How to use C in Go

Question

In the new Go language  how do I call C   code  In other words  how can I wrap my C   classes and use them in Go

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You might need to add -lc   to the LDFlags for Golang CGo to recognize the need for the standard library

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You re walking on uncharted territory here  Here is the Go example for calling C code  perhaps you can do something like that after reading up on C   name mangling and calling conventions  and lots of trial and error   If you still feel like trying it  good luck

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This can be achieved using command cgo   In essence   If the import of  C  is immediately preceded by a comment  that comment  called the preamble  is used as a header when compiling the C parts of the package  For example   source https   golang org cmd cgo        include  lt stdio h gt      include  lt errno h gt  import  C

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As of go1 2   cgo automatically incorporates and compiles C   code   http   golang org doc go1 2 cgo and cpp

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There s talk about interoperability between C and Go when using the gcc Go compiler  gccgo  There are limitations both to the interoperability and the implemented feature set of Go when using gccgo  however  e g   limited goroutines  no garbage collection

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Looks it s one of the early asked question about Golang   And same time answers to never update   During these three to four years   too many new libraries and blog post has been out   Below are the few links what I felt useful     SWIG and Go  Calling C   Code From Go With SWIG  On comparing languages  C   and Go  GoForCPPProgrammers

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Funny how many broader issues this announcement has dredged up  Dan Lyke had a very entertaining and thoughtful discussion on his website  Flutterby  about developing Interprocess Standards as a way of bootstrapping new languages  and other ramifications  but that s the one that is germane here

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The problem here is that a compliant implementation does not need to put your classes in a compile  cpp file  If the compiler can optimize out the existence of a class  so long as the program behaves the same way without it  then it can be omitted from the output executable   C has a standardized binary interface  Therefore you ll be able to know that your functions are exported  But C   has no such standard behind it

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I ve created the following example based on Scott Wales  answer  I ve tested it in macOS High Sierra 10 13 3 running go version go1 10 darwin amd64    1  Code for library hpp  the C   API we aim to call    pragma once class Foo    public    Foo int value      Foo      int value   const       private    int m value        2  Code for library cpp  the C   implementation    include  library hpp   include  lt iostream gt   Foo  Foo int value    m value value      std  cout  lt  lt    c    Foo  Foo    lt  lt  m value  lt  lt       lt  lt  std  endl     Foo   Foo     std  cout  lt  lt    c    Foo   Foo    lt  lt  m value  lt  lt       lt  lt  std  endl     int Foo  value   const     std  cout  lt  lt    c    Foo  value   is    lt  lt  m value  lt  lt  std  endl    return m value       3  Code for library-bridge h the bridge needed to expose a C API implemented in C   so that go can use it    pragma once  ifdef   cplusplus extern  C     endif  void  LIB NewFoo int value   void LIB DestroyFoo void  foo   int LIB FooValue void  foo     ifdef   cplusplus       extern  C   endif    4  Code for library-bridge cpp  the implementation of the bridge    include  lt iostream gt    include  library-bridge h   include  library hpp   void  LIB NewFoo int value      std  cout  lt  lt    c   bridge  LIB NewFoo    lt  lt  value  lt  lt       lt  lt  std  endl    auto foo   new Foo value     std  cout  lt  lt    c   bridge  LIB NewFoo    lt  lt  value  lt  lt     will return pointer                lt  lt  foo  lt  lt  std  endl    return foo        Utility function local to the bridge s implementation Foo  AsFoo void  foo    return reinterpret cast lt Foo  gt  foo      void LIB DestroyFoo void  foo      std  cout  lt  lt    c   bridge  LIB DestroyFoo    lt  lt  foo  lt  lt       lt  lt  std  endl    AsFoo foo - gt  Foo       int LIB FooValue void  foo      std  cout  lt  lt    c   bridge  LIB FooValue    lt  lt  foo  lt  lt       lt  lt  std  endl    return AsFoo foo - gt value         5  Finally  library go  the go program calling the C   API   package main      cgo LDFLAGS  -L  -llibrary     include  library-bridge h  import  C  import  unsafe  import  fmt   type Foo struct       ptr unsafe Pointer    func NewFoo value int  Foo       var foo Foo     foo ptr   C LIB NewFoo C int value       return foo    func  foo Foo  Free         C LIB DestroyFoo foo ptr     func  foo Foo  value   int       return int C LIB FooValue foo ptr      func main         foo    NewFoo 42      defer foo Free      The Go analog to C   s RAII     fmt Println   go    foo value        Using the following Makefile  liblibrary so  library cpp library-bridge cpp     clang   -o liblibrary so library cpp library-bridge cpp       -std c  17 -O3 -Wall -Wextra -fPIC -shared   I can run the example program as follows     make clang   -o liblibrary so library cpp library-bridge cpp       -std c  17 -O3 -Wall -Wextra -fPIC -shared   go run library go  c   bridge  LIB NewFoo 42   c    Foo  Foo 42   c   bridge  LIB NewFoo 42  will return pointer 0x42002e0  c   bridge  LIB FooValue 0x42002e0   c    Foo  value   is 42  go  42  c   bridge  LIB DestroyFoo 0x42002e0   c    Foo   Foo 42    Important  The comments above import  C  in the go program are NOT OPTIONAL  You must put them exactly as shown so that cgo knows which header and library to load  in this case       cgo LDFLAGS  -L  -llibrary     include  library-bridge h  import  C    Link to GitHub repo with the full example

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You can t quite yet from what I read in the FAQ      Do Go programs link with C C   programs       There are two Go compiler implementations  gc  the 6g program and friends  and gccgo  Gc uses a different calling convention and linker and can therefore only be linked with C programs using the same convention  There is such a C compiler but no C   compiler  Gccgo is a GCC front-end that can  with care  be linked with GCC-compiled C or C   programs       The cgo program provides the mechanism for a    foreign function interface    to allow safe calling of C libraries from Go code  SWIG extends this capability to C   libraries

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Seems that currently SWIG is best solution for this   http   www swig org Doc2 0 Go html  It supports inheritance and even allows to subclass C   class with Go struct so when overridden methods are called in C   code  Go code is fired   Section about C   in Go FAQ is updated and now mentions SWIG and no longer says  because Go is garbage-collected it will be unwise to do so  at least naively

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Update  I ve succeeded in linking a small test C   class with Go  If you wrap you C   code with a C interface you should be able to call your library with cgo  see the example of gmp in  GOROOT misc cgo gmp     I m not sure if the idea of a class in C   is really expressible in Go  as it doesn t have inheritance   Here s an example   I have a C   class defined as      foo hpp class cxxFoo   public    int a    cxxFoo int  a  a  a        cxxFoo        void Bar           foo cpp  include  lt iostream gt   include  foo hpp  void cxxFoo  Bar void     std  cout lt  lt this- gt a lt  lt std  endl      which I want to use in Go  I ll use the C interface     foo h  ifdef   cplusplus extern  C     endif   typedef void  Foo    Foo FooInit void     void FooFree Foo     void FooBar Foo    ifdef   cplusplus    endif    I use a void  instead of a C struct so the compiler knows the size of Foo   The implementation is     cfoo cpp  include  foo hpp   include  foo h  Foo FooInit       cxxFoo   ret   new cxxFoo 1     return  void  ret    void FooFree Foo f      cxxFoo   foo    cxxFoo  f    delete foo    void FooBar Foo f      cxxFoo   foo    cxxFoo  f    foo- gt Bar        with all that done  the Go file is      foo go package foo     include  foo h  import  C  import  unsafe  type GoFoo struct        foo C Foo    func New   GoFoo        var ret GoFoo       ret foo   C FooInit         return ret    func  f GoFoo Free         C FooFree unsafe Pointer f foo      func  f GoFoo Bar         C FooBar unsafe Pointer f foo        The makefile I used to compile this was      makefile TARG foo CGOFILES foo go include   GOROOT  src Make   GOARCH  include   GOROOT  src Make pkg foo o foo cpp     g      CGO CFLAGS   GOARCH   -fPIC -O2 -o    -c   CGO CFLAGS    lt  cfoo o cfoo cpp     g      CGO CFLAGS   GOARCH   -fPIC -O2 -o    -c   CGO CFLAGS    lt  CGO LDFLAGS  -lstdc     elem  foo so  foo cgo4 o foo o cfoo o     gcc    CGO CFLAGS   GOARCH      CGO LDFLAGS   GOOS   -o         CGO LDFLAGS    Try testing it with      foo test go package foo import  testing  func TestFoo t  testing T       foo    New        foo Bar        foo Free        You ll need to install the shared library with make install  then run make test  Expected output is   gotest rm -f  test foo a  gotest  6 6g -o  gotest  6 foo cgo1 go foo cgo2 go foo test go rm -f  test foo a gopack grc  test foo a  gotest  6  foo cgo3 6 1 PASS

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