Why Do C# P/Invoke Calls Using `Cdecl` Sometimes Crash When Interfacing with `__stdcall` C Functions?

Understanding P/Invoke Calling Convention Conflicts

Using P/Invoke to call C functions from C# can lead to crashes if the calling conventions don't match. This article explores the reasons behind these conflicts.

Calling Convention Mismatches and Their Effects

A common scenario involves a C function using the __stdcall convention, while the corresponding C# DllImport attribute specifies CallingConvention.Cdecl. This happens because:

• __stdcall: The standard calling convention for Windows APIs and many C functions. The called function (callee) is responsible for cleaning up the stack. This is the P/Invoke default.
• __cdecl: Used in C code. The calling function (caller) is responsible for stack cleanup.

The Problem: Stack Imbalances

When the caller and callee disagree on stack cleanup, stack imbalances result. This leads to crashes, unpredictable behavior, and unreliable debugging due to lost stack information.

Other Calling Conventions (and Why They Matter)

• __thiscall: Used in C for member functions. It's not directly supported in .NET P/Invoke due to the complexities of inheritance and compiler differences.
• __fastcall: Uses CPU registers for argument passing. Unsupported in managed code.
• __clrcall: The managed code convention, combining aspects of __stdcall, __cdecl, and __fastcall, with runtime safety checks.

Why the Default P/Invoke Convention Can Cause Problems

The default P/Invoke calling convention (stdcall) often clashes with C code compiled using __cdecl. This stems from the historical association of __stdcall with Windows APIs, leading to the (incorrect) assumption that all C functions use it. This mismatch causes the stack cleanup issues described above.

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