Unrecoverable Errors with panic!
Sometimes bad things happen in your code, and there’s nothing you can do about
it. In these cases, Rust has the panic!
macro. There are two ways to cause a
panic in practice: by taking an action that causes our code to panic (such as
accessing an array past the end) or by explicitly calling the panic!
macro.
In both cases, we cause a panic in our program. By default, these panics will
print a failure message, unwind, clean up the stack, and quit. Via an
environment variable, you can also have Rust display the call stack when a
panic occurs to make it easier to track down the source of the panic.
Unwinding the Stack or Aborting in Response to a Panic
By default, when a panic occurs the program starts unwinding, which means Rust walks back up the stack and cleans up the data from each function it encounters. However, walking back and cleaning up is a lot of work. Rust, therefore, allows you to choose the alternative of immediately aborting, which ends the program without cleaning up.
Memory that the program was using will then need to be cleaned up by the
operating system. If in your project you need to make the resultant binary as
small as possible, you can switch from unwinding to aborting upon a panic by
adding panic = 'abort'
to the appropriate [profile]
sections in your
Cargo.toml file. For example, if you want to abort on panic in release mode,
add this:
[profile.release]
panic = 'abort'
Let’s try calling panic!
in a simple program:
Filename: src/main.rs
fn main() { panic!("crash and burn"); }
When you run the program, you’ll see something like this:
$ cargo run
Compiling panic v0.1.0 (file:///projects/panic)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.25s
Running `target/debug/panic`
thread 'main' panicked at src/main.rs:2:5:
crash and burn
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
The call to panic!
causes the error message contained in the last two lines.
The first line shows our panic message and the place in our source code where
the panic occurred: src/main.rs:2:5 indicates that it’s the second line,
fifth character of our src/main.rs file.
In this case, the line indicated is part of our code, and if we go to that
line, we see the panic!
macro call. In other cases, the panic!
call might
be in code that our code calls, and the filename and line number reported by
the error message will be someone else’s code where the panic!
macro is
called, not the line of our code that eventually led to the panic!
call.
We can use the backtrace of the functions the panic!
call came from to figure
out the part of our code that is causing the problem. To understand how to use
a panic!
backtrace, let’s look at another example and see what it’s like when
a panic!
call comes from a library because of a bug in our code instead of
from our code calling the macro directly. Listing 9-1 has some code that
attempts to access an index in a vector beyond the range of valid indexes.
Filename: src/main.rs
fn main() { let v = vec![1, 2, 3]; v[99]; }
Here, we’re attempting to access the 100th element of our vector (which is at
index 99 because indexing starts at zero), but the vector has only three
elements. In this situation, Rust will panic. Using []
is supposed to return
an element, but if you pass an invalid index, there’s no element that Rust
could return here that would be correct.
In C, attempting to read beyond the end of a data structure is undefined behavior. You might get whatever is at the location in memory that would correspond to that element in the data structure, even though the memory doesn’t belong to that structure. This is called a buffer overread and can lead to security vulnerabilities if an attacker is able to manipulate the index in such a way as to read data they shouldn’t be allowed to that is stored after the data structure.
To protect your program from this sort of vulnerability, if you try to read an element at an index that doesn’t exist, Rust will stop execution and refuse to continue. Let’s try it and see:
$ cargo run
Compiling panic v0.1.0 (file:///projects/panic)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.27s
Running `target/debug/panic`
thread 'main' panicked at src/main.rs:4:6:
index out of bounds: the len is 3 but the index is 99
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
This error points at line 4 of our main.rs where we attempt to access index
99
of the vector in v
.
The note:
line tells us that we can set the RUST_BACKTRACE
environment
variable to get a backtrace of exactly what happened to cause the error. A
backtrace is a list of all the functions that have been called to get to this
point. Backtraces in Rust work as they do in other languages: the key to
reading the backtrace is to start from the top and read until you see files you
wrote. That’s the spot where the problem originated. The lines above that spot
are code that your code has called; the lines below are code that called your
code. These before-and-after lines might include core Rust code, standard
library code, or crates that you’re using. Let’s try getting a backtrace by
setting the RUST_BACKTRACE
environment variable to any value except 0
.
Listing 9-2 shows output similar to what you’ll see.
$ RUST_BACKTRACE=1 cargo run
thread 'main' panicked at src/main.rs:4:6:
index out of bounds: the len is 3 but the index is 99
stack backtrace:
0: rust_begin_unwind
at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:645:5
1: core::panicking::panic_fmt
at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/panicking.rs:72:14
2: core::panicking::panic_bounds_check
at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/panicking.rs:208:5
3: <usize as core::slice::index::SliceIndex<[T]>>::index
at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/slice/index.rs:255:10
4: core::slice::index::<impl core::ops::index::Index<I> for [T]>::index
at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/slice/index.rs:18:9
5: <alloc::vec::Vec<T,A> as core::ops::index::Index<I>>::index
at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/alloc/src/vec/mod.rs:2770:9
6: panic::main
at ./src/main.rs:4:6
7: core::ops::function::FnOnce::call_once
at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
That’s a lot of output! The exact output you see might be different depending
on your operating system and Rust version. In order to get backtraces with this
information, debug symbols must be enabled. Debug symbols are enabled by
default when using cargo build
or cargo run
without the --release
flag,
as we have here.
In the output in Listing 9-2, line 6 of the backtrace points to the line in our project that’s causing the problem: line 4 of src/main.rs. If we don’t want our program to panic, we should start our investigation at the location pointed to by the first line mentioning a file we wrote. In Listing 9-1, where we deliberately wrote code that would panic, the way to fix the panic is to not request an element beyond the range of the vector indexes. When your code panics in the future, you’ll need to figure out what action the code is taking with what values to cause the panic and what the code should do instead.
We’ll come back to panic!
and when we should and should not use panic!
to
handle error conditions in the “To panic!
or Not to
panic!
” section later in this
chapter. Next, we’ll look at how to recover from an error using Result
.