Calling Conventions

When you declare a procedure or function, you can specify a calling convention using one of the directives register, pascal, cdecl, stdcall, and safecall. For example, function MyFunction(X, Y: Real): Real; cdecl;

Calling conventions determine the order in which parameters are passed to the routine. They also affect the removal of parameters from the stack, the use of registers for passing parameters, and error and exception handling. The default calling convention is register.

The register and pascal conventions pass parameters from left to right; that is, the left most parameter is evaluated and passed first and the rightmost parameter is evaluated and passed last. The cdecl, stdcall, and safecall conventions pass parameters from right to left. For all conventions except cdecl, the procedure or function removes parameters from the stack upon returning. With the cdecl convention, the caller removes parameters from the stack when the call returns.

The register convention uses up to three CPU registers to pass parameters, while the other conventions pass all parameters on the stack.

The safecall convention implements exception 'firewalls.' On Win32, this implements interprocess COM error notification.

The table below summarizes calling conventions. Calling conventions

Directive

Parameter order

Clean-up

Passes parameters in registers?

register

Left-to-right

Routine

Yes

pascal

Left-to-right

Routine

No

cdecl

Right-to-left

Caller

No

stdcall

Right-to-left

Routine

No

safecall

Right-to-left

Routine

No

The default register convention is the most efficient, since it usually avoids creation of a stack frame. (Access methods for published properties must use register.) The cdecl convention is useful when you call functions from shared libraries written in C or C++, while stdcall and safecall are recommended, in general, for calls to external code. On Win32, the operating system APIs are stdcall and safecall. Other operating systems generally use cdecl. (Note that stdcall is more efficient than cdecl.)

The safecall convention must be used for declaring dual-interface methods. The pascal convention is maintained for backward compatibility.

The directives near, far, and export refer to calling conventions in 16-bit Windows programming. They have no effect in Win32, or in .NET applications and are maintained for backward compatibility only.

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