Talk about cgo! again.

This is a creation in Article, where the information may have evolved or changed.

Recently on the Kubernetes to integrate other team algorithm things, their algorithm is written in C, naturally I need to use CGO to invoke. This article has organized some functions in the process of using CGO, which are hereby collated and recorded.

1, passing a two-dimensional integer array to the C function

The C function needs to receive a two-dimensional integer array as a parameter,

extern bool xxx_func(int** _matrix);

The initial implementation is as follows:

goArray := [][]int{{1},{3,3},{3,3,2}}cArray := make([][]C.int, len(goArray))for i, _ := range goArray {     cArray[i] = make([]C.int, len(goArray[i]))     for j, _ := range goArray[i] {         cArray[i][j] = C.int(goArray[i][j])         }} C.xxx_func((**C.int)(unsafe.Pointer(&cArray[0][0]))) // 把该数组的首地址传递给c函数

Is that the way it's done? Of course not! Looking closely at the function, this function actually creates a slice, then stores n separate slices in it, and its memory address is not contiguous! Of course, this is true for go itself, but for C it is a big mistake, the memory address of the two-dimensional array in C must be contiguous (the two-dimensional array in C is actually a one-dimensional array, addressing differently). So it should be done as follows:

• Assigns a slice of length n*m (N and m for rows and columns of a two-dimensional array)
• Populate this slice with values from the two-dimensional array of Go
• Pass this slice pointer to the C function

The first type of code is implemented as follows:

n, m := len(goArray), 0for _, row := range goArray {    if len(row) > m {        m = len(row)    }}cArray := make([]C.int, n*m)for i, row := range goArray {    for j, v := range row {        a[i * n + j] = v    }  

Then run, found cannot use &cArray[0] (type *C.int) as type **C.int in argument to func literal that the above operation is actually to convert the two-dimensional array of go to a one-dimensional array and then passed to the C function, but the C function to receive is a int** two-dimensional array, so error, the best solution is to change the C function parameters to receive int* a one-dimensional array.
But what if we can't change the parameters of the C function?
The second type of code implementation:

        cArray := make([]*C.int, len(goArray))        for i, row := range goArray {                p := (*C.int)(C.malloc(C.size_t(C.sizeof_int * len(row))))                cArray[i] = p                pa := (*[1 << 30]C.int)(unsafe.Pointer(p))                for j, v := range row {                        (*pa)[j] = C.int(v)                }        }

Here, we assign a source-sized c.int array for each row of the Goarray, and give the pointer p to CArray,
The P type *C.int is then converted to *[1<<30]C.int a type (pointing to a large array of c.int types), but there are two questions to note:

• Memory release for C.malloc ()
• We row have allocated an array for each row, but the array length information is missing! There's no problem with go, but for C, an array is just a pointer, and it doesn't know the length of the memory area through this pointer, you have to tell it, or write a struct that contains the pointer and length, which depends on the specific implementation of C.

The first method is recommended, because the memory release problem of CGO and C can toss you for a few days, unless you know clearly where to release and where not to release.

2, passing a string array to the C function

This is much simpler, but you need to be aware of the memory release. The code is as follows:

goString := []string{"w0", "w1", "w2", "w3"} cString := make([]*C.char, len(goString)) for i, _ := range goString {         cString[i] = C.CString(goString[i])         // 注意这个内存释放，该不该在调用完c函数之后释放，依赖于c那边的实现，这里要小心使用！        defer C.free(unsafe.Pointer(cString[i])) } C.xxx_func2((**C.char)(unsafe.Pointer(&cString[0])))

Official documentation on C.cstring:

// Go string to C string// The C string is allocated in the C heap using malloc.// It is the caller's responsibility to arrange for it to be// freed, such as by calling C.free (be sure to include stdlib.h// if C.free is needed).func C.CString(string) *C.char

3, convert c string array to go string slice

//你必须清楚地知道返回的c字符串数组的长度func GoStrings(length int, argv **C.char) []string {        if argv == nil {                return nil        }        tmpslice := (*[1 << 30]*C.char)(unsafe.Pointer(argv))[:length:length]        gostrings := make([]string, length)        for i, s := range tmpslice {                gostrings[i] = C.GoString(s)        }        return gostrings}

C.gostring's Official document, really concise: (

// C string to Go stringfunc C.GoString(*C.char) string

Reference: https://golang.org/cmd/cgo/
CGO Wiki:https://github.com/golang/go/wiki/cgo
Google Big God more: https://groups.google.com/forum/#!topic/golang-nuts/Nb-nfVdAyF0