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+# Building `sys/unix`
+
+The sys/unix package provides access to the raw system call interface of the
+underlying operating system. See: https://godoc.org/golang.org/x/sys/unix
+
+Porting Go to a new architecture/OS combination or adding syscalls, types, or
+constants to an existing architecture/OS pair requires some manual effort;
+however, there are tools that automate much of the process.
+
+## Build Systems
+
+There are currently two ways we generate the necessary files. We are currently
+migrating the build system to use containers so the builds are reproducible.
+This is being done on an OS-by-OS basis. Please update this documentation as
+components of the build system change.
+
+### Old Build System (currently for `GOOS != "linux"`)
+
+The old build system generates the Go files based on the C header files
+present on your system. This means that files
+for a given GOOS/GOARCH pair must be generated on a system with that OS and
+architecture. This also means that the generated code can differ from system
+to system, based on differences in the header files.
+
+To avoid this, if you are using the old build system, only generate the Go
+files on an installation with unmodified header files. It is also important to
+keep track of which version of the OS the files were generated from (ex.
+Darwin 14 vs Darwin 15). This makes it easier to track the progress of changes
+and have each OS upgrade correspond to a single change.
+
+To build the files for your current OS and architecture, make sure GOOS and
+GOARCH are set correctly and run `mkall.sh`. This will generate the files for
+your specific system. Running `mkall.sh -n` shows the commands that will be run.
+
+Requirements: bash, go
+
+### New Build System (currently for `GOOS == "linux"`)
+
+The new build system uses a Docker container to generate the go files directly
+from source checkouts of the kernel and various system libraries. This means
+that on any platform that supports Docker, all the files using the new build
+system can be generated at once, and generated files will not change based on
+what the person running the scripts has installed on their computer.
+
+The OS specific files for the new build system are located in the `${GOOS}`
+directory, and the build is coordinated by the `${GOOS}/mkall.go` program. When
+the kernel or system library updates, modify the Dockerfile at
+`${GOOS}/Dockerfile` to checkout the new release of the source.
+
+To build all the files under the new build system, you must be on an amd64/Linux
+system and have your GOOS and GOARCH set accordingly. Running `mkall.sh` will
+then generate all of the files for all of the GOOS/GOARCH pairs in the new build
+system. Running `mkall.sh -n` shows the commands that will be run.
+
+Requirements: bash, go, docker
+
+## Component files
+
+This section describes the various files used in the code generation process.
+It also contains instructions on how to modify these files to add a new
+architecture/OS or to add additional syscalls, types, or constants. Note that
+if you are using the new build system, the scripts/programs cannot be called normally.
+They must be called from within the docker container.
+
+### asm files
+
+The hand-written assembly file at `asm_${GOOS}_${GOARCH}.s` implements system
+call dispatch. There are three entry points:
+```
+  func Syscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
+  func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
+  func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
+```
+The first and second are the standard ones; they differ only in how many
+arguments can be passed to the kernel. The third is for low-level use by the
+ForkExec wrapper. Unlike the first two, it does not call into the scheduler to
+let it know that a system call is running.
+
+When porting Go to a new architecture/OS, this file must be implemented for
+each GOOS/GOARCH pair.
+
+### mksysnum
+
+Mksysnum is a Go program located at `${GOOS}/mksysnum.go` (or `mksysnum_${GOOS}.go`
+for the old system). This program takes in a list of header files containing the
+syscall number declarations and parses them to produce the corresponding list of
+Go numeric constants. See `zsysnum_${GOOS}_${GOARCH}.go` for the generated
+constants.
+
+Adding new syscall numbers is mostly done by running the build on a sufficiently
+new installation of the target OS (or updating the source checkouts for the
+new build system). However, depending on the OS, you may need to update the
+parsing in mksysnum.
+
+### mksyscall.go
+
+The `syscall.go`, `syscall_${GOOS}.go`, `syscall_${GOOS}_${GOARCH}.go` are
+hand-written Go files which implement system calls (for unix, the specific OS,
+or the specific OS/Architecture pair respectively) that need special handling
+and list `//sys` comments giving prototypes for ones that can be generated.
+
+The mksyscall.go program takes the `//sys` and `//sysnb` comments and converts
+them into syscalls. This requires the name of the prototype in the comment to
+match a syscall number in the `zsysnum_${GOOS}_${GOARCH}.go` file. The function
+prototype can be exported (capitalized) or not.
+
+Adding a new syscall often just requires adding a new `//sys` function prototype
+with the desired arguments and a capitalized name so it is exported. However, if
+you want the interface to the syscall to be different, often one will make an
+unexported `//sys` prototype, and then write a custom wrapper in
+`syscall_${GOOS}.go`.
+
+### types files
+
+For each OS, there is a hand-written Go file at `${GOOS}/types.go` (or
+`types_${GOOS}.go` on the old system). This file includes standard C headers and
+creates Go type aliases to the corresponding C types. The file is then fed
+through godef to get the Go compatible definitions. Finally, the generated code
+is fed though mkpost.go to format the code correctly and remove any hidden or
+private identifiers. This cleaned-up code is written to
+`ztypes_${GOOS}_${GOARCH}.go`.
+
+The hardest part about preparing this file is figuring out which headers to
+include and which symbols need to be `#define`d to get the actual data
+structures that pass through to the kernel system calls. Some C libraries
+preset alternate versions for binary compatibility and translate them on the
+way in and out of system calls, but there is almost always a `#define` that can
+get the real ones.
+See `types_darwin.go` and `linux/types.go` for examples.
+
+To add a new type, add in the necessary include statement at the top of the
+file (if it is not already there) and add in a type alias line. Note that if
+your type is significantly different on different architectures, you may need
+some `#if/#elif` macros in your include statements.
+
+### mkerrors.sh
+
+This script is used to generate the system's various constants. This doesn't
+just include the error numbers and error strings, but also the signal numbers
+and a wide variety of miscellaneous constants. The constants come from the list
+of include files in the `includes_${uname}` variable. A regex then picks out
+the desired `#define` statements, and generates the corresponding Go constants.
+The error numbers and strings are generated from `#include <errno.h>`, and the
+signal numbers and strings are generated from `#include <signal.h>`. All of
+these constants are written to `zerrors_${GOOS}_${GOARCH}.go` via a C program,
+`_errors.c`, which prints out all the constants.
+
+To add a constant, add the header that includes it to the appropriate variable.
+Then, edit the regex (if necessary) to match the desired constant. Avoid making
+the regex too broad to avoid matching unintended constants.
+
+### internal/mkmerge
+
+This program is used to extract duplicate const, func, and type declarations
+from the generated architecture-specific files listed below, and merge these
+into a common file for each OS.
+
+The merge is performed in the following steps:
+1. Construct the set of common code that is identical in all architecture-specific files.
+2. Write this common code to the merged file.
+3. Remove the common code from all architecture-specific files.
+
+
+## Generated files
+
+### `zerrors_${GOOS}_${GOARCH}.go`
+
+A file containing all of the system's generated error numbers, error strings,
+signal numbers, and constants. Generated by `mkerrors.sh` (see above).
+
+### `zsyscall_${GOOS}_${GOARCH}.go`
+
+A file containing all the generated syscalls for a specific GOOS and GOARCH.
+Generated by `mksyscall.go` (see above).
+
+### `zsysnum_${GOOS}_${GOARCH}.go`
+
+A list of numeric constants for all the syscall number of the specific GOOS
+and GOARCH. Generated by mksysnum (see above).
+
+### `ztypes_${GOOS}_${GOARCH}.go`
+
+A file containing Go types for passing into (or returning from) syscalls.
+Generated by godefs and the types file (see above).