GCO and C symbols defined out of go code

[ingvagabund]

Given /usr/lib/golang/src/net/cgo_resnew.go from stdlib:

package net

/*
#include <sys/types.h>
#include <sys/socket.h>

#include <netdb.h>
*/
import "C"

import "unsafe"

func cgoNameinfoPTR(b []byte, sa *C.struct_sockaddr, salen C.socklen_t) (int, error) {
        gerrno, err := C.getnameinfo(sa, salen, (*C.char)(unsafe.Pointer(&b[0])), C.socklen_t(len(b)), nil, 0, C.NI_NAMEREQD)
        return int(gerrno), err

definition of some C.name symbols are not known in advance from the go code itself:

• C.struct_sockaddr
• C.socklen_t
• C.getnameinfo
• C.char
• C.socklen_t
• C.NI_NAMEREQD

Type of some of the symbols can be determined based on they context:

• C.struct_sockaddr and C.socklen_t are data types as they are part of a function signature definition
• C.getnameinfo is function as it has used in a call expression and has more than one argument
• C.char is a data type as char is C language keyword for char type

Some can not:

• C.socklen_t: is it a function call or a data type casting? Given it is used as an parameter type, it is a type
• C.NI_NAMEREQD: is it a variable or a pointer to a function?

Man page for getnameinfo says:

NAME
       getnameinfo - address-to-name translation in protocol-independent manner

SYNOPSIS
       #include <sys/socket.h>
       #include <netdb.h>

       int getnameinfo(const struct sockaddr *addr, socklen_t addrlen,
                       char *host, socklen_t hostlen,
                       char *serv, socklen_t servlen, int flags);

So one can deduce:

• C.NI_NAMEREQD is expected to be a variable of type in as there is no call expression

One can process header files sys/types.h, sys/socket.h and netdb.h to get a list of types, variables/constants and functions. E.g. for each C header that is used in a go file provide a special file (e.g. sys/types.g.yaml) with C symbol definitions (just for illustration, types and functions are defined across multiple header file):

header_file: "header/file/name.h"
symbols:
types:
  - type: struct_sockaddr
  - type: socklen_t
functions:
- function: getnameinfo

Or, the yaml file can be defined per cgo go file as well as cgo_resnew.go.yaml or per multiple cgo files as cgo.yaml.

Additional links

• https://github.com/AlekSi/cgo-by-example/blob/master/main.go
• https://golang.org/cmd/cgo/
• https://blog.golang.org/c-go-cgo
• https://groups.google.com/forum/#!topic/golang-nuts/o-l82ZHoRCA

[ingvagabund]

@pirat89