131 lines
3.2 KiB
Markdown
131 lines
3.2 KiB
Markdown
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---
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layout: post
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title: "Notes on cgo"
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date: 2015-03-29 00:28
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comments: true
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categories:
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- golang
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- c
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---
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## Referencing c from go.
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### Basic operation
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import "C" imports the C namespace into go, the comment just above this statement is treated as C.
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```c
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/*
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#include "add.h"
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*/
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import "C"
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```
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Everything you exported from c library is available in the C namespace.
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```go
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func main() {
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fmt.Println(
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C.add_two_numbers(3,4)
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)
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}
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```
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Building code is as easy as `go build`
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Accessing standard types are easy as `C.char`, `C.int`. For complex types, prepend the type name with an underscore at end – e.g `C.struct_stat`.
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### Pointers and memory operations
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Go provides an `unsafe` module. Go’s GC cannot manage the memory allocated in C code, you should call `C.free(unsafe.Pointer)` (`free` is defined in `<stdlib.h>` ensure that this library is imported) to deallocate.
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There are some special functions which convert Go types to C types and vice versa. (lifted from cgo documentation)
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```go
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// Go string to C string
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// The C string is allocated in the C heap using malloc.
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// It is the caller's responsibility to arrange for it to be
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// freed, such as by calling C.free (be sure to include stdlib.h
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// if C.free is needed).
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func C.CString(string) *C.char
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// C string to Go string
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func C.GoString(*C.char) string
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// C string, length to Go string
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func C.GoStringN(*C.char, C.int) string
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// C pointer, length to Go []byte
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func C.GoBytes(unsafe.Pointer, C.int) []byte
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```
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### Accessing complex objects
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Accessing structs from Go
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```go
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/*
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struct person {
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int age;
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*/
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// this can be used in go as
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var p C.struct_person
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p.age = 23
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```
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You can also pass pointers
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```go
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C.function_accepts_ptr_to_struct(&p)
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```
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Accessing Unions : No native counterpart, instead of converting it to a type, go treats them as a block of memory represented as byte array. Accessing data is done by casting unsafe.Pointer
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```go
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/*
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union quant {
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float weight;
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int count;
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};
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*/
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var q C.union_quant
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ptr := (*float32)(unsafe.Pointer(&q))
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*ptr = 3.14
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```
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## Extra goodies
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### Compiler Flags
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Compiler flags can be set using the `#cgo` directive. (`CFLAGS`, `LDFLAGS` etc)
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```go
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/*
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#cgo: LDFLAGS: -lmath
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#include <math.h>
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*/
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import "C"
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```
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### Restricting building
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In many cases using cgo breaks the portability of your app, use build constraints to specify compatibility in source. Build constraints are comments which begin with
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```go
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// +build
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```
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Now, build comment must appear before package clause and should be followed by a blank line To restrict building just to Linux with cgo,
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```go
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// +build linux,cgo !darwin
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package main
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```
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These are evaluated as `OR` of each constraint so the above line becomes `(linux AND cgo) OR (NOT darwin)`, also multiple build tags can be embedded in file.
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There are other methods to restrict builds, see references.
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## Reference
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- [Link to example code gist](https://gist.github.com/dbalan/ace29f0c43638ee4f81d)
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- [cgo Talk by Rajesh Ramachandran (Gophercon India)](https://www.youtube.com/watch?v=oeEVtyypYPg)
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- [Conditional compilation with go build](http://dave.cheney.net/2013/10/12/how-to-use-conditional-compilation-with-the-go-build-tool)
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- [cgo compiler directives](http://golang.org/cmd/cgo/)
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