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