在Linux中，单声道调用我的.so lib返回System.EntryPointNotFoundException

Question

here is my c++ code

#include "_app_switcher.h"

std::string c_meth(std::string str_arg) {
    return "prpr";
}

my mono code:

    [Test]
    public void TestDraft()
    {
        Console.WriteLine(c_meth("prpr"));
    }

    [DllImport("/home/roroco/Dropbox/cs/App.Switcher/c/app-switcher/lib/libapp-switcher-t.so")]
    private static extern string c_meth(string strArg);

the err output:

System.EntryPointNotFoundException : c_meth
  at (wrapper managed-to-native) Test.Ro.EnvTest.c_meth(string)
  at Test.Ro.EnvTest.TestDraft () [0x00001] in /home/roroco/Dropbox/cs/Ro/TestRo/EnvTest.cs:15 
  at (wrapper managed-to-native) System.Reflection.MonoMethod.InternalInvoke(System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x00032] in <71d8ad678db34313b7f718a414dfcb25>:0

I guess it's because my header file is not in /usr/include, so how to specific c++ header file in mono?

Answer 1

There are a couple of reasons why your code doesn't work:

1. The function c_meth does not exist within your shared library. The function that does exist is _Z6c_methNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE .
2. The C++ class std::string and the .NET class System.String are different and entirely unrelated. .NET only knows how to marshal System.String to const char* and vice versa.

C++ allows function overloading. That means it needs a way to tell void foo(int) apart from void foo(std::string) . To do this, it uses name mangling to generate a unique name for each overload. To disable name mangling for a function you declare it with the extern "C" specifier. This also limits you to C-like interfaces, so you can only pass and return primitive objects and pointers. No classes or references. That's fine though, since .NET doesn't know what to do with C++ classes. You need to accept a raw const char* parameter and return a const char* :

extern "C" const char* c_meth(const char* str_arg) {
    return "prpr";
}

---

Returning strings is also problematic. .NET will try to de-allocate the returned memory after copying the string to the managed heap. Since the string returned in this case wasn't allocated using the appropriate allocation method, that will fail. To avoid this you'll need to declare the imported method in C# to return an IntPtr and use Marshal.PtrToString(Ansi|Unicode) to convert to a System.String .

If you do need to return something other than a string constant, you have a couple options:

1. Allocate the memory for the string with the appropriate function. The function to use is platform-dependent. See Mono's documentation for info about which function to use.
2. Allocate the memory in C# and pass the buffer to the unmanaged function using a System.Text.StringBuilder :

C++ side:

extern "C" void c_meth(const char* str_arg, char* outbuf, int outsize) {
    std::string ret = someFunctionThatReturnsAString(str_arg);
    std::strncpy(outbuf, ret.c_str(), outsize);
}

C# side:

[Test]
public void TestDraft()
{
    StringBuilder sb = new StringBuilder(256)
    c_meth("prpr", sb, 256);
    Console.WriteLine(sb.ToString());
}

[DllImport("/home/roroco/Dropbox/cs/App.Switcher/c/app-switcher/lib/libapp-switcher-t.so")]
private static extern void c_meth(string strArg, StringBuilder outbuf, int outsize);