Does -Wpedantic make the program follow the -std=version?

Question

If I use gcc -std=c17, will -Wpedantic issue warnings for c17? Is this the same as ISO C?

Answer 1

Yes, in general the -std=cxx picks the C standard to follow, and is somewhat similar to the default -std=gnuxx setting, except the latter enables various language extensions. Basically this options picks which language features that will be available, but doesn't otherwise tell the compiler to be strict about it.

-Wpedantic-/-pedantic-errors is what makes the compiler strict. -std=c17 -Wpedantic will turn it into a strict (mostly) ISO C compliant compiler. It will remove various non-standard extensions from standard headers and also give diagnostic messages when you attempt various non-standard features. There are a few scenarios here and there where gcc fails to conform to the standard even in strict mode, but mostly it has very good conformance when these options are enabled.

Clang and icc compilers use the same settings too.

Answer 2

According to gcc manual, "c17", "c18", "iso9899:2017" and "iso9899:2018" all designate the C standard that was published by ISO in 2018.

According to the same manual, -Wpedantic depends on the version of the standard that is selected with the -std option:

Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any -std option used.

Important note from the same documentation:

Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.