调用C DLL函数用char *参数使用的P / Invoke [英] Calling a C DLL Function With char* Parameters With P/Invoke

问题描述

我读过这个其他类似的问题，但他们不解决这个特定的问题。我有一个旧的C库具有与toUpperCase功能（作为一个例子）。这需要一个char *，并返回一个char *。然而，返回的指针指向相同的字符串（不要问我，我没有写）

I've read other similar questions on this but they don't solve this particular problem. I have an old C-library with a touppercase function (as an example). This takes a char* and returns a char*. However, the pointer returned is a pointer to the same string (don't ask me I didn't write it).

函数看起来是这样的：

__declspec(dllexport)
char * __cdecl touppercase(char *ps_source)
{
    char *ps_buffer = NULL;

    assert (ps_source != NULL);

    ps_buffer = ps_source;
    while (*ps_buffer != '\0')
    {
        *ps_buffer = toupper(*ps_buffer);
        ps_buffer++;
    }
*ps_buffer = '\0';
    return (ps_source);
}

C＃代码来声明这个样子：

The C# code to declare this looks like:

    [DllImport("mydll.dll", EntryPoint = "touppercase",
               CharSet = CharSet.Ansi, ExactSpelling = true,
               CallingConvention = CallingConvention.Cdecl)]
    private static extern System.IntPtr touppercase(string postData);

在我的应用程序这个调用看起来像

The call to this in my app looks like

     string sTest2 = Marshal.PtrToStringAnsi(to_uppercase(sTest));

不过sTest2刚刚结束是一个随机字符串。

However sTest2 just ends up being a random string.

我添加了一个测试功能，使用相同的参数相同的dll但本地分配内存，复制字符串。这工作得很好。 ？为什么原来的版本现在的工作。

I added a test function to the same dll with the same parameters but this allocates memory locally and copies the string. This works fine. Why does the original version now work?

请注意：更新DLL库本身是不是一种选择

Note: Updating the dll libraries themselves isn't an option.

推荐答案

该功能修改的参考，所以你应该使用StringBuilder：

The function is modifying the reference so you should use a stringbuilder:

[DllImport("dll.dll", EntryPoint="touppercase",
CharSet = CharSet.Ansi, ExactSpelling = true,
CallingConvention = CallingConvention.Cdecl)]
private static extern System.IntPtr touppercase(StringBuilder postData);

说它是这样的：

Call it something like this:

    StringBuilder sTest = new StringBuilder("abs");
    touppercase(sTest);
    string result = sTest.ToString();

对于返回指针的解释 - >本福格特

For the explanation of the return pointer -> Ben Voigt

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