How to compare a stored string variable in GDB?

Question

I have a variable called x in GDB which I want to compare against a string.

gdb $ print $x
$1 = 0x1001009b0 "hello"

but a comparsion with

if $x == "hello"

doesn't work.

Answer 1

As @tlwhitec points out: You could also use the built-in $_streq(str1, str2) function:

(gdb) p $_streq($x, "hello")

This function does not require GDB to be configured with Python support, which means that they are always available.

More convenient functions can be found in https://sourceware.org/gdb/onlinedocs/gdb/Convenience-Funs.html. Or use

(gdb) help function

to print a list of all convenience functions.

---

For older gdb's that lack the built-in $_streq function, You can define your own comparison

(gdb) p strcmp($x, "hello") == 0
$1 = 1

If you are unfortunate enough to not have the program running (executing a core file or something), you can do something to the effect of the following if your gdb is new enough to have python:

(gdb) py print cmp(gdb.execute("output $x", to_string=True).strip('"'), "hello") == 0
True

or:

(gdb) define strcmp
>py print cmp(gdb.execute("output $arg0", to_string=True).strip('"'), $arg1)
>end
(gdb) strcmp $x "hello"
0