使用 Swig 在 C 和 java 之间传递数组

Question

我精通 C，但不精通 Java，我正在尝试使用 Swig 从 Java 调用 C。简单调用工作正常，但我无法使用 carrays.i 或 arrays_java.i 方法交换数组。两者都没有任何完整的例子。

AMItest.i:

%module AMItest
%{
#include "AMItest.h"
%}
%include "carrays.i"
%include "AMItest.h"

AMItest.h:

extern int AMItest(double *Array, int Start, int End);

AMItest.c

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <errno.h>
#include "AMItest.h"
int AMItest(double *Array, int Start, int End) {
    if (End<=Start) {
        fprintf(stderr, "Start must be lower than End\n");
        return EDOM;
    }
    for (int i=0; i<=End-Start; i++)
        Array[i]=sin(i/4.);
    return 0;
}

main.java:

public class main {
    public static void main(String argv[]) {
        System.loadLibrary("AMItest_swig"); // File is libAMItest_swig.so
        double[] myArray = new double[11];
        System.out.println(AMItest.AMItest(myArray, 10, 20));
        for (int i=0; i<myArray.length; i++)
            System.out.print(myArray[i] + " ");
    }
}

编译：

$ rm -f AMItest AMItest*.java SWIG*.java *.class *.o *.a *.so *_wrap.c
$ swig -java AMItest.i
$ gcc -fPIC -c AMItest.c AMItest_wrap.c -I/usr/lib/jvm/java-11-openjdk-amd64/include -I/usr/lib/jvm/java-11-openjdk-amd64/include/linux
$ gcc -shared AMItest.o  AMItest_wrap.o  -W  -o libAMItest_swig.so
$ javac main.java
main.java:7: error: incompatible types: double[] cannot be converted to SWIGTYPE_p_double
                System.out.println(AMItest.AMItest(myArray, 10, 20));

谷歌搜索 SWIGTYPE_p_* errors 导致的（无用的）链接少得惊人。我也尝试过使用 arrays_java.i 或将其添加到 .i 文件中：

%array_functions( double, double )
%include <typemaps.i>
%apply (double *OUTPUT, int, int) {(double *Array, int Start, int End)};

这是我的 .i 文件还是我的 Java 的问题？！？

Answer 1

像这样调整你的AMItest.i：

%module AMItest
%{
#include "AMItest.h"
%}
%include "arrays_java.i"
%apply double[] {double *};
%include "AMItest.h"

Sometimes a C function expects an array to be passed as a pointer... One of the ways to wrap this is to apply the Java array typemaps that come in the arrays_java.i library file ... The ANY size will ensure the typemap is applied to arrays of all sizes. You could narrow the typemap matching rules by specifying a particular array size. Now you can use a pure Java array and pass it to the C code.

在此处查看文档：http://www.swig.org/Doc4.0/SWIGDocumentation.html#Java_unbounded_c_arrays

测试

如上所示使用您提供的带有修改后的 AMItest.i 的示例，调用 java main 时您将获得以下输出：

0
0.0 0.24740395925452294 0.479425538604203 0.6816387600233342 0.8414709848078965 0.9489846193555862 0.9974949866040544 0.9839859468739369 0.9092974268256817 0.7780731968879213 0.5984721441039564

大型阵列的更高效替代方案

对于包含 11 个双精度元素的数组，您可能希望使用上面显示的方法。但我不想隐藏 SWIG 文档中显示的替代方法：

This approach is probably the most natural way to use arrays. However, it suffers from performance problems when using large arrays as a lot of copying of the elements occurs in transferring the array from the Java world to the C++ world. An alternative approach to using Java arrays for C arrays is to use an alternative SWIG library file carrays.i. This approach can be more efficient for large arrays as the array is accessed one element at a time.

您的示例需要在两个地方进行修改，如下所示：

AKITest.i:

%module AMItest
%{
#include "AMItest.h"
%}
%include "carrays.i"
%array_functions(double, doubleArray);
%include "AMItest.h"

你稍作修改后的 main.java 将如下所示：

public class main {
    public static void main(String argv[]) {
        System.loadLibrary("AMItest_swig"); // File is libAMItest_swig.so
        int size = 11;
        SWIGTYPE_p_double myArray = AMItest.new_doubleArray(size);
        AMItest.AMItest(myArray, 10, 20);
        for (int i=0; i<size; i++)
            System.out.print(AMItest.doubleArray_getitem(myArray, i) + " ");
    }
}

结果是一样的。

使用 Swig 在 C 和 java 之间传递数组

Passing arrays between C and java with Swig

c

java

arrays

swig