What C99 and C Code Differences Cause Compilation Errors?

C99 and C Compatibility Exceptions

While C is generally a subset of C , certain nuances in the C language prohibit the direct compilation of some valid C code. Here are some key differences to consider:

1. No Tentative Definitions

C allows multiple declarations of variables with the same name, but C does not. Consequently, the following code, which is valid in C, will not compile in C :

int n;
int n; // ill-formed: n already defined

2. Incompatible Array Types

In C, arrays can be declared with either int[] or int[N] syntax. However, C treats these differently. The following code, which is valid in C, will raise an error in C :

int a[1];
int (*ap)[] = &a; // ill-formed: a does not have type int[]

3. K&R Function Definition Style Disallowed

C supports the K&R style of function definition, but C does not. The following code, which is valid in C, will not compile in C :

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

4. Nested Struct Scope Differences

In C, nested structs have file scope, while in C , they have class scope. This means that the following code, which is valid in C, will not compile in C :

struct A { struct B { int a; } b; int c; };
struct B b; // ill-formed: b has incomplete type (*not* A::B)

5. No Default int

C does not allow defaulting integer variables to int, unlike C. Therefore, the following code, which is valid in C, will raise an error in C :

auto a; // ill-formed: type-specifier missing

Additional Exceptions in C99

In addition to these exceptions, C99 introduces further incompatibilities with C :

• No special handling of declaration specifiers in array dimensions of parameters.
• No variable length arrays.
• No flexible array member.
• No restrict qualifier for aliasing analysis.

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