A bare cc (or gcc or clang) invocation is not meant to be standard-conforming and will both reject standard-conforming code and leave out standard-mandated diag

A bare cc (or gcc or clang) invocation is not meant to be standard-conforming and will both reject standard-conforming code and leave out standard-mandated diagnostics. To accept all standard-conforming code, you need the -std=c[...] option e.g. to prevent it from picking up non-standard keywords. Without -std=c[...], GCC would flag a syntax error for the perfectly valid void asm(void) {}, because it detects asm as a keyword contrary to what the standard requires. Without -pedantic, no diagnostic will be issued for e.g. void f(int i) { int x[i]; } with cc -std=c90, even though C90 requires a diagnostic for an array where the length is not a constant. For GCC, this is documented at <https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Standards.html#C-Language> https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Standards.html....