Static linking

Author: aidanhs

src/doc/trpl/advanced-linking.md

@@ -30,3 +30,129 @@ meaning.
 It is highly recommended to *not* use this attribute, and rather use the more
 formal `#[link(...)]` attribute on `extern` blocks instead.
 
+# Static linking
+
+Static linking refers to the process of creating output that contain all
+required libraries and so don't need libraries installed on every system where
+you want to use your compiled project. Rust libraries are statically linked by
+default so you can use Rust-created binaries and libraries without installing
+the Rust runtime everywhere. By contrast, native libraries are dynamically
+linked by default, but sometimes it can be useful to change this.
+
+Linking is a very platform dependant topic - on some platforms, static linking
+may not be possible at all! This section assumes some basic familiarity with
+linking on your platform on choice.
+
+## Linux
+
+By default, all Rust programs on Linux will link to the system libc along with
+a number of other libraries Let's look at an example on a 64-bit linux machine
+with GCC and glibc (by far the most common libc on Linux):
+
+``` text
+$ cat example.rs
+fn main() {}
+$ rustc example.rs
+$ ldd example
+        linux-vdso.so.1 =>  (0x00007ffd565fd000)
+        libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fa81889c000)
+        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fa81867e000)
+        librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007fa818475000)
+        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fa81825f000)
+        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fa817e9a000)
+        /lib64/ld-linux-x86-64.so.2 (0x00007fa818cf9000)
+        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fa817b93000)
+```
+
+Dynamic linking on Linux can be undesirable if you wish to target older
+machines as applications compiled aginst newer versions glibc are not
+guaranteed to run against older versions.
+
+You can examine Rust linking arguments with an option to rustc. Newlines have
+been added for readability:
+
+``` text
+$ rustc example.rs -Z print-link-args
+"cc"
+    "-Wl,--as-needed"
+    "-m64"
+    [...]
+    "-o" "example"
+    "example.o"
+    "-Wl,--whole-archive" "-lmorestack" "-Wl,--no-whole-archive"
+    "-Wl,--gc-sections"
+    "-pie"
+    "-nodefaultlibs"
+    [...]
+    "-Wl,--whole-archive" "-Wl,-Bstatic"
+    "-Wl,--no-whole-archive" "-Wl,-Bdynamic"
+    "-ldl" "-lpthread" "-lrt" "-lgcc_s" "-lpthread" "-lc" "-lm" "-lcompiler-rt"
+```
+
+Arguments with a `-L` before them set up the linker search path and arguments
+ending with `.rlib` are linking Rust crates statically into your application.
+Neither of these are relevent for static linking so have been ommitted.
+
+The first step in being able to statically link is to obtain an object file.
+This can be achieved with `rustc --emit obj example.rs`, and creates a file
+called `example.o`, which you can see being passed in the command line above -
+rustc automatically deletes it when finished with it by default. As you now have
+the object file, you should be able to run the link command obtained with
+`print-link-args` to create perform the linking stage yourself.
+
+In order to statically link, there are a number of changes you must make. Below
+is the command required to perform a static link; we will go through them each
+in turn.
+
+``` text
+$ rustc example.rs -Z print-link-args
+"cc"
+    "-static"
+    "-m64"
+    [...]
+    "-o" "example"
+    "example.o"
+    "-Wl,--whole-archive" "-lmorestack" "-Wl,--no-whole-archive"
+    "-Wl,--gc-sections"
+    "-nodefaultlibs"
+    [...]
+    "-Wl,--whole-archive"
+    "-Wl,--no-whole-archive"
+    "-ldl" "-lpthread" "-lrt" "-lgcc_eh" "-lpthread" "-lc" "-lm" "-lcompiler-rt"
+```
+
+ - `-static` was added - this is the signal to the compiler to use a static
+   glibc, among other things
+ - `-Wl,--as-needed` was removed - this can be left in, but is unnecessary
+   as it only applies to dynamic librares
+ - `-pie` was removed - this is not compatible with static binaries
+ - both `-Wl,-B*` options were removed - everything will be linked statically,
+   so informing the linker of how certain libraries should be linked is not
+   appropriate
+ - `-lgcc_s` was changed to `-lgcc_eh` - `gcc_s` is the GCC support library,
+   which Rust uses for unwinding support. This is only available as a dynamic
+   library, so we must specify the static version of the library providing
+   unwinding support.
+
+By running this command, you will likely see some warnings like
+
+``` text
+warning: Using 'getpwuid_r' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
+```
+
+These should be considered carefully! They indicate calls in glibc which
+*cannot* be statically linked without significant extra effort. An application
+using these calls will find it is not as portable as 'static binary' would imply.
+Rust supports targeting musl as an alternative libc to be able to fully
+statically link these calls.
+
+As we are confident that our code does not use these calls, we can now see the
+fruits of our labour:
+
+```
+$ ldd example
+        not a dynamic executable
+```
+
+This binary can be copied to virtually any 64-bit Linux machine and work
+without requiring external libraries.