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A compile-time assert in pure standard C89 is possible, and a little bit of preprocessor trickery makes its usage look just as clean as the runtime usage of assert().

The key trick is to find a construct that can be evaluated at compile time and can cause an error for some values. One answer is the declaration of an array cannot have a negative size. Using a typedef prevents the allocation of space on success, and preserves the error on failure.

The error message itself will cryptically refer to declaration of a negative size (GCC says “size of array foo is negative”), so you should pick a name for the array type that hints that this error really is an assertion check.

A further issue to handle is that it is only possible to typedef a particular type name once in any compilation unit. So, the macro has to arrange for each usage to get a unique type name to declare.

My usual solution has been to require that the macro have two parameters. The first is the condition to assert is true, and the second is part of the type name declared behind the scenes. You might imagine that using token pasting and the __LINE__ predefined macro could form a unique name possibly without needing an extra argument.

Unfortunately, if the assertion check is in an included file, it can still collide with a check at the same line number in a second included file, or at that line number in the main source file. We could paper over that by using the macro __FILE__, but it is defined to be a string constant and there is no preprocessor trick that can turn a string constant back into part of an identifier name; not to mention that legal file names can contain characters that are not legal parts of an identifier.

So, I would propose the following code fragment:

    /** A compile time assertion check.
     *
     *  Validate at compile time that the predicate is true without
     *  generating code. This can be used at any point in a source file
     *  where typedef is legal.
     *
     *  On success, compilation proceeds normally.
     *
     *  On failure, attempts to typedef an array type of negative size. The
     *  offending line will look like
     *      typedef assertion_failed_file_h_42[-1]
     *  where file is the content of the second parameter which should
     *  typically be related in some obvious way to the containing file
     *  name, 42 is the line number in the file on which the assertion
     *  appears, and -1 is the result of a calculation based on the
     *  predicate failing.
     *
     *  \param predicate The predicate to test. It must evaluate to
     *  something that can be coerced to a normal C boolean.
     *
     *  \param file A sequence of legal identifier characters that should
     *  uniquely identify the source file in which this condition appears.
     */
    #define CASSERT(predicate, file) _impl_CASSERT_LINE(predicate,__LINE__,file)

    #define _impl_PASTE(a,b) a##b
    #define _impl_CASSERT_LINE(predicate, line, file) 
        typedef char _impl_PASTE(assertion_failed_##file##_,line)[2*!!(predicate)-1];

A typical usage might be something like:

    #include "CAssert.h"
    ...
    struct foo { 
        ...  /* 76 bytes of members */
    };
    CASSERT(sizeof(struct foo) == 76, demo_c);

In GCC, an assertion failure would look like:

$ gcc -c demo.c
demo.c:32: error: size of array `assertion_failed_demo_c_32' is negative
$

I’ve ported most of this post from an answer I gave to a question at Stack Overflow. Stack Overflow is a general programming question an answer site that I have found to be useful on a wide range of topics. They have grown to include related sites on many non-programming subjects as well.

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