diff --git a/include/ruby/internal/intern/sprintf.h b/include/ruby/internal/intern/sprintf.h
index f188529ab8..aedc0f9ab1 100644
--- a/include/ruby/internal/intern/sprintf.h
+++ b/include/ruby/internal/intern/sprintf.h
@@ -18,25 +18,141 @@
  *             Do not  expect for  instance `__VA_ARGS__` is  always available.
  *             We assume C99  for ruby itself but we don't  assume languages of
  *             extension libraries.  They could be written in C++98.
- * @brief      Our own private printf(3).
+ * @brief      Our own private `printf(3)`.
  */
 #include "ruby/internal/attr/format.h"
+#include "ruby/internal/attr/nonnull.h"
 #include "ruby/internal/dllexport.h"
 #include "ruby/internal/value.h"
 
 RBIMPL_SYMBOL_EXPORT_BEGIN()
 
 /* sprintf.c */
-VALUE rb_f_sprintf(int, const VALUE*);
 
+/**
+ * Identical to rb_str_format(), except how the arguments are arranged.
+ *
+ * @param[in]  argc  Number of objects of `argv`.
+ * @param[in]  argv  A format string, followed by its arguments.
+ * @return     A rendered new instance of ::rb_cString.
+ *
+ * @internal
+ *
+ * You can safely pass NULL to `argv`.  Doesn't make any sense though.
+ */
+VALUE rb_f_sprintf(int argc, const VALUE *argv);
+
+RBIMPL_ATTR_NONNULL((1))
 RBIMPL_ATTR_FORMAT(RBIMPL_PRINTF_FORMAT, 1, 2)
-VALUE rb_sprintf(const char*, ...);
-VALUE rb_vsprintf(const char*, va_list);
+/**
+ * Ruby's extended `sprintf(3)`.   We ended up reinventing  the entire `printf`
+ * business because we  don't want to depend on  locales.  OS-provided `printf`
+ * routines  might or  might  not,  which caused  instabilities  of the  result
+ * strings.
+ *
+ * The format  sequence is a  mixture of  format specifiers and  other verbatim
+ * contents.  Each  format specifier starts with  a `%`, and has  the following
+ * structure:
+ *
+ * ```
+ * %[flags][width][.precision][length]conversion
+ * ```
+ *
+ * This  function  supports  flags  of   ` `, `#`,  `+`,  `-`,  `0`,  width  of
+ * non-negative  decimal integer  and  `*`, precision  of non-negative  decimal
+ * integers and `*`, length of `L`,  `h`, `t`, `z`, `l`, `ll`, `q`, conversions
+ * of `A`,  `D`, `E`, `G`, `O`,  `U`, `X`, `a`,  `c`, `d`, `e`, `f`,  `g`, `i`,
+ * `n`, `o`, `p`, `s`, `u`, `x`, and `%`.  In case of `_WIN32` it also supports
+ * `I`.   And additionally,  it  supports magical  `PRIsVALUE`  macro that  can
+ * stringise arbitrary Ruby objects:
+ *
+ * ```CXX
+ * rb_sprintf("|%"PRIsVALUE"|", RUBY_Qtrue); // => "|true|"
+ * rb_sprintf("%+"PRIsVALUE, rb_stdin);      // => "#<IO:<STDIN>>"
+ * ```
+ *
+ * @param[in]  fmt  A `printf`-like format specifier.
+ * @param[in]  ...  Variadic number of contents to format.
+ * @return     A rendered new instance of ::rb_cString.
+ *
+ * @internal
+ *
+ * :FIXME:  We can improve this document.
+ */
+VALUE rb_sprintf(const char *fmt, ...);
 
+RBIMPL_ATTR_NONNULL((1))
+RBIMPL_ATTR_FORMAT(RBIMPL_PRINTF_FORMAT, 1, 0)
+/**
+ * Identical to rb_sprintf(), except it takes a `va_list`.
+ *
+ * @param[in]  fmt  A `printf`-like format specifier.
+ * @param[in]  ap   Contents to format.
+ * @return     A rendered new instance of ::rb_cString.
+ */
+VALUE rb_vsprintf(const char *fmt, va_list ap);
+
+RBIMPL_ATTR_NONNULL((2))
 RBIMPL_ATTR_FORMAT(RBIMPL_PRINTF_FORMAT, 2, 3)
-VALUE rb_str_catf(VALUE, const char*, ...);
-VALUE rb_str_vcatf(VALUE, const char*, va_list);
-VALUE rb_str_format(int, const VALUE *, VALUE);
+/**
+ * Identical to  rb_sprintf(), except  it renders the  output to  the specified
+ * object rather than creating a new one.
+ *
+ * @param[out]  dst            String to modify.
+ * @param[in]   fmt            A `printf`-like format specifier.
+ * @param[in]   ...            Variadic number of contents to format.
+ * @exception   rb_eTypeError  `dst` is not a String.
+ * @return      Passed `dst`.
+ * @post        `dst` has the rendered output appended to its end.
+ */
+VALUE rb_str_catf(VALUE dst, const char *fmt, ...);
+
+RBIMPL_ATTR_NONNULL((2))
+RBIMPL_ATTR_FORMAT(RBIMPL_PRINTF_FORMAT, 2, 0)
+/**
+ * Identical to  rb_str_catf(), except it  takes a  `va_list`.  It can  also be
+ * seen as a  routine identical to rb_vsprintf(), except it  renders the output
+ * to the specified object rather than creating a new one.
+ *
+ * @param[out]  dst            String to modify.
+ * @param[in]   fmt            A `printf`-like format specifier.
+ * @param[in]   ap             Contents to format.
+ * @exception   rb_eTypeError  `dst` is not a String.
+ * @return      Passed `dst`.
+ * @post        `dst` has the rendered output appended to its end.
+ */
+VALUE rb_str_vcatf(VALUE dst, const char *fmt, va_list ap);
+
+/**
+ * Formats a string.
+ *
+ * Returns  the string  resulting from  applying `fmt`  to `argv`.   The format
+ * sequence  is a  mixture of  format specifiers  and other  verbatim contents.
+ * Each format specifier starts with a `%`, and has the following structure:
+ *
+ * ```
+ * %[flags][width][.precision]type
+ * ```
+ *
+ * ...  which is  different from  that of  rb_sprintf().  Because  ruby has  no
+ * `short` or `long`, there is no way to specify a "length" of an argument.
+ *
+ * This function  supports flags  of ` `,  `#`, `+`, `-`,  `<>`, `{}`,  with of
+ * non-negative decimal integer and `$`, `*`, precision of non-negative decimal
+ * integer and `$`, `*`,  type of `A`, `B`, `E`, `G`, `X`,  `a`, `b`, `c`, `d`,
+ * `e`,  `f`, `g`,  `i`, `o`,  `p`,  `s`, `u`,  `x`,  `%`.  This  list is  also
+ * (largely the same but) not identical to that of rb_sprintf().
+ *
+ * @param[in]  argc           Number of objects in `argv`.
+ * @param[in]  argv           Format arguments.
+ * @param[in]  fmt            A printf-like format specifier.
+ * @exception  rb_eTypeError  `fmt` is not a string.
+ * @exception  rb_eArgError   Failed to parse `fmt`.
+ * @return     A rendered new instance of ::rb_cString.
+ * @note       Everything it takes must be Ruby objects.
+ *
+ */
+VALUE rb_str_format(int argc, const VALUE *argv, VALUE fmt);
 
 RBIMPL_SYMBOL_EXPORT_END()