Read/write a variable/symbol from inside GDB's python-interactive mode

Question

I'm experimenting with python-interactive mode in gdb, and I can't figure out how to change a variable from inside it. I know how to do it without python - set variable a = 10.

I'm using this test program:

#include <stdio.h>
int main(int argc, char *argv[]) {
    int a;
    printf("Enter a: ");
    scanf("%d", &a);
    printf("You entered: %d\n", a);
}

I've placed a breakpoint after the scanf(), and when it's hit I enter python interactive mode. Now I want to change the variable a to some other value. I tried using a = 10, but it wasn't changed, and the same value I entered in the scanf() (in this case it's 5) was printed instead.

(gdb) b main.c:6
Breakpoint 1 at 0x8048503: file main.c, line 6.
(gdb) r
Starting program: /home/sashoalm/Desktop/test/a.out 
Enter a: 5

Breakpoint 1, main (argc=1, argv=0xbffff1d4 "\214\363\377\277") at main.c:6
6       printf("You entered: %d\n", a);
(gdb) python-interactive 
>>> a = 10
>>> 
(gdb) c
Continuing.
You entered: 5
[Inferior 1 (process 26133) exited normally]

So what is the correct way to do it?

your main() prototype is incorrect. char *argv --> char **argv or char *argv[]. may be typo ? — Achal
– Achal, Commented Apr 13, 2018 at 1:51

sashoalm · Accepted Answer · 2016-06-06 10:03:34Z

3

After some searching through the Python API documentation I found the answer. I needed to use gdb.execute('set var a = 10'), which allows python scripts to execute gdb commands, and the commands are evaluated as if the user has written them.

I used this code to read a, add 5 to it and then set it:

symbol = gdb.lookup_symbol('a')[0]
frame = gdb.selected_frame()
value = symbol.value(frame)
gdb.execute('set var a = %d' % (int(value)+5))

answered Jun 6, 2016 at 10:03

sashoalm

80.5k138 gold badges485 silver badges830 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Read/write a variable/symbol from inside GDB's python-interactive mode

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related