Return 来自 Go 使用 cgo 的 String[]

Question

我必须从 Java 调用 Go 函数。我正在使用 cgo 和 JNA 来执行此操作。

Go 例程所做的唯一事情就是分配内存和 return char**。从 Java 方面，我正在使用 String[] 接收 char**，如 documentation 中所述。

以下是 C helper 和 Go 函数的详细信息：

static char** cmalloc(int size) {
    return (char**) malloc(size * sizeof(char*));
}

static void setElement(char **a, char *s, int index) {
    a[index] = s;
}

//export getSearchKeysA
func getSearchKeysA() **C.char {
    set_char := C.cmalloc(1)
    defer C.free(unsafe.Pointer(set_char))
    C.setElement(set_char, C.CString("hello world"), C.int(0))
    return set_char
}

Java方：

String[] getSearchKeysA();

我得到的错误是：

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x00007fff6b15323e, pid=92979, tid=0x0000000000000c07
#
# JRE version: Java(TM) SE Runtime Environment (8.0_192-b12) (build 1.8.0_192-b12)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (25.192-b12 mixed mode bsd-amd64 compressed oops)
# Problematic frame:
# C  [libsystem_kernel.dylib+0x723e]  __pthread_kill+0xa
#
# Failed to write core dump. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# An error report file with more information is saved as:
# /Users/dfb3/datafabric/pocs/go-java-connector/hs_err_pid92979.log
#
# If you would like to submit a bug report, please visit:
#   http://bugreport.java.com/bugreport/crash.jsp
# The crash happened outside the Java Virtual Machine in native code.
# See problematic frame for where to report the bug.
#

我注意到问题出现在 malloc 分配内存时。

我已经尝试执行 ulimit -c unlimited 并从方法中删除 defer C.free(unsafe.Pointer(set_char))。

错误的原因是什么，我该如何解决？有没有其他方法可以使用 JNA 从 Go 中 returning a []string？

由于打字错误和基于@PeterSO 回答的更新：

我最初写的是 malloc(0) 但它应该是 malloc(1)
func C.CString(string) *C.char，它应该给我分配内存，是不是吗？

Answer 1

你写：

func getSearchKeysA() **C.char {
    set_char := C.cmalloc(0)
    defer C.free(unsafe.Pointer(set_char))
    C.setElement(set_char, C.CString("hello world"), C.int(0))
    return set_char
}

可能执行为：

func getSearchKeysA() (retval **C.char) {
    set_char := C.cmalloc(42)
    C.setElement(set_char, C.CString("hello world"), C.int(1))
    retval = set_char
    C.free(unsafe.Pointer(set_char))
    return retval
}

你指的是 free 之后的 set_char 吗？

The Go Programming Language Specification Version of July 31, 2019

Defer statements

A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.

你写：

set_char := C.cmalloc(0)

static char** cmalloc(int size) {
    return (char**) malloc(size * sizeof(char*));
}

$ man malloc

The malloc() function allocates size bytes and returns a pointer to the allocated memory. The memory is not initialized. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().

为什么分配大小 0（零）？

malloc内存未初始化。

Answer 2

我终于可以 return 使用 cgo 从 GO 得到 String[]。

我将留下函数签名：

//export getSearchKeys
func getSearchKeys(numKeysByReference *C.int) **C.char {
  *numKeysByReference = // ... some value
  // Using the C helper defined above
  set_char := C.cmalloc(*numKeysByReference)
  // Logic allocating and populating C.char[i .. *numKeysByReference]
  // ...
  return set_char
}

使用 cgo 创建 **C.char 结构后，在 Java 端，我收到如下数据：

IntByReference intByReference = new IntByReference();
PointerByReference array = lib.getSearchKeys(intByReference);
String[] results = array.getPointer().getStringArray(0, intByReference.getValue());

正如@PeterSO 提到的，我们在使用它后调用了 defer C.free()。否则，它将在 returning 之后被释放。

Return 来自 Go 使用 cgo 的 String[]

Return String[] from Go using cgo

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