英文:
How to pass pointer to slice to C function in go
问题
背景：使用cgo从Golang调用C函数。
我想使用一个具有以下签名的C函数：int f(int *count, char ***strs)。
它会修改count和strs的数据，这就是它使用指针的原因。
count的值是strs的长度；strs是一个字符串数组；返回值只是一个（布尔）指示器，指示是否存在错误。
在Golang中，我可以通过使用C.f((*C.int)(&count))成功传递和修改count；通过使用[]*C.char传递[]string。示例代码如下：
/*
#include <stdio.h>
int f(int *c, char **str) {
int i;
printf("%d\n", *c);
for (i = 0; i < *c; i++) {
printf("%s\n", str[i]);
}
*c = (*c) + 1;
return 1;
}
*/
import "C"
func go_f(strs []string) int {
count := len(strs)
c_count := C.int(count)
c_strs := make([]*C.char, count)
for index, value := range strs {
c_strs[index] = C.CString(value)
defer C.free(unsafe.Pointer(c_strs[index]))
}
err := C.f(&c_argc, (**C.char)(&c_argv[0]))
return int(err)
}
如你所见，C函数目前是int f(int *c, char **str)，但我想要的是int f(int *c, char ***str)。
也就是说：我实际上想要的是在C中对字符串数组进行修改（例如调整大小），然后将其转换回Go的字符串切片，以便我仍然可以在Go中使用它。
如何做到这一点？我已经搜索和尝试了一段时间，但没有成功。
英文:
Background: using cgo to call C functions from Golang.
I want to use a C function which has this signature: int f(int *count, char ***strs).
It will modify the data of count and strs, which is the reason why it uses pointer to them.
The value of count is the length of strs; strs is an array of string; the return value is simply an (boolean) indicator which states whether there is an error or not.
In golang, I can successfully pass and modify count by using C.f((*C.int)(&count)); pass []string by using []*C.char. Sample code is like this:
/*
#include <stdio.h>
int f(int *c, char **str) {
int i;
printf("%d\n", *c);
for (i = 0; i < *c; i++) {
printf("%s\n", str[i]);
}
*c = (*c) + 1;
return 1;
}
*/
import "C"
func go_f(strs []string) int {
count := len(strs)
c_count := C.int(count)
c_strs := make([]*C.char, count)
for index, value := range strs {
c_strs[index] = C.CString(value)
defer C.free(unsafe.Pointer(c_strs[index]))
}
err := C.f(&c_argc, (**C.char)(&c_argv[0]))
return int(err)
}
As you can see, the C function is currently int f(int *c, char **str), but what I'd like is int f(int *c, char ***str).
This is to say: what I actually want is to enable the modification to the string array (e.g. resize) in C and turn it back to a Go string slice so I can still use it in Go.
How to do this? I've searched and experimented for a while but with no luck.
答案1
得分: 6
一个Go切片既在Go中分配，又是一个与C数组不同的数据结构，所以你不能将它传递给C函数（cgo也会阻止你这样做，因为切片包含一个Go指针）。
你需要在C中分配数组，以便在C中操作数组。就像使用C.CString一样，你还需要跟踪在哪里释放外部数组，特别是如果C函数可能分配一个新数组。
cArray := C.malloc(C.size_t(c_count) * C.size_t(unsafe.Sizeof(uintptr(0))))
// 将C数组转换为Go数组，以便我们可以对其进行索引
a := (*[1<<30 - 1]*C.char)(cArray)
for index, value := range strs {
a[index] = C.CString(value)
}
err := C.f(&c_count, (***C.char)(unsafe.Pointer(&cArray)))
英文:
A Go slice is both allocated in Go, and a different data structure than a C array, so you can't pass it to a C function (cgo will also prevent you from doing this because a slice contains a Go pointer)
You need to allocate the array in C in order to manipulate the array in C. Just like with C.CString, you will also need to track where to free the outer array, especially if the C function may possibly allocate a new array.
cArray := C.malloc(C.size_t(c_count) * C.size_t(unsafe.Sizeof(uintptr(0))))
// convert the C array to a Go Array so we can index it
a := (*[1<<30 - 1]*C.char)(cArray)
for index, value := range strs {
a[index] = C.CString(value)
}
err := C.f(&c_count, (***C.char)(unsafe.Pointer(&cArray)))
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论