c - How to detect if printf will support %a? -

i need losslessly represent double precision float in string, , using

sprintf(buf, "%la", x); 

this works fine on system, when built on mingw under windows, gives warning:

unknown conversion type character 'a' in format 

i coded workaround case, have trouble detecting when should use workaround -- tried #if __stdc_version__ >= 199901l, seems gcc/mingw defines if doesn't support %a. there macro check?

this doesn't answer question "how detect if printf support %a?" in general case, can modify compiler installation %a supported.

first of , use mingw-w64. up-to-date fork of mingw. original version of mingw not maintained , not fix bugs such experiencing (preferring blame microsoft or something).

using mingw-w64 4.9.2 in windows 10, following code works me:

#include <stdio.h>  int main() {     double x = 3.14;     printf("%a\n", x); } 

producing 0x1.91eb85p+1 correct. still deferring microsoft runtime.

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your question mentions %la, %a , %la both same , can used print either float or double argument.

if want print long double, microsoft runtime not support that; gcc , ms use different sizes of long double. have use mingw-w64's own printf implementation:

#define __use_mingw_ansi_stdio 1 #include <stdio.h>  int main() {     long double x = 3.14;     printf("%la\n", x); } 

which outputs 0xc.8f5c28f5c28f8p-2. same number 0x1.91eb85p+1 more precision , different placement of binary point, correct.