Don't use '!' for tests unless it's a boolean, e.g. use

For tests, always compare the value to the appropriate 0 instead of

checking it directly, unless the value is a boolean.

For pointers, use:

.Bd -literal

if (p != NULL)

.Ed

.Pp

not

.PP

.Bd -literal

if (!p)

.Ed

.Pp

For other values, use:

if (!*p)

if (*p)

.Ed

.Pp

unless the value is a boolean. In that case, use:

.Bd -literal

if (p)

.Ed

.Pp

and

.Bd -literal

if (!p)