In this post I'll show you the code path that Rust takes inside itsstandard library when you open a file. I wanted to learn how Rusthandles system calls and errno, and all the little subtleties of thePOSIX API. This is what I learned!
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When you open a file, or create a socket, or do anything else thatreturns an object that can be accessed like a file, you get a filedescriptor in the form of an int.
You get a nonnegative integer in case of success, or -1 in case of anerror. If there's an error, you look at errno, which gives you aninteger error code.
- Most programs on Linux are dynamically linked. So when you are creating a unikernel with OPS out of a linux application OPS goes out and finds all the libraries it's dynamically linked to and throws it onto the disk image. This works well if you are on linux - 99% of everything on linux is dynamically linked.
- Redox is a new operating system kernel entirely written in Rust. Update: If you want to see what a C-less libstd would look like, take a look at steed, an effort to reimplement Rust's libstd without C dependencies.
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Many system calls can return EINTR, which means 'interrupted systemcall', which means that something interrupted the kernel while itwas doing your system call and it returned control to userspace, withthe syscall unfinished. For example, your process may have received aUnix signal (e.g. you send it SIGSTOP by pressing Ctrl-Z on aterminal, or you resized the terminal and your process got aSIGWINCH). Most of the time EINTR means simply that you mustretry the operation: if you Control-Z a program to suspend it, andthen fg to continue it again; and if the program was in the middleof open()ing a file, you would expect it to continue at that exactpoint and to actually open the file. Software that doesn't check forEINTR can fail in very subtle ways!
Once you have an open file descriptor, you can read from it:
. and one has to remember that if read() returns 0, it means wewere at the end-of-file; if it returns less than the number of bytesrequested it means we were close to the end of file; if this is anonblocking socket and it returns EWOULDBLOCK or EAGAIN then onemust decide to retry the operation or actually wait and try againlater.
There is a lot of buggy software written in C that tries to use thePOSIX API directly, and gets these subtleties wrong. Most programswritten in high-level languages use the I/O facilities provided bytheir language, which hopefully make things easier.
Rust makes error handling convenient and safe. If you decide toignore an error, the code looks like it is ignoring the error(e.g. you can grep for unwrap() and find lazy code). Thecode actually looks better if it doesn't ignore the error andproperly propagates it upstream (e.g. you can use the ? shortcut topropagate errors to the calling function).
I keep recommending this article on error models to people; itdiscusses POSIX-like error codes vs. exceptions vs. more modernapproaches like Haskell's and Rust's - definitely worth studying overa few of days (also, see Miguel's valiant effort to move C# I/O awayfrom exceptions for I/O errors).
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So, what happens when one opens a file in Rust, from the toplevel APIdown to the system calls? Let's go down the rabbit hole.
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You can open a file like this:
This does not give you a raw file descriptor; it gives you anio::Result, which you must pick apart to see ifyou actually got back a File that you can operate on, or an error.
Let's look at the implementation of File::open() and File::create(). The interlude mac os.
Here, OpenOptions is an auxiliary struct that implements a 'builder'pattern. Instead of passing bitflags for the variousO_CREATE/O_APPEND/etc. flags from the open(2) system call, onebuilds a struct with the desired options, and finally calls .open()on it.
So, let's look at the implementation of OpenOptions.open():
Found (itch) mac os. See that fs_imp::File::open()? That's what we want: it's theplatform-specific wrapper for opening files. Let's lookat its implementation for Unix:
The first line, let path = cstr(path)? tries to convert a Pathinto a nul-terminated C string. The second line calls the following:
Here, let flags = . converts the OpenOptions we had in thebeginning to an int with bit flags. Atmk! mac os.
Then, it does let fd = cvt_r (LAMBDA), and that lambda functioncalls the actual open64() from libc (a Rust wrapper for the system'slibc): it returns a file descriptor, or -1 on error. Why is thisdone in a lambda? Let's look at cvt_r():
Okay! Here f is the lambda that calls open64(); cvt_r() callsit in a loop and translates the POSIX-like result into somethingfriendly to Rust. This loop is where it handles EINTR, which getstranslated into ErrorKind::Interrupted. I suppose cvt_r() standsfor convert_retry()? Let's look atthe implementation of cvt(), which fetches the error code:
(The IsMinusOne shenanigans are just a Rust-ism to help convertmultiple integer types without a lot of as casts.)
The above means, if the POSIX-like result was -1, return an Err() fromthe last error returned by the operating system. That should surelybe errno internally, correct? Let's look atthe implementation for io::Error::last_os_error():
Escape the earrape mac os. We don't need to look at Error::from_raw_os_error(); it's just aconversion function from an errno value into a Rust enum value.However, let's look at sys::os::errno():
Here, errno_location() is an extern function defined in GNU libc(or whatever C library your Unix uses). It returns a pointer to theactual int which is the errno thread-local variable. Since non-Ccode can't use libc's global variables directly, there needs to be away to get their addresses via function calls - that's whaterrno_location() is for.
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And on Windows?
Remember the internal File.open()? This is what it lookslike on Windows:
CreateFileW() is the Windows API function to open files. Theconversion of error codes inside Error::last_os_error() happensanalogously - it calls GetLastError() from the Windows API andconverts it.
The above means, if the POSIX-like result was -1, return an Err() fromthe last error returned by the operating system. That should surelybe errno internally, correct? Let's look atthe implementation for io::Error::last_os_error():
Escape the earrape mac os. We don't need to look at Error::from_raw_os_error(); it's just aconversion function from an errno value into a Rust enum value.However, let's look at sys::os::errno():
Here, errno_location() is an extern function defined in GNU libc(or whatever C library your Unix uses). It returns a pointer to theactual int which is the errno thread-local variable. Since non-Ccode can't use libc's global variables directly, there needs to be away to get their addresses via function calls - that's whaterrno_location() is for.
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And on Windows?
Remember the internal File.open()? This is what it lookslike on Windows:
CreateFileW() is the Windows API function to open files. Theconversion of error codes inside Error::last_os_error() happensanalogously - it calls GetLastError() from the Windows API andconverts it.
Can we not call C libraries?
The Rust/Unix code above depends on the system's libc for open() anderrno, which are entirely C constructs. Libc is what actually doesthe system calls. There are efforts to make the Rust standard librarynot use libc and use syscalls directly.
As an example, you can look atthe Rust standard library for Redox. Redox is a new operatingsystem kernel entirely written in Rust. Fun times!
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Update: If you want to see what a C-less libstd would looklike, take a look at steed, an effort to reimplement Rust's libstdwithout C dependencies.
Rust is very meticulous about error handling, but it succeeds inmaking it pleasant to read. I/O functions give you back anio::Result<>, which you piece apart to see if it succeeded or got anerror.
Internally, and for each platform it supports, the Rust standardlibrary translates errno from libc into an io::ErrorKind Rustenum. The standard library also automatically handles Unix-isms likeretrying operations on EINTR.
I've been enjoying reading the Rust standard library code: ithas taught me many Rust-isms, and it's nice to see how thehairy/historical libc constructs are translated into clean Rustidioms. I hope this little trip down the rabbit hole for theopen(2) system call lets you look in other interesting places, too.
