Basic understanding of calling kernel functions in NSIS - nsis

Question

In NSIS, I was going through some API which calls some functions of Windows.
For example:
Kernel32::SetEnvironmentVariable(t, t)i ("VAR1", "$R0").r0
Kernel32::GetLocaleInfo(i,i,t,i)i(2048,0x2,.r0,${NSIS_MAX_STRLEN})i
As I am a newbie, can anyone please explain what is happening here ? What is t , i , .ro etc. ?
I know that kernel32 is a DLL and SetEnvironmentVariable & GetLocaleInfo are the functions defined in this DLL.

Answer 1

The documentation will tell you what t and i are.
The basic syntax for System::Call is module::function(parameters)return. Parameters and return are optional and they both follow the same basic format: type input output. Type is not optional if you need input and/or output.
Parameters and return can be repeated multiple times, this is only useful when the function prototype exists as a define but you sometimes see this syntax on the NSIS Wiki as well. Kernel32::SetEnvironmentVariable(t, t)i ("VAR1", "$R0").r0 is exactly the same as Kernel32::SetEnvironmentVariable(t "VAR1", t "$R0")i.r0. When using a define it would look like this:
!define SetEnvironmentVariable "Kernel32::SetEnvironmentVariable(t, t)i"
System::Call '${SetEnvironmentVariable}("VAR1", "$R0").r0'
It is helpful if you know a language like C/C++ or Delphi when using the System plug-in because you need to understand the basic Windows types and how functions are typically used.
If for example you wanted to call the GetWindowsDirectory function you would first look at the function on MSDN:
UINT WINAPI GetWindowsDirectory(_Out_ LPTSTR lpBuffer, _In_ UINT uSize);
Translating this to NSIS gives you a initial skeleton of Kernel32::GetWindowsDirectory(t, i)i (WINAPI is the default calling convention). t maps to the Windows TCHAR* type and i is a 32-bit integer, the same as ULONG, LONG, DWORD, INT, UINT, INT32 and UINT32 in the Windows SDK.
The only missing piece is the input and output. The MSDN declaration is decorated with SAL annotations so it is easy to see which parameters are input and which are output.
A working example might look something like this:
System::Call 'Kernel32::GetWindowsDirectory(t .r1, i ${NSIS_MAX_STRLEN})i .r0'
DetailPrint "Return=$0 Output1=$1"
. can be used as a "nothing" placeholder when there is no input. In this case we use it twice. The return value never has input when using System::Call and we also have one output-only parameter.
r1 and r0 are aliases for the $1 and $0 NSIS registers and these aliases must be used when you need the output of something. The plain NSIS register can be used as input but then the variable expansion happens inside NSIS and not in the plug-in and this can cause issues with quotes in strings and is not really recommended if the string might contain quotes or legal System plug-in syntax. I would therefore recommend that your first example is rewritten as Kernel32::SetEnvironmentVariable(t "VAR1", t R0)i.r0.
In C your second example would look something like
char mybuf[1024];
GetLocaleInfo(MAKELCID(LANG_NEUTRAL,SUBLANG_SYS_DEFAULT), LOCALE_SLANGUAGE, mybuf, sizeof(mybuf));