mirror of https://github.com/swig/swig
166 lines
3.6 KiB
HTML
166 lines
3.6 KiB
HTML
<html>
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<title>SWIG:Examples:java:pointer</title>
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</head>
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<body bgcolor="#ffffff">
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<tt>SWIG/Examples/java/pointer/</tt>
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<hr>
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<H2>Simple Pointer Handling</H2>
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<p>
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This example illustrates a couple of techniques for handling
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simple pointers in SWIG. The prototypical example is a C function
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that operates on pointers such as this:
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<blockquote>
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<pre>
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void add(int *x, int *y, int *r) {
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*r = *x + *y;
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}
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</pre>
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</blockquote>
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By default, SWIG wraps this function exactly as specified and creates
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an interface that expects pointer objects for arguments.
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SWIG wraps a C pointer with a type wrapper class, for example, SWIGTYPE_p_int for an int*.
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The only problem is how does one go about creating these objects from a Java program?
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<p>
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<h2>Possible Solutions</h2>
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<ul>
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<li>Write some helper functions to explicitly create objects. For
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example:
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<blockquote>
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<pre>
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int *new_int(int ivalue) {
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int *i = (int *) malloc(sizeof(ivalue));
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*i = ivalue;
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return i;
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}
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int get_int(int *i) {
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return *i;
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}
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void delete_int(int *i) {
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free(i);
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}
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</pre>
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</blockquote>
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<p>
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<li>The SWIG pointer library provides an easier way. <br>
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For example, in the interface file
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you would do this:
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<blockquote>
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<pre>
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%include cpointer.i
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%pointer_functions(int, intp);
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</pre>
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</blockquote>
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and from Java you would use pointers like this:
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<blockquote>
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<pre>
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SWIGTYPE_p_int a = example.new_intp();
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SWIGTYPE_p_int b = example.new_intp();
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SWIGTYPE_p_int c = example.new_intp();
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example.intp_assign(a,37);
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example.intp_assign(b,42);
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// Note that getCPtr() has package access by default
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System.out.println(" a =" + Long.toHexString(SWIGTYPE_p_int.getCPtr(a)));
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System.out.println(" b =" + Long.toHexString(SWIGTYPE_p_int.getCPtr(b)));
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System.out.println(" c =" + Long.toHexString(SWIGTYPE_p_int.getCPtr(c)));
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// Call the add() function with some pointers
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example.add(a,b,c);
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// Now get the result
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int res = example.intp_value(c);
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System.out.println(" 37 + 42 =" + res);
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// Clean up the pointers
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example.delete_intp(a);
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example.delete_intp(b);
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example.delete_intp(c);
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</pre>
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</blockquote>
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<p>
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<li>Use the SWIG typemap library. This library allows you to completely
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change the way arguments are processed by SWIG. For example:
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<blockquote>
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<pre>
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%include "typemaps.i"
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void add(int *INPUT, int *INPUT, int *OUTPUT);
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</pre>
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</blockquote>
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And in a Java program:
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<blockquote>
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<pre>
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int[] r = {0};
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example.sub(37,42,r);
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System.out.println("Result =" + r[0]);
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</pre>
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</blockquote>
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Needless to say, this is substantially easier although a bit unusual.
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<p>
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<li>A final alternative is to use the typemaps library in combination
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with the %apply directive. This allows you to change the names of parameters
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that behave as input or output parameters. For example:
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<blockquote>
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<pre>
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%include "typemaps.i"
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%apply int *INPUT {int *x, int *y};
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%apply int *OUTPUT {int *r};
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void add(int *x, int *y, int *r);
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void sub(int *x, int *y, int *r);
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void mul(int *x, int *y, int *r);
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... etc ...
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</pre>
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</blockquote>
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</ul>
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<h2>Example</h2>
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The following example illustrates the use of these features for pointer
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extraction.
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<ul>
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<li> <a href="example.c">example.c</a> (C Source)
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<li> <a href="example.i">example.i</a> (SWIG interface)
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<li> <a href="runme.java">runme.java</a> (Java program)
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</ul>
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<h2>Notes</h2>
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<ul>
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<li>Since pointers are used for so many different things (arrays, output values,
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etc...) the complexity of pointer handling can be as complicated as you want to
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make it.
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<p>
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<li>More documentation on the typemaps.i and cpointer.i library files can be
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found in the SWIG user manual. The files also contain documentation.
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</ul>
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<hr>
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</body>
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</html>
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