environment variable when running the go tool: set it to 1 to enable
the use of cgo, and to 0 to disable it. The go tool will set the
build constraint "cgo" if cgo is enabled. The special import "C"
implies the "cgo" build constraint, as though the file also said
"//go:build cgo". Therefore, if cgo is disabled, files that import
"C" will not be built by the go tool. (For more about build constraints

pkg syscall (openbsd-386-cgo), const EBADMSG = 92 #67998
pkg syscall (openbsd-386-cgo), const ELAST = 95 #67998
pkg syscall (openbsd-386-cgo), const ENOTRECOVERABLE = 93 #67998
pkg syscall (openbsd-386-cgo), const ENOTRECOVERABLE Errno #67998
pkg syscall (openbsd-386-cgo), const EOWNERDEAD = 94 #67998
pkg syscall (openbsd-386-cgo), const EOWNERDEAD Errno #67998
pkg syscall (openbsd-386-cgo), const EPROTO = 95 #67998

pkg syscall (freebsd-arm64-cgo), const AF_ISO = 7
pkg syscall (freebsd-arm64-cgo), const AF_ISO ideal-int
pkg syscall (freebsd-arm64-cgo), const AF_LAT = 14
pkg syscall (freebsd-arm64-cgo), const AF_LAT ideal-int
pkg syscall (freebsd-arm64-cgo), const AF_LINK = 18
pkg syscall (freebsd-arm64-cgo), const AF_LINK ideal-int
pkg syscall (freebsd-arm64-cgo), const AF_LOCAL = 1

pkg log/syslog (linux-386-cgo), const LOG_LOCAL7 Priority
pkg log/syslog (linux-386-cgo), const LOG_LPR = 48
pkg log/syslog (linux-386-cgo), const LOG_LPR Priority
pkg log/syslog (linux-386-cgo), const LOG_MAIL = 16
pkg log/syslog (linux-386-cgo), const LOG_MAIL Priority
pkg log/syslog (linux-386-cgo), const LOG_NEWS = 56
pkg log/syslog (linux-386-cgo), const LOG_NEWS Priority

pkg syscall (freebsd-386-cgo), type Dirent struct, Off int64
pkg syscall (freebsd-386-cgo), type Dirent struct, Pad0 uint8
pkg syscall (freebsd-386-cgo), type Dirent struct, Pad1 uint16
pkg syscall (freebsd-386-cgo), type Stat_t struct, Atim_ext int32
pkg syscall (freebsd-386-cgo), type Stat_t struct, Blksize int32
pkg syscall (freebsd-386-cgo), type Stat_t struct, Btim_ext int32

pkg syscall (openbsd-386-cgo), const SOCK_CLOEXEC = 32768
pkg syscall (openbsd-386-cgo), const SOCK_CLOEXEC ideal-int
pkg syscall (openbsd-386-cgo), const SOCK_NONBLOCK = 16384
pkg syscall (openbsd-386-cgo), const SOCK_NONBLOCK ideal-int
pkg syscall (openbsd-386-cgo), const SYS_ACCEPT4 = 93
pkg syscall (openbsd-386-cgo), const SYS_ACCEPT4 ideal-int
pkg syscall (openbsd-386-cgo), const SYS_PIPE2 = 101

The stanza

	// #include <gmp.h>
	import "C"

is a signal to cgo.  The doc comment on the import of "C" provides
additional context for the C file.  Here it is just a single #include
but it could contain arbitrary C definitions to be imported and used.

Cgo recognizes any use of a qualified identifier C.xxx and uses gcc to
find the definition of xxx.  If xxx is a type, cgo replaces C.xxx with

pkg syscall (freebsd-386-cgo), type SysProcAttr struct, Ctty int
pkg syscall (freebsd-386-cgo), type SysProcAttr struct, Foreground bool
pkg syscall (freebsd-386-cgo), type SysProcAttr struct, Pgid int
pkg syscall (freebsd-amd64), type SysProcAttr struct, Ctty int
pkg syscall (freebsd-amd64), type SysProcAttr struct, Foreground bool
pkg syscall (freebsd-amd64), type SysProcAttr struct, Pgid int
pkg syscall (freebsd-amd64-cgo), type SysProcAttr struct, Ctty int

pkg syscall (linux-386-cgo), type SysProcAttr struct, GidMappings []SysProcIDMap
pkg syscall (linux-386-cgo), type SysProcAttr struct, UidMappings []SysProcIDMap
pkg syscall (linux-386-cgo), type SysProcIDMap struct
pkg syscall (linux-386-cgo), type SysProcIDMap struct, ContainerID int
pkg syscall (linux-386-cgo), type SysProcIDMap struct, HostID int
pkg syscall (linux-386-cgo), type SysProcIDMap struct, Size int

## Tools {#tools}

### Go command {#go-command}