32 KiB
Error Handing
This is a factor of life in software, Rust has a number of features for handling errors. One feature is that Rust requires you to acknowledge the possibility of an error and take some action before our code will compile.
This requirement ensures that errors are handled before the possibility could arise
This can be split into two major categories
- Recoverable - File a file not found, just need to report the problem to the user and retry the operation
- Unrecoverable - A symptom of bugs, like trying to access a location beyond the end of an array. Need to immediately stop the program
Many languages don't distinguish between the two kinds of errors and handle them the same way using mechanisms such as exceptions
Rust does not have exceptions
Instead it has the type Result< T, E> for recoverable errors
It has the panc! macro to stop execution when an unrecoverable error occurs
Unrecoverable Errors
When bad things happen in your code and nothing you can do nothing about it then Rust has the panc! macro
There are two ways to cause a panic:
- by taking an action that causes the code to panic (like accessing an array past the end)
- explicitly calling
panic!macro
By default these print a failure message, unwind, clean up the stack and then quit.
Using an environment variable you can also have Rust display the call stack when a panic occurs. This can make it easier to track down the source of the panic
When a call to panic! occurs the error message will be contained in the last two lines. The first line will contain our message and the second is when the source of this panic occurred
example
fn main() {
panic!("crash and burn");
}
This will output
thread 'main' panicked at src/main.rs:2:5:
crash and burn
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
This indicates that the panic occured in the file main.rs at the 2nd line on the 5th character
In this example it indicates that it is part of our source cdoe, looking there will show the panic! macro
In other cases the panic! call might be reported as someone else code where the panic! macro was called
You can also use the traceback functions of the panic call to figure ot the part of our code that caused the problem
To understand this an example will be used
fn main() {
let v = vec![1, 2, 3];
v[99];
}
Here we are tryin to access the 100th element, this is out of range and therefore Rust will initiate a error
In C, attempting to read beyond hte end of a data structure is undefined behavior, and you might get whatever is at the memory location, this would be something "random"
This is considered a buffer overread and can lead to security vulnerabilities, this would allow an attacker to be able to manipulate the index in such a way that they shouldnt be allowed to sore in that data structure.
Rust protects yo from this kind of vulnerability by casuing a panic if you try to read something out of range.
The note: line tells us that we can set the RUST_BACKTRACE environment variable to get a backtrace to show exactly what happened to casue the error.
The key to reading a backtrace is to start at the top and read until you see the files you wrote, that is where the problem originates.
The lines above that spot are code that our code has called, and the lines below are the code that called your code. These before-and-after lines might include core Rust code, std lib code or crates that you are using
You can set the backtrace by setting the RUST_BACKTRACE environment variable to any value except 0
Example
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.
In order to a backtarace with this info, debug symbols must be enabled
Debug symbols are enabled by defualt when using cargo build or cargo run without the --release flag
In line 6 of the backtrace points to the line in our project that causes the problem, that would be line 4 of src/main.rs
If we dont want our program to panc thne we sould start our investigation at the inidcated line we wrote
Unwinding the Stack or Aborting in Response to a Panic
unwinding in rust means that it walks back up the stack and cleans up the data form each function it encounters.
However walking back and cleaning up is a lot of work
Rust also allows yo to choose the alternative of immediately aborting, which means ends the program without cleaning up
Memory that the program was using will thne be clean up by the OS
If yo need your project's resultant binary as small as possible you can switch from unwinfing to aborting upon a panic
This can be done by adding panic = 'abort' to the appropriate [profile] section in your Cargo.toml
example of this
[profile.release]
panic = 'abort'
Recoverable Errors
You can use the enum Result to handle most errors becasue they are not serious enough to warrant a panic
One example of this of a non serious error is opening a file and that operation fails becasue that file doesnt exist, you may want to create the file instead of terminating the process
the enum Result is defined as
Which has two variants Ok and Err
enum Result<T, E> {
Ok(T),
Err(E),
}
T and E are generic type parameters
T reperesents the tpye of value that will be returned in a success case within th Ok variant
E reperesents the tpye of the error that will be returned in a failure case within the Err variant
Because Result has these generic type parameters we can use the Result type and the functions defined on it in many different situations where the success value and error value we want to reutrn may differ
Lets use a function that retunrs a Result value because the function could fail
use std::fs::File;
fn main () {
let greeting_file_result = File::open("hello.txt");
}
The return type of File::open is a Result<T, E>
The generic parameter T has been filled in by the implementation of File::open with the type of the success value is a file handle (std::fs::File)
The E parameter is used in the error value which is std::io::Error
This return type indicates that call may succeed and reutnr a file handle that we can read and write to or it may fail if it doesnt exist, or not having the correct permissions
File::open function needs a way to tell us whether it succeeded or failed hence the use of Result enum which conveys this message of failure or success
When File::open succeeds, the value in greeting_file_result will be an instance of Ok that contains a file handle
When it fails the value in greeting_file_result will be an intance of Er that contains more info about the kind of error that occurred
We need to add a match in order to use Result here is one option
use std::fs::File;
fn main() {
let greeting_file_result = File::open("hello.txt");
let greeting_file = match greeting_file_result {
Ok(file) => file,
Err(error) => panic!("Problem opening the file: {error:?}"),
};
}
This is like the Option enum
Result enum and its variants have been brought into scope by the prelude, so you dont need to specifiy Result:: before the Ok and Err variants in the match arm
When the result is Ok the code will reutrn the file value out itself and then we can assign that file handle to the greeting_file. After machh we can use the file for reading or writing.
The other arm in match handles the Err value we get from File::open, in this case we choose to call the panic! macro with more details about the error
Matching on Different Errors
You can take different actions depending on the type of failure
For example if a file doesnt open because it doesnt exist then maybe you want you want to create the file first instead of panicing and exiting the program
One way we can do this is by using a match for the returned value Result<T, E>
Example
use std::fs::File;
use std::io::ErrorKind;
fn main() {
let greeting_file_result = File::open("hello.txt");
let greeting_file = match greeting_file_result {
Ok(file) => file,
Err(error) => match error.kind() {
ErrorKind::NotFound => match File::create("hello.txt") {
Ok(fc) => fc,
Err(e) => panic!("Problem creating the file: {e:?}"),
},
other_error => {
panic!("Problem opening the file: {other_error:?}");
}
},
};
}
The tpye that ``File::openreutrn inside thrErrvarian isio:Error`, this is a struct provided by the std library
This struct has a method kind which can be called to get an io::ErrorKind value
The enum io::ErrorKind is also provided by a std library which has vairants representing the different kinds of errors that may result from an io operation
The error we care about is Error::NotFound, this indicates that the file that we try to open doesnt exist.
In this case we need to have a match on the outher thne use that on an inner match on error.kind()
In this inner match we create a file which can also fail hence in 2nd inner match to each return a file handle or panic out of the program
Alternatives to Using match with Result<T, E>
this match expression is very useful but also primitive and boilerplate
one example to condese this code is
use std::fs::File;
use std::io::ErrorKind;
fn main() {
let greeting_file = File::open("hello.txt").unwrap_or_else(|error| {
if error.kind() == ErrorKind::NotFound {
File::create("hello.txt").unwrap_or_else(|error| {
panic!("Problem creating the file: {error:?}");
})
} else {
panic!("Problem opening the file: {error:?}");
}
});
}
This accomplishes the same thing as the the example above but it uses if and else and the unwrap_or_else
this is much cleaner to read even though it odesnt use and match statements
look at the exact definition unwrap_or_else in the std library document
Shortcuts for Panic on Error: unwrap and expect
match works well, but it can be a bit verbose and doesnt communicate clearly what the intent is
Result<T, E> has many helper methods to do specific tasks
The unwrap method that is a shortcut for the match expression that was used before
If the Result value is a Ok variant, unwrap will return the value inside the Ok
If the Result is the Err variant, unwrap will call the panc! macro for us
Here is an example of its use
use std::fs::File;
fn main() {
let greeting_file = File::open("hello.txt").unwrap();
}
If this code without a hello.txt it will panic with an error like this
thread 'main' panicked at src/main.rs:4:49:
called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }
You can also use the expect method lets us also choose the panic! error message
This is good for providing clearer error mesages and make tracking down the source of the panic easier
Here is an example of an expect in use
use std::fs::File;
fn main() {
let greeting_file = File::open("hello.txt")
.expect("hello.txt should be included in this project");
}
expect is used in the same way as unwrap to retunr the file handle or call the panic! macro
The message is used by expect in its call to panic! as the paramter that is passed to expect rather than a defualt message
expect is maninly used in production-quality code over unwrap becasue it gives more context about why the opertaion is always expected to succeed
Propagating Errors
When a operation fails you may want to pass back the error instead of handling the error within the function itself
This is used when you want the code that calls it to handle the error
here is an example is of this
use std::fs::File;
use std::io::{self, Read};
fn read_username_from_file() -> Result<String, io::Error> {
let username_file_result = File::open("hello.txt");
let mut username_file = match username_file_result {
Ok(file) => file,
Err(e) => return Err(e),
};
let mut username = String::new();
match username_file.read_to_string(&mut username) {
Ok(_) => Ok(username),
Err(e) => Err(e),
}
}
This could be written in an even smaller way, but this is good for exploration
For example is has a return type of Result<String, io::Error> this is also a vaule of Result<T, E> where the values are filled in with String and io:Error, also note thta the ownership will transfer
If the function succeeds without any problems then the cod that calls it will receive an Ok value that holds a String - the username that this function read from the file
If this function encounters any problems then the code that calls this will recive an Err value that holds an instance of io::Error that contains more info aobut what the problems were.
io:Error as the return type was chosne becuase that happens to be the tpye of the error value returned from both operations we're callin in this function's body that might fail these functions that could fail are File::open and read_to_string method.
the last term does not need the return keyword do to it being both the last item and a expression
its up to the calling code how to hanlde the Err and Ok value, for example it could call a panic! if it gets a Err value
This pattern of propagting errors is so commin in Rust that Rust provides the question mark operator ? to make it easier
A Shorcut for Popagation Errors: the ? Operator
Here is an implementation that is a bit smaller but deos the same thing as above
use std::fs::File;
use std::io::{self, Read};
fn read_username_from_file() -> Result<String, io::Error> {
let mut username_file = File::open("hello.txt")?;
let mut username = String::new();
username_file.read_to_string(&mut username)?;
Ok(username)
}
The ? is placed adter a Result value is defined in almost the same way as the match expressions were dfeined in the previous example
If the Result value is an Ok, the value inside the Ok will get returned form this expression and it will continue
If the value is an Err, the Err will be returned from the whole function as if we had used the return keyword so that the error value gets propagated to te calling code
The difference between what match and what ? does is that error values that have thr ? operator called on them go through the from function, defined in thr From trait in the std library which is used to convert values fomr one type to another.
When ? operator calls the from function, the error type is received is converted into the error type defined in the return type of the current function
This is useful when a function returns one error type to represent all the ways a functon might fail, even if parts might fail for many different reasons
In the context of the example, the ? at the end of the File::open call will return the value inside an Ok to the variable useranme_file
If an error occurs the ? operator will retunr early out of the whole function and give any Err value to the calling code.
This same thing applies to the ? at the end of the read_to_string call
The ? operartor eliminates a lot of boilerplate and make this function's implementation simpler
Here is an even shorter version of this code
use std::fs::File;
use std::io::{self, Read};
fn read_username_from_file() -> Result<String, io::Error> {
let mut username = String::new();
File::open("hello.txt")?.read_to_string(&mut username)?;
Ok(username)
}
Instead of creating a veriable username_file it has been chained to the call to read_to_string directly from the result of File::open("hello.txt")?
We still need a ? at the end of read_to_string call, which still allows it to return an Ok value containg username when both File::open and read_to_string succeed rather than returning errors
The functionality is still the same as the two examples above
Here is an even shorter veriosn ising fs::read_to_string
use std::fs;
use std::io;
fn read_username_from_file() -> Result<String, io::Error> {
fs::read_to_string("hello.txt")
}
Reading a file into a string is so common that the std library provides the convenient fs::read_to_string function that opens a file, creates a new String, reads the contents of the file which puts the contents into that String, and returns it
But this doesnt show how to explain ho wto use it
Where The ? Operator Can Be Used
The ? operator can only be used in functions whose return type is compatible with the value in the ? is used on
Because the ? is defined to perform an early return of a value out of the function, in the same manner as the match expression.
In the match case there was an arm that retunred an Err(e) value that is compatible with its return
You can use ? operator in main due to another crate being able to be used in a different crate
This requires that the retun type is compatable, () is not compatable with Result
For example this is not compatable
use std::fs::File;
fn main() {
let greeting_file = File::open("hello.txt")?;
}
But it will outbut something like this
$ cargo run
Compiling error-handling v0.1.0 (file:///projects/error-handling)
error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
--> src/main.rs:4:48
|
3 | fn main() {
| --------- this function should return `Result` or `Option` to accept `?`
4 | let greeting_file = File::open("hello.txt")?;
| ^ cannot use the `?` operator in a function that returns `()`
|
= help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
help: consider adding return type
|
3 ~ fn main() -> Result<(), Box<dyn std::error::Error>> {
4 | let greeting_file = File::open("hello.txt")?;
5 +
6 + Ok(())
7 + }
|
For more information about this error, try `rustc --explain E0277`.
error: could not compile `error-handling` (bin "error-handling") due to 1 previous error
If you notice this states that you are only allowed to use ? if it returns Result, Option or another type that implements FromResidual
To fix this error we have two options:
- Change your return tpye of your function to be compatible with the value yo're using the
?operator on, so long as there are no other restrictions preventing this - Use a
matchor one of theResult<T, E>methos to handle theResult<T, E>in the may most appropriate
The eror message also mentioned that ? can be used with the Option<T> values as well
The behavior when ? is called with an Option<T> is the same as Result<T, E>
If the value is None, this will be returned early as None
If the value is Some, the value inside the Some is the resultant value of the expression and the function continues
Here is an example where the function finds the last character of the first line in the given text
fn last_char_of_first_line(text: &str) -> Option<char> {
text.lines().next()?.chars().last()
}
This function returns Option<char> because its possible that there is a character there, but it is also possible that there is not
This code taskes a string slice and calls the lines method on it, which returns an iterator over the lines in the string. Becasue this function wants to examine the first line, it calls next on the iterator to get the first value from the iterator
If the string is emtpy then next will return None, if this is the case then yo can use ? to stop and return None form last_char_of_first_line
If its not then next will return a Some value that contains a string slice of the first line in the string slice argument
The ? extracts the string slice, then we can call chars on that string slice to get an iterator of its characters.
Since we want the last character in this first line, so we call last to return the last item in the iterator
This is an Option becasue it is possible that the first line is empty string
For example the string slice could contain "\nhi"
If there is a value then a Some will be returned and the ? in this case would not be needed due to it being the last part of the function and it would do essentially nothing
Note that ou can use the ? on a Result in a fuction that returns Reuslt and you can do the same on a function that returns a Option
You cannot mix and match
The ? operator won't automatically convert a Result to an Option or vice versa
In those cases you can use methods like the ok method on Result or the ok_or method on Option to do the conversion explicitly
If you can implement this kind of logic then you would need to use other method calls or a match expression
Main can either return a () or one of the possibilities is to return a Result<(), E>
Here is a concerte example that return a Result<(), Box<dyn Error>> and added a return value Ok(()) to the end
This ensures that the code will compile
use std::error::Error;
use std::fs::File;
fn main() -> Result<(), Box<dyn Error>> {
let greeting_file = File::open("hello.txt")?;
Ok(())
}
The Box<dyn Error> type is a trait object, which for now means "any kind of error"
Using v? on a Result value in with the error type Box<dyn Error> is allowed becuase it allows any Err value to be returned early.
Even though the body of this main function will only ever return errors of the type std::io::Eror by specifying Box<dyn Error>, this will continue to be correct even if more code that returns other errors is added to the body of main
When a main returns a Result<(), E> the executable will exit with a value of 0 if main returns Ok(()) and will return a nonzero value if main returns an Err value.
In C executalbes return a integer when they exit, programs that successfully return the 0 integer and reutrn an interger other than 0
Rust also returns integers form executables to be compatible with this convention
main can return any types that implement the std::proccess::Termination trait which contains a function report that returns an ExitCode
See the std library documentation for more info on implementing the Termination trait for custom types developed by you
To panic! or Not to panic!
When to call panic! and when to return Result
When the code panics there is no way to recover
you could call panic! for any erro sutuation whether there is a possible way to recover or not but then you make the decision that it is completely unrecoverable on behalf of the code
Or you could return a Result value and give the calling code option to handle it or attempt to recover in a way that's appropriate for its situation or you could decide that an speicfic Err value is unrecoverable and choose to call a panic! to turn a recoverable one into an unrecoverable
This makes returning a Result is a good default cohice for when yo are defining a function that may fail
In situations like prototype code, and tests its more appropriate to write code that panics instead of returning a Result
Examples, Prototype Code and Tests
When writing an example to illustrate some concept, also inclduing robust error-handling code can make examples less clear
In examples its clear that a call to a mthod like unwrap that could panic is meant as a placeholder for the way you want your app to handle erros
Similarly the unwrap and expect methods are vary handy when prototyping, before you are ready to decide how to hanlde errors.
This leaves clear markers in your code for when you are ready to make your app more robust
If a method call fails in a test, you want the whole test to fail, even if that method isnt the functionality under the test
panic! is how a test is marked as a failure, calling unwrap or expect is exactly what should happen
Cases in Which You Have More Info Than the Compiler
Its appropraite to call unwrap or expect whne you have some other logic that enusres the Result will have an Ok value, ut the logic is not something the compiler understands
You can sometimes still have a Result value even though the value inside should always be Ok
This is the case whenever you call a function or operation has a possibility of failing in generally, even though it is logically imposible in your situation.
This case happens when you can manually inspect the code and you would never expect an Err variant
It is unacceptable to call unwrap in these situations, it is better to document the reason you think you would never have an Err varaint in the expect text
Here is an example
use std::net::IpAddr;
let home: IpAddr = "127.0.0.1"
.parse()
.expect("Hardcoded IP address should be valid");
In this example it is creating an IpAddr instance by parsing a hard coded string
We can see that the string is a valid IP address, so it's acceptable to use expect here
Having a hardcoded, valid string doesnt change the return type of the parse method, we still get a Result value
The compiler will force us to handle the Result as if the Err value is a possiblity
This is because the compiler cannot see the logic from a hard coded string which doesnt have the possibility of failure
Mentioning the assumption that this IP address is hardcoded will prompt us to change expect to better error-handing code if in the future, we need to get the IP address from some other source instead
Guidelines for Error Handling
Its advisable to have your code panic when its possible to get ino a bad state
A bad state is when some assumption, guarantee, contract, or invariant has been broken such as when invalid values, contradictory values, or missing values are passed to your code plus one or more of the following:
- The bad state is somthing that is unexpected which is not something that is a likely problem (a user entering data in the wrong format)
- The code needs to rely on not being in a bad state, rather than checking at every step
- There is not a good way to encode this info in the types you use If someoneelse calls your code and passes in bad values or values that dont make sense, its best to return an error if you can, so that the lbirary's user can decide what they want to do in that case
In some cases where continuing could be insecure or harmful, the best choice might bbe to call a panic! and alert the library user to the bug in thier code so that they can ifx it during development
panic! is so often appropriate if you are calling external cod that is out of your control and it retunrs an invaild state that you have no way of fixing
If a failure is expected then it is more appropriate to return a Result than to make a panic call
Examples of this include a parser being being malformed data or an HTTP requst retunring a status that indicates ou have hit a rate limit.
In these caes it would be more appropriate to return a Result indicating that a failure is an expected possiblity that the calling code must decide how to handle.
When your code performs an opertation that could put a user at risk if is called using invalid values, your code should verify the values first then paic if the values aren't valid
This is mostly for safety reasons, attempting to operate on invalid data that could expose your code to vulnerabilities
This would be the main reason the std library will call panic! if you attempt an out-of-bounds memory access
Trying to access memory that doesnt belong to the current data struct, this is a common security problem.
Functions often have contract, their behavior is only guaranteed if the inputs meet particlar requirements.
Panicking when the cotnract is violated makes sense because a contract violation always indicates a caller-side bug and its not a kind of error you want the calling code to have to handle explicitly.
In this case there is no reasonable way for the calling code to reacover or fix the code becasue the programmer needs to dix the code.
Contracts for a fnction, especially when a violation will cause a panic, this should be explained in the API docs for the function.
Having lots of error chcks in all of your functions would be verbos and annoying.
Instead you can use Rust's type system and thus the compiler to do many of the checks for you.
If your function has a particular tpe as a parameter, you can proceed with your code's logic knowing that the compiler will ensure that you have a vaild value
For example, using u32 instead of i32 enusres you will never have a negative number, or you cant pass a Option into a function with a conrete tpye, the code won't even compile becuase it cant take in an Option
Creating Custom Types for Validation
This is useful for enchancing or guiding the user toward valid data, and have different behavior when a user give an almost invalid data
Having too many checks can impact performance if you had many many functions with a requirement that is the same.
Instead you can make a new type and put the validations in a function to create an instance of the tpye rather than needing to repeat the validations everywhere
This makes it more safe for functions to use the new type in their signatures and confidently use the values they recieve.
Here is a way to implement this kind of type for the original guessing game from ch2
pub struct Guess {
value: i32,
}
impl Guess {
pub fn new(value: i32) -> Guess {
if value < 1 || value > 100 {
panic!("Guess value must be between 1 and 100, got {value}.");
}
Guess { value }
}
pub fn value(&self) -> i32 {
self.value
}
}
The guess struct can only hold a i32 value that is private
In its associated functions there is a new function or a contructor that first checks if the value is between 1-100 and reutrns a new instance of the function if it is valid otherwise it calls a panic!
It can take in an i32 but it still calls a panic! if it is out of range, the panic is ued to alert the progammer who is writing thr calling ocde that they have a bug to fix becuae calling a value outside of the range would violate the contact that Guess::new depends on.
The conditions in which Guess:new might panic should be told in its public-facing API docs.
A function that has a parameter or returns only numbers between 1 and 100 could declarre in its signature that it takes in or returns a Guess rather than an i32 and you wouldnt need to addtionaly checks in its body