Manual Pages  — KASSERT

The KASSERT() macro tests the given boolean expression. If expression evaluates to false, and the kernel is compiled with options INVARIANTS, the panic(9) function is called. This terminates the running system at the point of the error, possibly dropping into the kernel debugger or initiating a kernel core dump. The second argument, msg, is a printf(9) format string and its arguments, enclosed in parentheses. The formatted string will become the panic string.

In a kernel that is built without options INVARIANTS, the assertion macros are defined to be no-ops. This eliminates the runtime overhead of widespread assertions from release builds of the kernel. Therefore, checks which can be performed in a constant amount of time can be added as assertions without concern about their performance impact. More expensive checks, such as those that output to console, or verify the integrity of a chain of objects are generally best hidden behind the DIAGNOSTIC kernel option.

The MPASS() macro (read as: "must-pass") is a convenience wrapper around KASSERT() that automatically generates a simple assertion message including file and line information.

void
foo_dealloc(struct foo *fp)
{

        KASSERT((fp->foo_flags & FOO_ACTIVE) == 0,
            ("%s: fp %p is still active, flags=%x", __func__, fp,
            fp->foo_flags));
        ...
}

This assertion provides the full flag set for the object, as well as the memory pointer, which may be used by a debugger to examine the object in detail ( for example with a 'show foo' command in ddb(4) ).

The assertion

MPASS(td == curthread);

located on line 87 of a file named foo.c would generate the following panic message:

panic: Assertion td == curthread failed at foo.c:87

This is a simple condition, and the message provides enough information to investigate the failure.

The assertion

MPASS(td == curthread && (sz >= SIZE_MIN && sz <= SIZE_MAX));

is NOT useful enough. The message doesn't indicate which part of the assertion was violated, nor does it report the value of sz, which may be critical to understanding why the assertion failed.

According to the guidelines above, this would be correctly expressed as:

MPASS(td == curthread);
KASSERT(sz >= SIZE_MIN && sz <= SIZE_MAX,
    ("invalid size argument: %u", sz));