[英]Calling variadic C function from Swift with pointers
我试图弄清楚如何调用从 Swift 写入指针的可变参数 C 函数,例如vsscanf
,但我不明白如何实际构造指向 Swift 变量的指针列表。
我认为如果我有一个字符串,我可以获得一个UnsafePointer<CChar>
并对其调用vsscanf
,但是......我如何告诉它实际写入数据的位置? 如何构造CVaListPointer
以传递给vsscanf
?
var a: Int
var b: Float
"(5, 3.14)".withCString{buffer in
let r = vsscanf(buffer, "(%d, %f)", /* how do I put a and b here? */)
}
基本上,做与此处相同的事情 (C):
#include <stdio.h>
#include <stdarg.h>
int parse(const char *buffer, char *format, ...)
{
va_list args;
va_start(args, format);
int result = vsscanf(buffer, format, args);
va_end(args);
return result;
}
int main(int argc, char const *argv[])
{
int a;
float b;
char s[] = "(5, 3.14)";
int r = parse(s, "(%d, %f)", &a, &b);
printf("a: %d, b: %f\n", a, b);
// "a: 5, b: 3.140000"
return 0;
}
更新:多亏了到目前为止的答案,我觉得我更好地理解了它是如何工作的,但我仍在努力解决的问题是以更简单的方式填充CVaListPointer
。 下面是从字符串中解析 Double 三元组的示例:
func parseVec3(s: String) -> (Double, Double, Double)? {
var x: Double = 0
var y: Double = 0
var z: Double = 0
let r = s.withCString { buffer in
withUnsafeMutablePointer(to: &x) { ptrx in
withUnsafeMutablePointer(to: &y) { ptry in
withUnsafeMutablePointer(to: &z) { ptrz in
withVaList([ptrx, ptry, ptrz]) { va in
return vsscanf(buffer, "(%lf %lf %lf)", va)
}
}
}
}
}
return r == 3 ? (x, y, z) : nil
}
if let v = parseVec3(s: "(1 2 3)") {
print(v)
}
现在,这确实有效。 但我的问题是我正在解析一个文件,其中大部分行(成千上万行)是六组数字的三元组。 withUnsafeMutablePointer
的高耸结构看起来非常荒谬。 我确定我可以使用更多 Swift 原生方法(或只是正则表达式)来解析它,但我希望只使用vsscanf
,因为在 C 中解析这个文件非常简单:
int main(int argc, char const *argv[])
{
char s[] = "(1 2 3) (5 9 1) (0 5 8)";
Vec3 a, b, c = {0, 0, 0};
sscanf(s,
"(%f %f %f) (%f %f %f) (%f %f %f)",
&(a.x), &(a.y), &(a.z),
&(b.x), &(b.y), &(b.z),
&(c.x), &(c.y), &(c.z)
);
printf("a: (x: %f, y: %f, z: %f)\n", a.x, a.y, a.z);
printf("b: (x: %f, y: %f, z: %f)\n", b.x, b.y, b.z);
printf("c: (x: %f, y: %f, z: %f)\n", c.x, c.y, c.z);
return 0;
}
如上所述使用 Swift 中的withUnsafeMutablePointer
方法执行此操作将导致11 with<Whatever>
范围,这只是解析的浮点数的一半......
我想我应该可以做这样的事情,但我不知道如何获得指向其他结构成员的指针偏移量:
func parseVec3_3(s: String) -> Vector3? {
var output = Vector3(x: 0, y: 0, z: 0)
var r: CInt = 0
s.withCString { buffer in
withUnsafeMutablePointer(to: &output) { ptr in
withVaList([ptr, /* offset pointer here */]) { va in
r = vsscanf(buffer, "(%lf %lf %lf)", va)
}
}
}
return r == 3 ? output : nil
}
像这样:
var a: Int = 0
var b: Float = 0
withUnsafePointer(to: &a) { pointerToA in
withUnsafePointer(to: &b) { pointerToB in
withVaList([pointerToA, pointerToB]) { va_list in
"(5, 3.14)".withCString { buffer in
let r = vsscanf(buffer, "(%d, %f)", va_list)
}
}
}
}
print(a)
print(b)
产出
5
3.14
更新
OP编辑的答案包括关于Vector3(x: 0, y: 0, z: 0)
的问题应该变成:
public struct Vector3: Equatable {
public var x: CGFloat
public var y: CGFloat
public var z: CGFloat
public init(x: CGFloat, y: CGFloat, z: CGFloat) {
self.x = x
self.y = y
self.z = z
}
}
func parseVec3_3(s: String) -> Vector3? {
var output = Vector3(x: 0, y: 0, z: 0)
var r: CInt = 0
s.withCString { buffer in
withUnsafePointer(to: &output) { (outputPointer: UnsafePointer<Vector3>) in
outputPointer.withMemoryRebound(to: CGFloat.self, capacity: 3) {
withVaList([OpaquePointer($0), OpaquePointer($0.advanced(by: 1)), OpaquePointer($0.advanced(by: 2))]) { va in
r = vsscanf(buffer, "(%lf %lf %lf)", va)
}
}
}
}
return r == 3 ? output : nil
}
if let vector: Vector3 = parseVec3_3(s: "(1.0 2.0 3.0)") {
print(vector)
}
产出
Vector3(x: 1.0, y: 2.0, z: 3.0)
来自CVarArgs 的文档:
要为
c_api
函数创建包装器,请编写一个接受CVarArg
参数的函数,然后使用withVaList(_:_:)
函数调用导入的 C 函数。Swift 只导入使用
va_list
作为参数的 C 可变参数函数。 使用可变参数语法的 C 函数不会被导入,因此不能使用...
参数CVarArg
。
您的包装函数可能如下所示:
func vsscanfSwiftWrapper(
buffer: UnsafePointer<CChar>,
format: UnsafePointer<CChar>,
_ arguments: CVarArg...
) -> CInt {
withVaList(arguments) { vaList in
vsscanf(buffer, format, vaList)
}
}
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