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#define ARRAY_SIZE(a)       (sizeof(a) / sizeof((a)[0]))

#define sbprintf(buff, written, fmt, ...)                               \
        swnprintf(buff, written, ARRAY_SIZE(buff), fmt, ##__VA_ARGS__);

#define ARRAY_SIZE(a)       (sizeof(a) / sizeof((a)[0]))

#define sbprintf(buff, written, fmt, ...)                               \
        swnprintf(buff, written, ARRAY_SIZE(buff), fmt, ##__VA_ARGS__)

EDIT:

When using __attribute__((warn_unused_result)), the macro sbprintf silences the warning because of the way it works; to warn the user, the following code can be used:

#define ARRAY_SIZE(a)       (sizeof(a) / sizeof((a)[0]))

#define sbprintf(buff, written, fmt, ...)                               \
        swnprintf(buff, written, ARRAY_SIZE(buff), fmt, ##__VA_ARGS__);

Note: GCC 8 warns (-Werror=sizeof-pointer-div) if buff is not an array without needing a static_assert, when using ARRAY_SIZE defined this way. If using an old version of GCC, you will need to use some tricks to block compilation.

The return value should always be used:

__attribute__((warn_unused_result))

Add checks to the format and varargs:

__attribute__((format(printf, 4, 5)))