Why is a Rust executable large?


Update (2016-10-07): The official FAQ has an entry for this exact problem now. I'll leave this post as more curiously minded people.

See also the /r/rust discussion and Hacker News thread at the time of writing.

Do you want a quick takeaway? Go to the end of post.

Suppose that you are a programmer primarily working with compiled languages. Somehow you’ve gotten tired of those languages, for multiple valid reasons, and heard of a trendy new programming language called Rust. Looking at some webpages and the official forum, it looks great and you decide to try it out. It seems that Rust was a bit cumbersome to install in the past, but thanks to rustup that problem seems gone now. Cargo seems to be great, so you follow the first sections of the Book and put together a small greeting to the new language:

fn main() {
    println!("Hello, world!");

Amazingly cargo run runs without a hassle. It is kind of a miracle as you are used to configuring a build script, Makefile, project, or whatever before building things. Impressed, you notice that the executable is available in target/debug/hello. You instinctively type out ls -al (or is it dir?) and you cannot believe your eyes:

$ ls -al target/debug/hello
-rwxrwxr-x 1 lifthrasiir 650711 May 31 20:00 target/debug/hello*

650 kilobytes to print anything?! You remember that Rust is probably the sole language that may possibly displace C++, and C++ is noted for its code bloat; would that mean Rust failed to fix one of C++’s big problems? Out of curiosity, you make the same program in C and compile it. The result is eye-opening:

$ cat hello-c.c
#include <stdio.h>
int main() {
    printf("Hello, World!\n");
$ make hello-c
$ ls -al hello-c
-rwxrwxr-x 1 lifthrasiir 8551 May 31 20:03 hello-c*

Maybe C has the benefit of having bare-metal libraries, you think. This time you try a C++ program using iostream, which should be much safer than C’s naive printf. But surprisingly it still seems tiny compared to Rust:

$ cat hello-cpp.cpp
#include <iostream>
using namespace std;
int main() {
    cout << "Hello, World!" << endl;
$ make hello-cpp
$ ls -al hello-cpp
-rwxrwxr-x 1 lifthrasiir 9168 May 31 20:06 hello-cpp*

What is wrong with Rust?

It seems that the surprisingly large size of a Rust binary is a massive concern for many. This question is by no means new; there is a well-known, albeit year-old, question on StackOverflow, and searching for “why is rust binary large” gives several more. Given the frequency of such questions, it is a bit surprising that we don’t yet have a definitive article or page dealing with them. So this is my attempt to provide one.

Just to be cautious: Is it a valid question to ask in the first place? We have hundreds of gigabytes of storage, if not terabytes, and people should be using decent ISPs nowadays, so the binary size should not be a concern, right? The answer is that it may still matter (though not as much as before):

For wondering readers: All examples are tested in Rust 1.9.0 and 1.11.0-nightly (a967611d8 2016-05-30). Unless noted, the primary operating system used is Linux 3.13.0 on x86-64. Your mileage may vary.

Optimization Level

If asked about the above, virtually every experienced Rust user would ask you back:

Have you enabled the release build?

It turns out that Cargo distinguishes the debug build (default) from the release build (--release). The Cargo documentation explains the exact differences between them, but in general the release build gets rid of development-only routines and data and enables tons of optimizations. It is not the default because, well, the debug build is more frequently requested than the release build.

Note that Jorge Aparicio has correctly pointed out that the release build does not produce the smallest possible binary. That’s because the release build defaults to optimization level 3 (-C opt-level=3), which may sacrifice some size for performance. The size-optimizing level (-C opt-level=s or -C opt-level=z) has recently landed, so you may use that later. For now, however, we'll stick to the default.

Let’s try the release build!

$ cargo build --release
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 646467 May 31 20:10 target/release/hello*

And that didn’t really make a difference! This is because the optimization is only run over the user code, and we don’t have much user code. Almost all of the binary is from the standard library, and it's not clear whether we can do anything about that...

Link-time Optimization (LTO)

…except that we can. Enter the world of link-time optimization.

So the story is as follows: We can individually optimize each crate, and in fact all standard libraries ship in the optimized form. Once the compiler produces an optimized binary, it gets assembled to a single executable by a program called “the linker”. But we don’t need the entirety of standard library: a simple “Hello, world” program definitely does not need std::net for example. Yet, the linker is so stupid that it won’t try to remove unused parts of crates; it will just paste them in.

There is actually a good reason that the traditional linker behaves like this. The linker is commonly used in the C and C++ languages among others, and each file is compiled individually. This is a sharp difference from Rust where the entire crate is compiled altogether. Unless required functions are scattered throughout the files, the linker can fairly easily get rid of unused files all at once. It’s not perfect, but reasonably approximates what we want: removing unused functions. One disadvantage is that the compiler is unable to optimize function calls pointing to other files; it simply lacks the required information.

C and C++ folks had been fine with that approximation for decades, but in the recent decades they decided they'd had enough and started to provide an option to enable link-time optimization (LTO). In this scheme the compiler produces optimized binaries from each file without looking at others, and then the linker actively looks at them all and tries to optimize the binary. It is much harder than working with (internally simplified) sources, and it hugely increases the compilation time, but it is worth trying if a smaller and/or faster executable is needed.

So far we have talked about C and C++, but the LTO is much more beneficial for Rust. Cargo.toml has an option to enable LTO:

lto = true

Did that work? Well, sort of:

$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 615725 May 31 20:17 target/release/hello*

It had a larger effect than the optimization level, but not much. Maybe it is time to look at the executable itself.

So what’s in my executable?

There are several tools for directly working with executables, but the most useful one is probably GNU binutils. It is available for all Unix-like systems and on Windows (MinGW has a standalone install for example).

There are many utilities in binutils, but strings is probably the simplest. It simply crawls the binary to find a sequence of printable characters terminated by a zero byte, a typical representation of C string. Thus it tries to extract readable strings out of a binary, which is quite helpful for us. So let’s try that, and prepare for the scroll:

$ strings target/release/hello | head -n 10
 B ,

And, wow, it already has something we didn’t expect: pthreads. (More on that later, though.) There are indeed tons of strings in our executable:

$ strings target/release/hello | wc -c

Huh, one sixth of our executable is for strings we don’t really use! At the closer inspection, this observation is not correct as strings also give many false positives, but there are some significant strings as well:

Why do we have those strings in the first place? They are debug symbols, which give an appropriate (possibly human-readable) name for the otherwise-machine-processed binary. Do you remember libbacktrace above? It needs those debug symbols to print any useful information. Yet, since we are really making a release build we may choose not to include them. Rust itself doesn't have this option, since they are typically stripped by an external utility called strip. So let’s look at what can be done about them.

Debug symbols, get off my lawn!

So we have three goals: no jemalloc, no libbacktrace, and no debug symbols. I’ve mentioned that strip strips debug symbols, so let’s do that first. Note that strip also comes with binutils, so you can just run that.

$ strip target/release/hello
$ target/release/hello
Hello, world!
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 347648 May 31 20:23 target/release/hello*

Now that IS smaller! About a half of the entire executable was for debugging symbols. Note that, having stripped our symbols, we cannot have a nice backtrace nor panic recovery:

$ sed -i.bak s/println/panic/ src/main.rs
$ cat src/main.rs
fn main() {
    panic!("Hello, world!");

$ cargo build --release && strip target/release/hello
$ RUST_BACKTRACE=1 target/release/hello
thread '<main>' panicked at 'Hello, world!', src/main.rs:2
stack backtrace:
   1:     0x7fde451c1e41 - <unknown>
Illegal instruction
$ mv src/main.rs.bak src/main.rs     # tidy up

…and it somehow aborted. Probably a libbacktrace issue, I don’t know, but that doesn’t harm much anyway.

Knocking jemalloc down

We have knocked debug symbols down, now let’s get rid of the remaining libraries. Some bad news: From this point on you are entering the realm of nightly Rust. This realm is not as scary as you might think, as it doesn't break in your face, but it may break in smaller ways (which is why we have nightlies!). That’s the major reason that we don’t yet have nightly features in stable– they may change. Fortunately, the features we are going to use have been quite stable and you can probably follow the remainder of this post with more recent nightlies. But for posteriority, I will stick to a particular nightly version.

Some good news: Installing nightlies (either the latest or any specific version) is very simple with rustup.

$ rustup override set nightly-2016-05-31
$ cargo build --release
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 620490 May 31 20:35 target/release/hello*
$ strip target/release/hello
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 351520 May 31 20:35 target/release/hello*

Okay, the size hadn’t changed much since the last time we used strip. Let’s knock jemalloc down first— it is well documented in the Book, but the gist is that it just takes two additional lines to change an allocator:

extern crate alloc_system;

fn main() {
    println!("Hello, world!");

And that makes quite a difference:

$ cargo build --release
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 210364 May 31 20:39 target/release/hello*
$ strip target/release/hello
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 121792 May 31 20:39 target/release/hello*

Okay! We are down from 600 whooping kilobytes to about 120 KB. Jemalloc indeed is a big library; it really has tons of configuration so that you can fine-tune its performance, and that has to go somewhere.

No panic, no gain

We are now left with libbacktrace. Our code doesn't panic, so we don’t need to print a backtrace, right? Well, we have reached a limit: libbacktrace is deeply integrated into the standard library, and the only way to avoid it is to not use libstd. Quite a dead end.

But that is not the end of story. Panicking gives us a backtrace, but also the ability to unwind anything. And unwinding is supported by yet another bit of code called libunwind. It turns out that we can get rid of this by disabling unwinding. Put this into Cargo.toml:

lto = true
panic = 'abort'

And we can see some effect:

$ cargo build --release
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 200131 May 31 20:44 target/release/hello*
$ strip target/release/hello
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 113472 May 31 20:44 target/release/hello*

And that’s it! It is about the end of story if you don’t want to change the code.

Intermission: Linkage

Before looking at more obscure areas, this is perfect time to admit that I was cheating with the size of C and C++ binaries. The fair (well, fairer) comparison would be as follows:

$ touch *.c *.cpp
$ make hello-c CFLAGS='-Os -flto -Wl,--gc-sections -static -s'
cc -Os -flto -Wl,--gc-sections -static    hello-c.c   -o hello-c
$ make hello-cpp CXXFLAGS='-Os -flto -Wl,--gc-sections -static -static-libstdc++ -s'
g++ -Os -flto -Wl,--gc-sections -static -static-libstdc++    hello-cpp.cpp   -o hello-cpp
$ ls -al hello-c hello-cpp
-rwxrwxr-x 1 lifthrasiir  758704 May 31 20:50 hello-c*
-rwxrwxr-x 1 lifthrasiir 1127784 May 31 20:50 hello-cpp*

(Note: all these options are required to be in line with Rust equivalents. -Wl,--gc-sections is probably the only option missing; it is a simple-minded cousin of LTO which does not optimize but just removes unused code sections—huge thanks to Alexis Beingessner and Corey Richardson for pointing out that it was missing. Rust has that implied by default, and it can be applied independently from LTO, so a fair comparison also needs that.)

Also, the Rust binary needs to be rebuilt:

$ cargo rustc --release -- -C link-args=-static
$ strip target/release/hello
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 105216 May 31 20:51 target/release/hello*

Okay, so it seems that Rust was actually far, far better than C and C++. But… why is it “fair”? Isn’t a 1 MB executable too much for such a simple program regardless of the language?

A binary executable is not a simple data format. It is normally processed and often altered by an OS routine called a “dynamic linker” (not to be confused with the aforementioned “linker”). The use of a dynamic linker allows programs to dynamically link to other (often common) libraries including the system ones, and until now we were implicitly linking to C and C++’s standard libraries—glibc and libstdc++ in this case! Rust does not (well, barely) make use of them however, so the entire comparison was unfair to Rust.

This kind of dynamic linking is a double-edged sword. It makes it trivial to update libraries used by multiple programs and in theory the total binary size should be reduced. There is a strong minority against dynamic linkage though, as it also makes it trivial to break libraries (because the dynamic linker is not like Cargo, what a pity) and its effect on the total binary size have been exaggerated. To elaborate on the latter, earlier in this post I’ve mentioned a problem that the LTO eventually solves, and the dynamic linkage suffers from the same problem but without any solution—LTO on dynamic library would ruin its advantage.

But in spite of those problems, dynamic linking remains a popular choice for many platforms and especially C and C++ standard libraries are often only available as a dynamic library. Yeah, we instructed the linker to statically link everything, but glibc lies and some functions still require dynamic libraries:

$ # restore debug symbols
$ touch *.c
$ make hello-c CFLAGS='-Os -flto -static'
cc -Os -flto -static    hello-c.c   -o hello-c
$ strings hello-c | grep dlopen
shared object cannot be dlopen()ed
invalid mode for dlopen()

It is actually a clever way to make a near-static library with some dynamic-only features (iconv for example). Of course, if you depend on those features you are doomed. That aside, however, you need to enable static linking in the linker side to avoid biases; the -static linker option is therefore necessary. It is very interesting to see that the Rust binary is actually smaller after the static linking. It does depend on the C standard library but only on a little bit (standard I/O completely bypasses stdio, for example), so it did manage to escape the weight of glibc somehow.

The picture has changed so much that the comparison still looks biased. It is all the fault of glibc after all! There are several libc alternatives, and musl is a promising one. It results in very compact binary even after static linkage:

$ touch *.c
$ make hello-c CFLAGS='-Os -flto -static -s' CC=musl-gcc
musl-gcc -Os -flto -static -s    hello-c.c   -o hello-c
$ ls -al hello-c
-rwxrwxr-x 1 lifthrasiir 5328 May 31 20:59 hello-c*

Can we use musl in Rust? Of course. You can install standard libraries for other targets with rustup. This time -C link-args=-static is not necessary as musl links statically by default.

$ rustup target install x86_64-unknown-linux-musl
$ cargo build --release --target=x86_64-unknown-linux-musl
$ ls -al target/x86_64-unknown-linux-musl/release/hello
-rwxrwxr-x 1 lifthrasiir 263743 May 31 21:07 target/x86_64-unknown-linux-musl/release/hello*
$ strip target/x86_64-unknown-linux-musl/release/hello
$ ls -al target/x86_64-unknown-linux-musl/release/hello
-rwxrwxr-x 1 lifthrasiir 165448 May 31 21:07 target/x86_64-unknown-linux-musl/release/hello*

Okay, so this finally looks fair. The entirety of this 160 KB executable can be properly attributed to Rust’s “bloat”; it contains libbacktrace and libunwind (weighing about 50 KB combined), and libstd is still hard to completely optimize out (having 40 KB of pure Rust code, referencing various bits of libc). This is the status quo of the Rust standard library, and has to be worked out. Well, at least this is one time cost per executable, so practically they wouldn’t matter much anyway.

There is yet another approach to being more fair. What if we can use dynamic linking for Rust? This completely ruins the distribution until every OS ships with the Rust standard library, but well, it's worth trying. Note that LTO, the custom allocator, and the different panic strategy are not compatible with dynamic linkage, so you have to leave them as the defaults.

$ cargo rustc --release -- -C prefer-dynamic
$ ls -al target/release/hello
-rwxrwxr-x 1 lifthrasiir 8831 May 31 21:10 target/release/hello*

So this is comparable to ordinary C/C++ programs with dynamic linkage. Mostly a curiosity in this case though.

Say goodbye to libstd

We have so far looked at reducing the executable size without changing your code a lot. But if you are willing to pay that cost, it may result in a much smaller executable. Note: This is not recommended in general, this section is going to be a strict cost-effect analysis.

We all know that libstd is friendly and convenient, but sometimes it is too much (and it is a source of libbacktrace and libunwind that we don’t really use). Start by avoiding libstd. The Book has a whole section about no stdlib mode, so let’s follow that. The new source code is as follows:

#![feature(lang_items, start)]

extern crate libc;

fn start(_argc: isize, _argv: *const *const u8) -> isize {
    // since we are passing a C string,
    // the final null character is mandatory
    const HELLO: &'static str = "Hello, world!\n\0";
    unsafe { libc::printf(HELLO.as_ptr() as *const _); }

#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "panic_fmt"] extern fn panic_fmt() -> ! { loop {} }

And you need to add a dependency to the libc crate in Cargo.toml:

libc = { version = "0.2", default-features = false }

And that’s it! It is very close to what you would get with a C program: 8503 bytes before strip, 6288 bytes after strip. (I’m tried of faking the console output (note the modified time), so I’ll omit them from now on.) musl goes further, but you need to give an additional option to properly link the libc crate to musl:

$ export RUSTFLAGS='-L native=/usr/lib/x86_64-linux-musl'
$ cargo build --release --target=x86_64-unknown-linux-musl
$ strip target/x86_64-unknown-linux-musl/release/hello

Now we are down to 5360 bytes. Only 32 bytes behind the best C program! How can we do better? Instead of using stdio, you can use the direct system call (on Unix only, of course):

fn start(_argc: isize, _argv: *const *const u8) -> isize {
    const HELLO: &'static str = "Hello, world!\n";
    unsafe {
                    HELLO.as_ptr() as *const _, HELLO.len());

This strips yet more bits off the binary and weighs 5072 bytes after strip.

One may argue that the above is not real Rust code, but a Rust-esque code depending on Unix system calls. Fine. No real Rust code would look like that. Instead we can make our own Stdout type, wired directly to system calls, and use ordinary formatting macros:

use core::fmt::{self, Write};

struct Stdout;
impl Write for Stdout {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        let ret = unsafe {
                        s.as_ptr() as *const _, s.len())
        if ret == s.len() as isize {
        } else {

fn start(_argc: isize, _argv: *const *const u8) -> isize {
    let _ = writeln!(&mut Stdout, "Hello, world!");

This is indeed much more natural Rust code. It weighs 9248 bytes after strip, suggesting that the minimal formatting code costs about 4 KB. Other types impose more overhead though; printing 3.14f64 for example costs about 25 KB of additional binary, as floating-to-decimal conversion is hard. Also mind that this never buffers (except for the internal kernel buffer for pipes).

And another thing...

We can go much further even from this point, since our executable still contains seemingly unused chunk of bytes. Welcome to the realm of actual low-level programming (in contrast to the lower-level programming we normally do with C/C++). This area has been very thoroughly explored by pioneers however, so I will end the trip by linking to them:

This post is intended as a tour of various techniques (not necessarily all practical) of reducing program size, but if you demand some conclusion, the pragmatic takeaway is that:

That’s all, folks!

List of articles

  1. Worklog 2015-03-06
  2. Worklog 2015-02-19
  3. Summary of Korean Rust Meetup #4
  4. Worklog 2015-01-17
  5. Worklog 2015-01-14
  6. Worklog 2015-01-13
  7. Worklog 2015-01-10
  8. Worklog 2014-12-20
  9. Worklog 2014-12-13
  10. Worklog 2014-12-09
  11. Worklog 2014-12-08
  12. Worklog 2014-12-06
  13. The Beginning