Let's try it. We prepare the following Python script.
$ cat hello.py
#!/usr/bin/env python
import sys
sys.stdout.write('Hello, my first script.\n')
Launch GDB and run the script(gdb) source hello.py Hello, my first script.
Use of GDB functions from a Python script
The following is the example to use GDB functions from a Python script. gdb.execute() can run operations used in the GDB command line.The following example uses target.cc as a debug target.
[target.cc]
#include <cstdio>
#include <cstdlib>
#include <time.h>
int getval(int div)
{
return time(NULL) / div;
}
int main(void)
{
printf("val: %d\n", getval(5));
return EXIT_SUCCESS;
}
show-bt.py will perform the following 4 operations.
- Set a break point at the head of getval() function
- Run the program
- Show the bracktrace at the break point
- Show rax register at the break point
[show-bt.py]
gdb.execute('b getval')
gdb.execute('run')
gdb.execute('bt')
gdb.execute('p/x $rax')
The following is an example to run a script in the command line.
The command line argument '-q' disables to show startup messages of GDB such as copyright. '-x' specifies the script to be executed.
$ gdb -q -x show-bt.py target Reading symbols from /home/foo/target...done. Breakpoint 1 at 0x400627: file target.cc, line 7. Breakpoint 1, getval (div=5) at target.cc:7 7 return time(NULL) / div; #0 getval (div=5) at target.cc:7 #1 0x0000000000400656 in main () at target.cc:12 #2 0x00007ffff72d2ead in __libc_start_main (main=, argc= , ubp_av= , init= , fini= , rtld_fini= , stack_end=0x7fffffffe128) at libc-start.c:228 #3 0x0000000000400539 in _start () $1 = 0x7ffff763aee8
Get output of a command as a Python string
The above example only executes normal GDB commands. Let's get the output as a Python string. It can be achived by passing True to to_string parameter of get.execute() function. Python is also good at string processing, so you can easily extract and display specific strings in a formatted format.The following exmaple shows the frame number and the last word from the stacktrace.
$ cat show-bt-and-format.py
gdb.execute('b getval')
gdb.execute('run')
bt = gdb.execute('bt', to_string=True)
for line in bt.split('\n'):
words = line.split(' ')
print '%s %s' % (words[0], words[-1])
$ gdb -q -x show-bt-and-format.py target Reading symbols from /home/foo/target...done. Breakpoint 1 at 0x400627: file target.cc, line 7. Breakpoint 1, getval (div=5) at target.cc:7 7 return time(NULL) / div; #0 target.cc:7 #1 target.cc:12 #2 libc-start.c:228 #3 ()
