How to make an expect script to input commands into GDB? - linux

Question

I want to write an Expect script that will simply input commands into GDB regardless of its output. Then I want to take certain parts of the output of GDB and extract information from it using shell commands such as grep and sed. Then I want to use this information to input more commands into GDB.
For example, I would initiate a back trace by sending the command "bt" to GDB from the expect script. Then I would grep for a word such as "pardrivr" and get the line number associated with it. Then I would input "f lineNumberOfPardrivr" into GDB. This process would be repeated until the correct information is eventually extracted.
Is this possible. If so what is the best way to go about doing this?
Thanks

Answer 1

My $0.02: I'd use a coprocess or named pipe under ksh/bash/zsh. Much easier. See: https://unix.stackexchange.com/questions/86270/how-do-you-use-the-command-coproc-in-bash
Also, consider tee'ing the output of gdb into a named pipe that you cat in another xterm. Makes it much easier to debug what your script is reading if you can see a copy of the gdb output.
Edited to add:
Still can't post comments. *sigh*
gdb in batch mode, or via a simple shell redirect, won't let us define commands on the fly based upon current gdb output. A coprocess or named pipe approach is much the same technique, but it lets us create new input dynamically at will based upon gdb's output processed through grep/sed/awk/perl/whatever. Python or Perl might be even easier to use with their facilities for regular expressions and subprocesses. E.g. (perl) open("|gdb ...")
http://perldoc.perl.org/functions/open.html
Edited again to add:
A named pipe is a FIFO (first in first out) that exists much like a file in the filesystem. It's not really a file of course. It's just something that can be used like a file. Anything that you write to it can be read back out, within the limits of the OS buffering. (Otherwise writes will block.)
FIFO's are available under Unix, Linux, & Macs, but not windows. You create them with mkfifo. Any process can write to it. Any process can read from it. From that link I posted up above:
mkfifo in out
cmd <in >out &
exec 3> in 4< out
echo data >&3
read var <&4
From my own playing around to demo this...
#in BASH
mkfifo IN OUT
#or mkfifo IN OUT ERR
gdb < IN > OUT 2>&1 &
#or gdb < IN > OUT 2> ERR &
#or gdb < IN > OUT &
exec 3> IN
exec 4< OUT
echo "help bt" >&3
while read -t 0.001 var <&4 ; do echo $var; done
echo "help stack" >&3
while read -t 0.001 var <&4 ; do echo $var; done
#don't forget to kill the gdb process when you are done...
echo "quit" >&3
while read -t 0.001 var <&4 ; do echo $var; done

Answer 2

I want to write an Expect script that will simply input commands into GDB regardless of its output.
For non interactive control you don't need expect as gdb has a -batch mode and is able to read (-x) commands from a file.
Moreover, as gdbreads input from stdin and produces output to stdout standard redirection might do the trick.
For example, I wrote a simple C program:
sh$ cat hello.c
#include <stdio.h>
int main() {
char msg[] = "Hello world";
printf("%s\n", msg);
return 0;
}
sh$ gcc -ggdb hello.c -o hello
I'm able to "script" the gdb session like that:
sh$ gdb -q hello | awk '$2=="$1" { print "Var was #" $NF }'
br 6
r
print &msg
c
quit
EOF
warning: no loadable sections found in added symbol-file system-supplied DSO at 0x7ffff7ffa000
Var was #0x7fffffffe230