[英]Return String[] from Go using cgo
I have to call a Go function from Java.我必须从 Java 调用 Go 函数。 I am using
cgo
and JNA
for doing this.我正在使用
cgo
和JNA
来做这件事。
The only thing that the Go routine does is allocating memory and return a char**
. Go 例程所做的唯一一件事就是分配内存并返回一个
char**
。 From Java side, I am receiving the char**
using String[]
as is mentioned in the documentation .从 Java 方面,我正在使用文档中提到的
String[]
接收char**
。
Below are the details of the C helper and Go functions:下面是 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
}
The Java side: Java方面:
String[] getSearchKeysA();
The error I am getting is: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.
#
What I have noticed is that the issue appears when malloc is allocating memory.我注意到问题是在 malloc 分配内存时出现。
I already tried doing ulimit -c unlimited
and removing the defer C.free(unsafe.Pointer(set_char))
from the method.我已经尝试执行
ulimit -c unlimited
并从方法中删除defer C.free(unsafe.Pointer(set_char))
。
What can be the cause of the error and how can I solve it?错误的原因是什么,我该如何解决? Is there any other way of returning a
[]string
from Go using JNA
?有没有其他方法可以使用
JNA
从 Go 返回[]string
?
Update because of typo and base on @PeterSO answer :由于打字错误和基于@PeterSO 回答而更新:
func C.CString(string) *C.char
, it should allocate the memory for me, is not? func C.CString(string) *C.char
,它应该为我分配内存,不是吗?I could finally return a String[]
from GO
using cgo
.我终于可以使用
cgo
从GO
返回一个String[]
。
I will leave the functions signatures: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
}
After creating the **C.char
structure using cgo
, on Java
side, I am receiving the data as follow:使用
cgo
创建**C.char
结构后,在Java
端,我收到的数据如下:
IntByReference intByReference = new IntByReference();
PointerByReference array = lib.getSearchKeys(intByReference);
String[] results = array.getPointer().getStringArray(0, intByReference.getValue());
As @PeterSO mentioned, we have call defer C.free()
after using it.正如@PeterSO 提到的,我们在使用之后调用了
defer C.free()
。 Otherwise, it will be deallocated after returning.否则,它将在返回后被释放。
You write: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
}
Which likely executes as:这可能执行为:
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
}
Are you referring to set_char
after free
?你指的是
set_char
after free
吗?
The Go Programming Language Specification Version of July 31, 2019
2019 年 7 月 31 日Go 编程语言规范版本
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.
“defer”语句调用一个函数,该函数的执行被推迟到周围函数返回的那一刻,因为周围函数执行了一个 return 语句,到达了它的函数体的末尾,或者因为相应的 goroutine 处于恐慌状态。
You write:你写:
set_char := C.cmalloc(0)
static char** cmalloc(int size) {
return (char**) malloc(size * sizeof(char*));
}
$ man malloc
$ man malloc
The malloc() function allocates size bytes and returns a pointer to the allocated memory.
malloc() 函数分配 size 字节并返回指向已分配内存的指针。 The memory is not initialized.
内存未初始化。 If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().
如果 size 为 0,则 malloc() 返回 NULL 或稍后可以成功传递给 free() 的唯一指针值。
Why allocate size 0 (zero)?为什么分配大小 0(零)?
malloc
memory is not initialized. malloc
内存未初始化。
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