Contracts

Contracts are optional pre- and post-condition checks that the compiler may use for static analysis, runtime checks and optimization. Note that conforming C3 compilers are not obliged to use pre- and post-conditions at all.

However, violating either pre- or post-conditions is unspecified behaviour, and a compiler may optimize code as if they are always true – even if a potential bug may cause them to be violated.

In safe mode, pre- and post-conditions are checked using runtime asserts.

Pre-conditions

Pre-conditions are usually used to validate incoming arguments. Each condition must be an expression that can be evaluated to a boolean. Pre-conditions use the @require annotation, and optionally can have an error message to display after them.

<*
 @require foo > 0, foo < 1000 : "optional error msg"
*>
fn int test_foo(int foo)
{
    return foo * 10;
}

If we now write the following code:

fn void main()
{
    test_foo(0);
}

With c3c (the standard C3 compiler) we will get a compile time error, saying that the contract is violated. However, expressions requiring more static analysis are often only caught at runtime.

Post conditions

Post conditions are evaluated to make checks on the resulting state after passing through the function. The post condition uses the @ensure annotation. Where return is used to represent the return value from the function.

<*
 @require foo != null
 @ensure return > foo.x
*>
fn uint check_foo(Foo* foo)
{
    uint y = abs(foo.x) + 1;
    // If we had row: foo.x = 0, then this would be a runtime contract error.
    return y * abs(foo.x);
}

Parameter annotations

@param supports [in] [out] and [inout]. These are only applicable for pointer arguments. [in] disallows writing to the variable, [out] disallows reading from the variable. Without an annotation, pointers may both be read from and written to without checks. If an & is placed in front of the annotation (e.g. [&in]), then this means the pointer must be non-null and is checked for null.

Type	readable?	writable?	use as “in”?	use as “out”?	use as “inout”?
no annotation	Yes	Yes	Yes	Yes	Yes
in	Yes	No	Yes	No	No
out	No	Yes	No	Yes	No
inout	Yes	Yes	Yes	Yes	Yes

However, it should be noted that the compiler might not detect whether the annotation is correct or not! This program might compile, but will behave strangely:

fn void bad_func(int* i)
{
    *i = 2;
}

<*
 @param [&in] i
*>
fn void lying_func(int* i)
{
    bad_func(i); // The compiler might not check this!
}

fn void test()
{
    int a = 1;
    lying_func(&a);
    io::printfn("%d", a); // Might print 2!
}

However, compilers will usually detect this:

<*
 @param [&in] i
*>
fn void bad_func(int* i)
{
    *i = 2; // <- Compiler error: cannot write to "in" parameter
}

Pure in detail

The pure annotation allows a program to make assumptions in regard to how the function treats global variables. Unlike for const, a pure function is not allowed to call a function which is known to be impure.

However, just like for const the compiler might not detect whether the annotation is correct or not! This program might compile, but will behave strangely:

int i = 0;

fn void bad_func()
{
    i = 2;
}

<*
 @pure
*>
fn void lying_func()
{
    bad_func() @pure; // Call bad_func by assuring it is pure!
}

fn void main()
{
    i = 1;
    lying_func();
    io::printfn("%d", i); // Might print 2!
}

Circumventing “pure” annotations will cause the compiler optimize under the assumption that globals are not affected, even if this isn’t true.

Pre-conditions for macros

In order to check macros, it’s often useful to use the builtin $defined function which returns true if the code inside would pass semantic checking.

<*
 @require $defined(resource.open, resource.open()) : `Expected resource to have an "open" function`
 @require resource != nil
 @require $assignable(resource.open(), void*)
*>
macro open_resource(resource)
{
    return resource.open();
}

Contract support

A C3 compiler may have different levels of contract use:

Level	Behaviour
0	Contracts are only semantically checked
1	@require may be compiled into asserts inside of the function. Compile time violations detected through constant folding should not compile
2	As Level 1, but @ensures are also checked
3	@require is added at caller side as well
4	Static analysis is extended beyond compile time folding

The c3c compiler is currently does level 3 checking.