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C3

Changes From C

Although C3 is trying to improve on C, this does not only mean addition of features, but also removal, or breaking changes:

No mandatory header files

There is a C3 interchange header format for declaring interfaces of libraries, but it is only used for special applications.

Removal of the old C macro system

The old C macro system is replaced by a new C3 macro system.

Import and modules

C3 uses module imports instead of header includes to link modules together.

Member access using . even for pointers

The -> operator is removed, access uses dot for both direct and pointer access. Note that this is just single access: to access a pointer of a pointer (e.g. int**) an explicit dereference would be needed.

Different operator precedence

Notably bit operations have higher precedence than +/-, making code like this: a & b == c evaluate like (a & b) == c instead of C’s a & (b == c). See the page about precedence rules.

Removal of the const type qualifier

The const qualifier is only retained for actual constant variables. C3 uses a special type of post condition for functions to indicate that they do not alter in parameters.

<*
 This function ensures that foo is not changed in the function.
 @param [in] foo
 @param [out] bar
*>
fn void test(Foo* foo, Bar* bar)
{
    bar.y = foo.x;
    // foo.x = foo.x + 1 - compile time error, can't write to 'in' param.
    // int x = bar.y     - compile time error, can't read from an 'out' param.
}

Rationale: const correctness requires littering const across the code base. Although const is useful, it provides weaker guarantees that it appears.

Fixed arrays do not decay and have copy semantics

C3 has three different array types. Variable arrays and slices decay to pointers, but fixed arrays are value objects and do not decay.

int[3] a = { 1, 2, 3 };
int[4]* b = &a; // No conversion
int* c = a; // ERROR
int* d = &a; // Valid implicit conversion
int* e = b; // Valid implicit conversion
int[3] f = a; // Copy by value!
Removal of multiple declaration syntax with initialization

Only a single declaration with initialization is allowed per statement in C3:

int i, j = 1; // ERROR
int a = 1;    // Ok
int b, c;     // Ok

In conditionals, a special form of multiple declarations are allowed but each must then provide its type:

for (int i = 0, int j = 1; i < 10; i++, j++) { ... }
Integer promotions rules and safe signed-unsigned comparisons

Promotion rules for integer types are different from C. C3 allows implicit widening only where there is only a single way to widen the expression. To explain the latter: take the case of long x = int_val_1 + int_val_2. In C this would widen the result of the addition: long x = (long)(int_val_1 + int_val_2), but there is another possible way to widen: long x = (long)int_val_1 + (long)int_val_2. so in this case, the widening is disallowed. However, long x = int_val_1 is unambiguous, so C3 permits it just like C (read more on the conversion page.

C3 also adds safe signed-unsigned comparisons: this means that comparing signed and unsigned values will always yield the correct result:

// The code below would print "Hello C3!" in C3 and "Hello C!" in C.
int i = -1;
uint j = 1;
if (i < j)
{
    printf("Hello C3!\n");
}
else
{
    printf("Hello C!\n");
}
Goto removed

goto is removed and replaced with labelled break and continue together with the nextcase statement that allows you to jump between cases in a switch statement.

Rationale: It is very difficult to make goto work well with defer and implicit unwrapping of optional results. It is not just making the compiler harder to write, but the code is harder to understand as well. The replacements together with defer cover many if not all usages of goto in regular code.

Implicit break in switches

Empty case statements have implicit fall through in C3, otherwise the nextcase statement is needed nextcase can also be used to jump to any other case statement in the switch.

switch (h)
{
    case 1:
        a = 1;
        nextcase; // Fall through
    case 2:
        b = 123;
    case 3:
        a = 2;
        nextcase 2; // Jump to case 2
    default:
        a = 111;
}
Locals variables are implicitly zeroed

In C global variables are implicitly zeroed out, but local variables aren’t. In C3 local variables are zeroed out by default, but may be explicitly undefined (using the @noinit attribute) if you wish to match the C behaviour.

Rationale for this change
  • In the “zero-is-initialization” paradigm, zeroing variables, in particular structs, is very common. By offering zero initialization by default this avoids a whole class of vulnerabilities.
  • Another alternative that was considered for C3 was mandatory initialization, but this adds a lot of extra boilerplate.
  • C3 also offers a way to opt out of zero-initialization, so the change comes at no performance loss.
Compound literal syntax changed
// C style:
call_foo((Foo) { 1, 2, 3 });


// C++ style (1):
call_foo(Foo(1, 2, 3));


// C++ style (2):
call_foo(Foo { 1, 2, 3 });


// C3:
call_foo(Foo { 1, 2, 3 } );


// C3 with inference:
call_foo({ 1, 2, 3 });
Bitfields replaced by bitstructs

Bitfields are replaced by bitstructs that have a well-defined encapsulating type, and an exact bit layout.

// C
struct Foo
{
    int a : 3;
    unsigned b : 4;
    MyEnum c : 7;
};


struct Flags
{
    bool has_hyperdrive : 1;
    bool has_tractorbeam : 1;
    bool has_plasmatorpedoes : 1;
}


// C3
bitstruct Foo : short
{
    int a : 0..2;
    uint b : 3..6;
    MyEnum c : 7..13;
}


// Simple form, only allowed when all fields are bools.
struct Flags : char
{
    bool has_hyperdrive;
    bool has_tractorbeam;
    bool has_plasmatorpedoes;
}
Evaluation order is well-defined

Evaluation order is left-to-right, and in assignment expressions, assignment happens after expression evaluation.

Signed overflow is well-defined

Signed integer overflow always wraps using 2s complement. It is never undefined behaviour.

Octal removed

The old 0777 octal syntax is removed and replaced by a 0o prefix, e.g. 0o777. Strings do not support octal sequences aside from '\0'.