How Does C 's Stricter Rules Differ from C's More Lenient Approach?

C 's Incompleteness: Where C Diverges from Its Subset

Despite frequent claims that C is wholly contained within C , subtle distinctions render this assertion incomplete. This article highlights instances where code acceptable in C becomes incompatible in C , shedding light on the differences that set them apart.

Intricate Definiteness

C enforces stricter rules for variable definitions, prohibiting tentativeness. Declaring the same variable repeatedly, as in int n; int n;, is impermissible in C , unlike in C.

Array Compatibility Disparity

C introduces type compatibility constraints that don't exist in C. In C, an array int a[1] can be assigned to a pointer int (*ap)[] = &a, despite being of different types (int[] vs. int[1]). This assignment would fail in C .

Obsolete Function Definitions

C abandons the K&R style of function definitions, rendering constructions like int b(a) int a; { } syntactically incorrect.

Nested Struct Scope Difference

Nested structs in C have class scope, unlike in C. Consequently, declaring a nested struct outside of an enclosing struct definition, such as struct B b;, results in an incomplete type error in C .

Explicit Type Specifiers Required

C mandates explicit type specifiers for all declarations. Omitting type information, as in auto a;, leads to syntax errors.

C99 Contributes Further Discrepancies

C99 introduces additional incompatibilities:

• Array Dimensions: Declaration specifiers in array dimensions of parameter lists must now align with C syntax (void f(int p[static 100]) { }).
• Variable Length Arrays: C prohibits non-constant expressions for array dimension calculations (int n = 1; int an[n];).
• Flexible Array Members: C disallows flexible array members (struct A { int a; int fam[]; }).
• Restrict Qualifiers: C introduces restrict qualifiers for alias analysis, which are absent from C (void copy(int *restrict src, int *restrict dst);).

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