This is the code that Toby Speight suggested in his answer:

/*
 * int  sbprintf(buff[restrict], int *restrict written,
 *                              const char *restrict format, ...);
 */
#define sbprintf(buff, written, format, ...)    (                       \
{                                                                       \
        int     len_;                                                   \
                                                                        \
        alx_static_assert_array(buff);                                  \
                                                                        \
        len_    = snprintf(buff, sizeof(buff), format, ##__VA_ARGS__);  \
        if (written != NULL)                                            \
                *written = len_;                                        \
        len_ >= (int)sizeof(buff);                                      \
}                                                                       \
)

I have tested it and works as expected:

• If written is NULL it doesn't write into it.
• The return value is true when the string is truncated and false otherwise.
• It doesn't compile if buff is not an array.
• It accepts a variable number of arguments after format, including no arguments.

The comment above the macro is the prototype that a user should see, to better understand the usage, although a real function with that prototype wouldn't work because of the array decaying to a pointer.

Comments about the style:

I tried to follow the Linux Kernel Coding Style, but there have been exceptions:

if (written != NULL) is used instead of if (written) to avoid the compiler complaining:

main.c:22:23: error: the address of ‘w1’ will always evaluate as ‘true’ [-Werror=address]
  if (alx_sbprintf(b1, &w1, test))
                       ^
.../libalx/base/stdio/sbprintf.h:36:6: note: in definition of macro ‘alx_sbprintf’
  if (written)       \
      ^~~~~~~