From: Keith.S.Thompson+u@gmail.com   
      
   David Brown  writes:   
   > On 13/01/2026 01:06, Keith Thompson wrote:   
   >> David Brown  writes:   
   >> [...]   
   >> Context: %wN and %wfN printf length modifier, new in C23.   
   >>   
   >>> gcc has supported the format, along with much of C23, since gcc 13,   
   >>> and ARM's gcc-based toolchain version 13.2 is from October 2023.  (The   
   >>> current version is 15.2 from December 2025.)  But I don't know about   
   >>> library support - that is a very different matter.  (Compiler support   
   >>> for printf really just means checking the format specifiers match the   
   >>> parameters.)   
   >> Of course printf is implemented in the library, not in the compiler.   
   >   
   > Primarily, yes.  But like all standard library functions, compilers   
   > can have special handling in some ways.  This is more obvious for   
   > functions like memcpy, where the compiler can often generate   
   > significantly better code (specially for small known sizes).  As far   
   > as I know, the only optimisation gcc does on printf is turn something   
   > like printf("Hello\n") into puts("Hello").  Hypothetically, there is   
   > nothing to stop a compiler being a great deal more sophisticated than   
   > that, and doing the format-string interpretation directly in some way.   
      
   Sure, but in practice the library support for printf is the only thing   
   that matters.  If your library doesn't support %wN, having the compiler   
   recognize it doesn't help.  I'm not sure why you even mentioned gcc in   
   this context.   
      
   >> gcc has had format checking for %wN and %wfN since release 13, but   
   >> that's useless in the absence of library support.   
   >   
   > Yes.   
   >   
   >> Support in glibc was added 2023-06-19 and released in version 2.39.   
   >> Other C library implementations may or may not support it.   
   >   
   > glibc is not particularly relevant for non-Linux embedded   
   > systems. newlib (and newlib-nano) are a common choice for such   
   > systems, but I have no idea if it currently has support for those   
   > formats.   
      
   Of course, that's why I mentioned other C library implementations.   
   glibc happens to be the one about which I had some relevant   
   information.   
      
   The versions of musl and dietlibc that I have on my Ubuntu system   
   don't support %wN.  The version of newlib I have on Cygwin also   
   doesn't support it.  PellesC on Windows does.   
      
   --   
   Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com   
   void Void(void) { Void(); } /* The recursive call of the void */   
      
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