I know that for C11, I can test #if(__STDC_VERSION >= 20112L). But for -std=c1x
what macro and/or value should I test it?
what's the nomenclature of this standard? or maybe a informal name, if any. I hope this is clear. Thanks in advance.
gcc's -std=c1x option is synonymous with -std=c11, and it sets __STDC_VERSION__ to the same value, 201112L (note: not 20112L).
Before it was released, the new C standard was referred to as "C1X" (since it wasn't known exactly when it would come out), and gcc added a -std=c1x option to enable partial support for the upcoming standard. When the standard was released as C11, gcc added -std=c11 to enable (still partial) support, but has kept the -std=c1x option for compatibility.
(Due to an editing error, the released 2011 ISO C standard doesn't specify the value of __STDC_VERSION__, but the editor has stated that 201112L is correct; see this question.)
Jens Gustedt's comment makes a good point. gcc -std=c11 sets __STDC_VERSION__ to 201112LL, which is supposed to imply C11 conformance, but in fact it's still missing a log of C11 features. We can expect this to improve in future releases.
Similarly, -std=c99 sets __STDC_VERSION__ to 199901L, but it doesn't quite conform to the C99 standard (the current status is documented here. gcc's C90 conformance (with -ansi or -std=c90) conforms quite well to the C90 standard.