Use the C language code instance in Golang _golang

CGO makes it possible to use C code in Golang.

Hello World

To have a more intuitive understanding, let's look at a simple example of creating a file Main.go:

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Package Main

/*
#include <stdio.h>

void Sayhi () {
printf ("Hi");
}
*/
Import "C"

Func Main () {
C.sayhi ()
}

Execution procedure:

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Go Run main.go

The program executes and outputs hi (more examples can be found $GOROOT/misc/cgo).

Preparations under Windows

If you want to use CGO on Windows, you need to install the GCC compiler, where I use MINGW-W64.

Set up compilation and link flags

We use import "C" to introduce a pseudo package (pseudo-package), through which we use C code. Before import "C", comments that follow the import "C" can include:

1. Compiler and linker flags
2.C Code

We can set compiler and linker flags by #cgo directives, for example:

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#cgo Cflags:-dpng_debug=1
#cgo AMD64 386 cflags:-dx86=1
#cgo Ldflags:-lpng
#include <png.h>
Import "C"

Incidentally, these directives can contain build constraints (builds constraint), as detailed in the following: Http://golang.org/pkg/go/build/.

The commonly used #cgo directives are:

The 1.CPPFLAGS, cflags instruction is used to compile C files in the current package (any. C,. S,. S file)
2.CPPFLAGS, cxxflags directives are used to compile C + + files in the current package (any. cpp,. cc,. cxx files)
The 3.LDFLAGS directive is used to specify the linker flag
The 4.pkg-config directive is used to get compiler and linker flags through the Pkg-config tool (for example: #cgo pkg-config:png Cairo)

Golang Reference C

The points to be noted on the structural body:

1.C structure of the domain name if the Golang keyword, the access needs to be in front of the domain name plus _. For example, there is a struct variable x in C, and there is a field type in the corresponding structure of this variable, then the Type field needs to be accessed through X._type in Golang
2. The structure body's bit field, the misaligned data and so on cannot be expressed in the Golang, will be ignored
C-type fields cannot be used in 3.Golang structures

The standard C numeric type corresponds to:

1.c.char
2.c.schar (signed Char)
3.c.uchar (unsigned char)
4.c.short
5.c.ushort (unsigned short)
6.c.int
7.c.uint (unsigned int)
8.c.long
9.c.ulong (unsigned long)
10.c.longlong (Long Long)
11.c.ulonglong (unsigned long Long)
12.c.float
13.c.double

Any C function (including a void function) can return a return value and C's errno variable (as an error):

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N, Err: = C.sqrt (-1)
_, Err: = C.voidfunc ()

The direct call to the C function pointer is not yet supported.

There are special functions that can be used to convert between C type and Golang type (by copy of data), and the pseudo definition is as follows:

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Golang string to C string
C's string is allocated using malloc, so the caller of this function
Need to invoke C.free to free memory
Func c.cstring (String) *c.char

Convert C string to Golang string
Func c.gostring (*c.char) string

Converts a certain length of a C string to a Golang string
Func c.gostringn (*c.char, C.int) string

Converts a chunk of C memory to a Golang byte array
Func c.gobytes (unsafe. pointer, c.int) []byte

Other points to note:

The void* in the 1.C language corresponds to unsafe. Pointer
Structures, unions, enum types (not variables) in the 2.C language, and Struct_, Union_, and enum_ prefixes are required to be accessed in Golang. Because this data type is not federated in Golang, the union of C is represented as an array of bytes in Golang
3. Those types that are equivalent to the C language are not exportable