Calling C functions with variable arguments on x64
Posted: Fri Jul 20, 2018 7:18 am
There are some problems calling C functions with variable arguments on x64 if they contain floating point arguments.
When it comes to functions with variable arguments on x64, the calling conventions mention ...
On Windows, the third argument (arg.d) is passed in register xmm2 which would be fine for a function without variable arguments.
Since sprintf is a function using variable arguments, a copy of this value should also be passed in register r8 which PB doesn't do.
On Linux/Mac, PB doesn't pass the correct value in the rax register as this isn't required for a function with a fixed amount of arguments.
A workaround would be setting up the call yourself using asm instructions if you need this functionality but it's not very convenient.
I wanted to explain the problem.
It's not a bug since PB just doesn't support functions with variable arguments.
When it comes to functions with variable arguments on x64, the calling conventions mention ...
Windows wrote:For floating-point values only, both the integer and the floating-point register will contain the float value in case the callee expects the value in the integer registers.
For example the c function sprintf.Linux/Mac wrote:When a function taking variable-arguments is called, %rax must be set to the total number of floating point parameters passed to the function in vector registers.
Code: Select all
ImportC ""
sprintf(*buffer, format.p-utf8, arg.d)
EndImport
Global Dim buf.c(128)
sprintf(@buf(), "%e", 3.14)
Debug PeekS(@buf(), -1, #PB_UTF8)Since sprintf is a function using variable arguments, a copy of this value should also be passed in register r8 which PB doesn't do.
On Linux/Mac, PB doesn't pass the correct value in the rax register as this isn't required for a function with a fixed amount of arguments.
A workaround would be setting up the call yourself using asm instructions if you need this functionality but it's not very convenient.
I wanted to explain the problem.
It's not a bug since PB just doesn't support functions with variable arguments.