Does GDB have a built in scripting mechanism, should I code up an expect script, or is there an even better solution out there?
I'll be sending the same sequence of commands every time and I'll be saving the output of each command to a file (most likely using GDB's built-in logging mechanism, unless someone has a better idea).
I was just going through something similar, and came up with a basic example - and knowing I'll forget about it soon, I thought I'd better post it :)
So I'll post it here, as it looks related to the question.
Basically, in this example I wanted to get some variable values in particular places of the code; and have them output until the program crashes. So here is first a little program which is guaranteed to crash in a few steps, test.c
:
#include <stdio.h>
#include <stdlib.h>
int icount = 1; // default value
main(int argc, char *argv[])
{
int i;
if (argc == 2) {
icount = atoi(argv[1]);
}
i = icount;
while (i > -1) {
int b = 5 / i;
printf(" 5 / %d = %d \n", i, b );
i = i - 1;
}
printf("Finished\n");
return 0;
}
The only reason the program accepts command-line arguments is to be able to choose the number of steps before crashing - and to show that gdb
ignores --args
in batch mode. This I compile with:
gcc -g test.c -o test.exe
Then, I prepare the following script - the main trick here is to assign a command
to each breakpoint
, which will eventually continue
(see also Automate gdb: show backtrace at every call to function puts). This script I call test.gdb
:
# http://sourceware.org/gdb/wiki/FAQ: to disable the
# "---Type <return> to continue, or q <return> to quit---"
# in batch mode:
set width 0
set height 0
set verbose off
# at entry point - cmd1
b main
commands 1
print argc
continue
end
# printf line - cmd2
b test.c:17
commands 2
p i
p b
continue
end
# int b = line - cmd3
b test.c:16
commands 3
p i
p b
continue
end
# show arguments for program
show args
printf "Note, however: in batch mode, arguments will be ignored!\n"
# note: even if arguments are shown;
# must specify cmdline arg for "run"
# when running in batch mode! (then they are ignored)
# below, we specify command line argument "2":
run 2 # run
#start # alternative to run: runs to main, and stops
#continue
Note that, if you intend to use it in batch mode, you have to "start up" the script at the end, with run
or start
or something similar.
With this script in place, I can call gdb
in batch mode - which will generate the following output in the terminal:
$ gdb --batch --command=test.gdb --args ./test.exe 5
Breakpoint 1 at 0x804844d: file test.c, line 10.
Breakpoint 2 at 0x8048485: file test.c, line 17.
Breakpoint 3 at 0x8048473: file test.c, line 16.
Argument list to give program being debugged when it is started is "5".
Note, however: in batch mode, arguments will be ignored!
Breakpoint 1, main (argc=2, argv=0xbffff424) at test.c:10
10 if (argc == 2) {
$1 = 2
Breakpoint 3, main (argc=2, argv=0xbffff424) at test.c:16
16 int b = 5 / i;
$2 = 2
$3 = 134513899
Breakpoint 2, main (argc=2, argv=0xbffff424) at test.c:17
17 printf(" 5 / %d = %d \n", i, b );
$4 = 2
$5 = 2
5 / 2 = 2
Breakpoint 3, main (argc=2, argv=0xbffff424) at test.c:16
16 int b = 5 / i;
$6 = 1
$7 = 2
Breakpoint 2, main (argc=2, argv=0xbffff424) at test.c:17
17 printf(" 5 / %d = %d \n", i, b );
$8 = 1
$9 = 5
5 / 1 = 5
Breakpoint 3, main (argc=2, argv=0xbffff424) at test.c:16
16 int b = 5 / i;
$10 = 0
$11 = 5
Program received signal SIGFPE, Arithmetic exception.
0x0804847d in main (argc=2, argv=0xbffff424) at test.c:16
16 int b = 5 / i;
Note that while we specify command line argument 5, the loop still spins only two times (as is the specification of run
in the gdb
script); if run
didn't have any arguments, it spins only once (the default value of the program) confirming that --args ./test.exe 5
is ignored.
However, since now this is output in a single call, and without any user interaction, the command line output can easily be captured in a text file using bash
redirection, say:
gdb --batch --command=test.gdb --args ./test.exe 5 > out.txt
There is also an example of using python for automating gdb in c - GDB auto stepping - automatic printout of lines, while free running?
Hope this helps,
Cheers!