Given a Rust Vector (potentially, a long one), I would like output a single element while debugging my program. Although indexing the vector directly is desirable, I would also appreciate an approach, which treats the Vec a structure.

Here are my attempts, which I try to keep as close as possible to this issue, which goes back to 2020: .

Here are my attempts (in Command line and in gdb):

$ cargo new test_gdb_vec --bin
$ cd test_gdb_vec

$ cat src/main.rs 
#->
fn main() {
    let x: Vec<usize> = vec![1, 2, 3];
    println!("{:?}", x);
    }

$ cargo build
#->
Compiling test_gdb_vec v0.1.0 (***)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.16s

$ cargo run
#->
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/test_gdb_vec`
[1, 2, 3]

$ rust-gdb target/debug/test_gdb_vec

(gdb) start
...
Enable debuginfod for this session? (y or [n]) n
...

Temporary breakpoint 1, test_gdb_vec::main () at src/main.rs:2
2       let x: Vec<usize> = vec![1,2,3];

(gdb) next

(gdb) p x
$1 = Vec(size=3) = {1, 2, 3}

(gdb) p x[0]
Cannot subscript non-array type

(gdb) p x.buf.ptr.pointer[0]
There is no member named ptr.

(gdb) p x.buf.inner.ptr.pointer.pointer[0]
$2 = 1

(gdb) p x.buf.inner.ptr.pointer.pointer[1]
$3 = 0

(gdb) p x.buf.inner.ptr.pointer.pointer@3
$4 = [0x5555555afb10, 0x3, 0x7ffff7ffb360 <__x86_cpu_features_p>]

We can see that neither direct indexing works (like it did not in 2020), nor does the instruction copied from the original post (p x.buf.ptr.pointer[0]). Its direct adaptation does not produce the desired result (1, 2, 3) either. Any help relative to getting the value of 1 element of a Vec is welcome!

Thanks for Your attention!

Additional information:

rustc 1.84.1 (e71f9a9a9 2025-01-27)
GNU gdb (Ubuntu 15.0.50.20240403-0ubuntu1) 15.0.50.20240403-git

Given a Rust Vector (potentially, a long one), I would like output a single element while debugging my program. Although indexing the vector directly is desirable, I would also appreciate an approach, which treats the Vec a structure.

Here are my attempts, which I try to keep as close as possible to this issue, which goes back to 2020: https://github/rust-lang/rust/issues/66482 .

Here are my attempts (in Command line and in gdb):

$ cargo new test_gdb_vec --bin
$ cd test_gdb_vec

$ cat src/main.rs 
#->
fn main() {
    let x: Vec<usize> = vec![1, 2, 3];
    println!("{:?}", x);
    }

$ cargo build
#->
Compiling test_gdb_vec v0.1.0 (***)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.16s

$ cargo run
#->
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/test_gdb_vec`
[1, 2, 3]

$ rust-gdb target/debug/test_gdb_vec

(gdb) start
...
Enable debuginfod for this session? (y or [n]) n
...

Temporary breakpoint 1, test_gdb_vec::main () at src/main.rs:2
2       let x: Vec<usize> = vec![1,2,3];

(gdb) next

(gdb) p x
$1 = Vec(size=3) = {1, 2, 3}

(gdb) p x[0]
Cannot subscript non-array type

(gdb) p x.buf.ptr.pointer[0]
There is no member named ptr.

(gdb) p x.buf.inner.ptr.pointer.pointer[0]
$2 = 1

(gdb) p x.buf.inner.ptr.pointer.pointer[1]
$3 = 0

(gdb) p x.buf.inner.ptr.pointer.pointer@3
$4 = [0x5555555afb10, 0x3, 0x7ffff7ffb360 <__x86_cpu_features_p>]

We can see that neither direct indexing works (like it did not in 2020), nor does the instruction copied from the original post (p x.buf.ptr.pointer[0]). Its direct adaptation does not produce the desired result (1, 2, 3) either. Any help relative to getting the value of 1 element of a Vec is welcome!

Thanks for Your attention!

Additional information:

rustc 1.84.1 (e71f9a9a9 2025-01-27)
GNU gdb (Ubuntu 15.0.50.20240403-0ubuntu1) 15.0.50.20240403-git

• debugging
• rust
• vector
• gdb

Share Improve this question Follow asked Mar 24 at 20:03 Alexandre AksenovAlexandre Aksenov 13355 bronze badges

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1 Answer 1

Sorted by: Reset to default 3

Cast before you index

p (x.buf.inner.ptr.pointer.pointer as *const usize)[0]

In order to reduce code bloat from monomorphizing generics, the internals of Vec were refactored to use RawVecInner which is type agnostic, leading to x.buf.inner.ptr.pointer.pointer being of type *mut u8. Hence, when indexing x.buf.inner.ptr.pointer.pointer directly, you get the values of the individual bytes.

Thus, you need to first cast the pointer to the appropriate type *[const|mut] T before indexing, and then not only will gdb step by size_of::<T>() when it indexes (rather than byte by byte), but it'll also display a T, not a u8.

---

For the curious:

• x is of type Vec<T, A> (see source).
• x.buf is of type RawVec<T, A> (see source).
• x.buf.inner is of type RawVecInner<A> (see source).
• x.buf.inner.ptr is of type Unique<u8> (see source).
• x.buf.inner.ptr.pointer is of type NonNull<u8> (see source).
• x.buf.inner.ptr.pointer.pointer is of type *mut u8.