Text file src/cmd/vendor/golang.org/x/sys/unix/README.md
1 # Building `sys/unix`
2
3 The sys/unix package provides access to the raw system call interface of the
4 underlying operating system. See: https://godoc.org/golang.org/x/sys/unix
5
6 Porting Go to a new architecture/OS combination or adding syscalls, types, or
7 constants to an existing architecture/OS pair requires some manual effort;
8 however, there are tools that automate much of the process.
9
10 ## Build Systems
11
12 There are currently two ways we generate the necessary files. We are currently
13 migrating the build system to use containers so the builds are reproducible.
14 This is being done on an OS-by-OS basis. Please update this documentation as
15 components of the build system change.
16
17 ### Old Build System (currently for `GOOS != "linux"`)
18
19 The old build system generates the Go files based on the C header files
20 present on your system. This means that files
21 for a given GOOS/GOARCH pair must be generated on a system with that OS and
22 architecture. This also means that the generated code can differ from system
23 to system, based on differences in the header files.
24
25 To avoid this, if you are using the old build system, only generate the Go
26 files on an installation with unmodified header files. It is also important to
27 keep track of which version of the OS the files were generated from (ex.
28 Darwin 14 vs Darwin 15). This makes it easier to track the progress of changes
29 and have each OS upgrade correspond to a single change.
30
31 To build the files for your current OS and architecture, make sure GOOS and
32 GOARCH are set correctly and run `mkall.sh`. This will generate the files for
33 your specific system. Running `mkall.sh -n` shows the commands that will be run.
34
35 Requirements: bash, go
36
37 ### New Build System (currently for `GOOS == "linux"`)
38
39 The new build system uses a Docker container to generate the go files directly
40 from source checkouts of the kernel and various system libraries. This means
41 that on any platform that supports Docker, all the files using the new build
42 system can be generated at once, and generated files will not change based on
43 what the person running the scripts has installed on their computer.
44
45 The OS specific files for the new build system are located in the `${GOOS}`
46 directory, and the build is coordinated by the `${GOOS}/mkall.go` program. When
47 the kernel or system library updates, modify the Dockerfile at
48 `${GOOS}/Dockerfile` to checkout the new release of the source.
49
50 To build all the files under the new build system, you must be on an amd64/Linux
51 system and have your GOOS and GOARCH set accordingly. Running `mkall.sh` will
52 then generate all of the files for all of the GOOS/GOARCH pairs in the new build
53 system. Running `mkall.sh -n` shows the commands that will be run.
54
55 Requirements: bash, go, docker
56
57 ## Component files
58
59 This section describes the various files used in the code generation process.
60 It also contains instructions on how to modify these files to add a new
61 architecture/OS or to add additional syscalls, types, or constants. Note that
62 if you are using the new build system, the scripts/programs cannot be called normally.
63 They must be called from within the docker container.
64
65 ### asm files
66
67 The hand-written assembly file at `asm_${GOOS}_${GOARCH}.s` implements system
68 call dispatch. There are three entry points:
69 ```
70 func Syscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
71 func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
72 func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
73 ```
74 The first and second are the standard ones; they differ only in how many
75 arguments can be passed to the kernel. The third is for low-level use by the
76 ForkExec wrapper. Unlike the first two, it does not call into the scheduler to
77 let it know that a system call is running.
78
79 When porting Go to an new architecture/OS, this file must be implemented for
80 each GOOS/GOARCH pair.
81
82 ### mksysnum
83
84 Mksysnum is a Go program located at `${GOOS}/mksysnum.go` (or `mksysnum_${GOOS}.go`
85 for the old system). This program takes in a list of header files containing the
86 syscall number declarations and parses them to produce the corresponding list of
87 Go numeric constants. See `zsysnum_${GOOS}_${GOARCH}.go` for the generated
88 constants.
89
90 Adding new syscall numbers is mostly done by running the build on a sufficiently
91 new installation of the target OS (or updating the source checkouts for the
92 new build system). However, depending on the OS, you make need to update the
93 parsing in mksysnum.
94
95 ### mksyscall.go
96
97 The `syscall.go`, `syscall_${GOOS}.go`, `syscall_${GOOS}_${GOARCH}.go` are
98 hand-written Go files which implement system calls (for unix, the specific OS,
99 or the specific OS/Architecture pair respectively) that need special handling
100 and list `//sys` comments giving prototypes for ones that can be generated.
101
102 The mksyscall.go program takes the `//sys` and `//sysnb` comments and converts
103 them into syscalls. This requires the name of the prototype in the comment to
104 match a syscall number in the `zsysnum_${GOOS}_${GOARCH}.go` file. The function
105 prototype can be exported (capitalized) or not.
106
107 Adding a new syscall often just requires adding a new `//sys` function prototype
108 with the desired arguments and a capitalized name so it is exported. However, if
109 you want the interface to the syscall to be different, often one will make an
110 unexported `//sys` prototype, an then write a custom wrapper in
111 `syscall_${GOOS}.go`.
112
113 ### types files
114
115 For each OS, there is a hand-written Go file at `${GOOS}/types.go` (or
116 `types_${GOOS}.go` on the old system). This file includes standard C headers and
117 creates Go type aliases to the corresponding C types. The file is then fed
118 through godef to get the Go compatible definitions. Finally, the generated code
119 is fed though mkpost.go to format the code correctly and remove any hidden or
120 private identifiers. This cleaned-up code is written to
121 `ztypes_${GOOS}_${GOARCH}.go`.
122
123 The hardest part about preparing this file is figuring out which headers to
124 include and which symbols need to be `#define`d to get the actual data
125 structures that pass through to the kernel system calls. Some C libraries
126 preset alternate versions for binary compatibility and translate them on the
127 way in and out of system calls, but there is almost always a `#define` that can
128 get the real ones.
129 See `types_darwin.go` and `linux/types.go` for examples.
130
131 To add a new type, add in the necessary include statement at the top of the
132 file (if it is not already there) and add in a type alias line. Note that if
133 your type is significantly different on different architectures, you may need
134 some `#if/#elif` macros in your include statements.
135
136 ### mkerrors.sh
137
138 This script is used to generate the system's various constants. This doesn't
139 just include the error numbers and error strings, but also the signal numbers
140 an a wide variety of miscellaneous constants. The constants come from the list
141 of include files in the `includes_${uname}` variable. A regex then picks out
142 the desired `#define` statements, and generates the corresponding Go constants.
143 The error numbers and strings are generated from `#include <errno.h>`, and the
144 signal numbers and strings are generated from `#include <signal.h>`. All of
145 these constants are written to `zerrors_${GOOS}_${GOARCH}.go` via a C program,
146 `_errors.c`, which prints out all the constants.
147
148 To add a constant, add the header that includes it to the appropriate variable.
149 Then, edit the regex (if necessary) to match the desired constant. Avoid making
150 the regex too broad to avoid matching unintended constants.
151
152
153 ## Generated files
154
155 ### `zerror_${GOOS}_${GOARCH}.go`
156
157 A file containing all of the system's generated error numbers, error strings,
158 signal numbers, and constants. Generated by `mkerrors.sh` (see above).
159
160 ### `zsyscall_${GOOS}_${GOARCH}.go`
161
162 A file containing all the generated syscalls for a specific GOOS and GOARCH.
163 Generated by `mksyscall.go` (see above).
164
165 ### `zsysnum_${GOOS}_${GOARCH}.go`
166
167 A list of numeric constants for all the syscall number of the specific GOOS
168 and GOARCH. Generated by mksysnum (see above).
169
170 ### `ztypes_${GOOS}_${GOARCH}.go`
171
172 A file containing Go types for passing into (or returning from) syscalls.
173 Generated by godefs and the types file (see above).
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