如何使用GDB在给定函数的范围内声明变量？

Question

I know that gdb allows for an already declared variable to be set using the set command.

Is it possible for gdb to dynamically declare a new variable inside the scope of a given function?

Answer 1

You can dynamically allocate some space and use it to store a new variable. Depending on what you mean by "scope of the current function" it may not be what you want.

But here is how it looks like, when you have function func() that takes a pointer to an output parameter:

set $foo = malloc(sizeof(struct funcOutStruct))
call func($foo)
p *$foo
call free($foo)

Answer 2

For C (and probably C++) code, that would be very hard, since doing so in most implementations would involve shifting the stack pointer, which would make the function's exit code fail due to it no longer matching the size of the stack frame. Also all the code in the function that accesses local variables would suddenly risk hitting the wrong location, which is also bad.

So, I don't think so, no.

Answer 3

that's how I used to print variables

(gdb) set $path=((ngx_path_t     **)ngx_cycle->paths.elts)[2]
(gdb) print *$path
    $16 = {
        name = {
            len = 29,
            data = 0x80ed15c "/usr/local/nginx/fastcgi_temp"
            },
        len = 5,
        level = {1, 2, 0},
        manager = 0,
        loader = 0,
        data = 0x0,
        conf_file = 0x0,
        line = 0
    }