From: david.brown@hesbynett.no   
      
   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.   
      
   >   
   > 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.   
      
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