_LOCAL0);
> 
> So I really need to look at all arguments to WriteLog() to see if
> mysql_error() have returned something weird to cause strlen() to
> misbehave. My fmt argument is good, as can be seen in the gdb
> output.
> 
> Unfortunately I haven't found a way to reproduce the crash (it
> happens very rarely), which is why I really need to try and track
> the bug down using this core dump. To further complicate things,
> it's taken from a production machine with all the difficulties
> that can bring (no debug libraries etc.).
> 
> (Apologies if this is the wrong place to ask, and if the formatting
> gets screwed up.)