Why Does Some C Code Fail to Compile in C ?

C as a Subset of C : Exceptions Explained

While it's often stated that C is a subset of C , there are certain instances where code may compile seamlessly in C but fail in C .

Cases where Code Fails in C

• Tentative Definitions:
  C allows multiple declarations of the same variable name, but C prohibits this with the error "n already defined."

  int n;
  int n; // error: redeclaration of 'n'

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• Incompatible Array Types:
  C allows assigning arrays to pointers with different types (e.g., int[1] to int *()), while C disallows such conversions.

  int a[1];
  int (*ap)[] = &a; // error: 'a' does not have type 'int[]'

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• K&R Function Definition Style:
  C permits function definitions using the syntax int b(a) int a; { }, but C considers it a grammar error.

  int b(a) int a; { } // error: grammar error

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• Nested Struct Scope:
  C enables declaring nested structs with class-scope, but C treats them as local variables instead.

  struct A { struct B { int a; } b; int c; };
  struct B b; // error: 'b' has incomplete type

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• Default Declarations:
  C allows declaring variables without specifying their type (e.g., auto a;), but C requires an explicit type-specifier.

  auto a; // error: type-specifier missing

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Additional Exceptions in C99

• Array Dimensions in Parameters:
  C99 prohibits specifiers like static in array dimensions of function parameters.

  void f(int p[static 100]) { } // error: invalid syntax

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• Variable Length Arrays:
  C99 does not support variable length arrays (VLAs) where array sizes are computed at runtime.

  int n = 1;
  int an[n]; // error: 'n' is not a constant expression

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• Flexible Array Members:
  C99 lacks flexible array members (FAMs) that can have variable-sized data members within structs.

  struct A { int a; int fam[]; }; 
  // error: 'fam' has incomplete type

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• Restrict Qualifier:
  C99 allows the restrict qualifier to specify non-aliasing of memory, which C does not support.

  void copy(int *restrict src, int *restrict dst); // error: two names for one parameter

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