This mode can be recognized by the macro __STRICT_ANSI__.
From man gcc:
-ansi
In C mode, this is equivalent to -std=c89. In C++ mode, it is equivalent to
-std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90
(when compiling C code), or of standard (when compiling code), such as the
asm and typeof keywords, and predefined macros such as unix and vax that
identify the type of system you are using. It also enables the undesirable
and rarely used ISO trigraph feature. For the C compiler, it disables
recognition of style // comments as well as the inline keyword.
The alternate keywords _ _asm_ _, _ _extension_ _, _ _inline_ _ and
_ _typeof_ _ continue to work despite -ansi. You would not want to use them
in an ISO C program, of course, but it is useful to put them in header files
that might be included in compilations done with -ansi. Alternate predefined
macros such as _ _unix_ _ and _ _vax_ _ are also available, with or without
-ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously.
For that, -pedantic is required in addition to -ansi.
The macro _ _STRICT_ANSI_ _ is predefined when the -ansi option is used.
Some header files may notice this macro and refrain from declaring certain
functions or defining certain macros that the ISO standard doesn't call for;
this is to avoid interfering with any programs that might use these names for
other things.
Functions that would normally be built in but do not have semantics defined
by ISO C (such as alloca and ffs) are not built-in functions when -ansi is
used.
(cherry picked from commit da7f3bf1c1fcbbaee48bed464e132ea52aa961ab)
