erts: Clearify erl_nif documentation about badarg exception

Also state that maximum atom length is 255 characters.

1 files changed, 27 insertions, 17 deletions

diff --git a/erts/doc/src/erl_nif.xml b/erts/doc/src/erl_nif.xml
index feba6daaa0..afeec69f02 100644
--- a/erts/doc/src/erl_nif.xml
+++ b/erts/doc/src/erl_nif.xml
@@ -899,26 +899,34 @@ typedef enum {
       <fsummary>Create an atom term</fsummary>
       <desc><p>Create an atom term from the null-terminated C-string <c>name</c>
       with iso-latin-1 encoding. If the length of <c>name</c> exceeds the maximum length
-      allowed for an atom, <c>enif_make_atom</c> returns a <c>badarg</c> exception.</p></desc>
+      allowed for an atom (255 characters), <c>enif_make_atom</c> invokes
+      <seealso marker="#enif_make_badarg">enif_make_badarg</seealso>.
+      </p></desc>
     </func>
     <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_atom_len(ErlNifEnv* env, const char* name, size_t len)</nametext></name>
       <fsummary>Create an atom term</fsummary>
       <desc><p>Create an atom term from the string <c>name</c> with length <c>len</c>.
       Null-characters are treated as any other characters. If <c>len</c> is greater than the maximum length
-      allowed for an atom, <c>enif_make_atom</c> returns a <c>badarg</c> exception.</p></desc>
+      allowed for an atom (255 characters), <c>enif_make_atom</c> invokes
+      <seealso marker="#enif_make_badarg">enif_make_badarg</seealso>.
+      </p></desc>
     </func>
     <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_badarg(ErlNifEnv* env)</nametext></name>
       <fsummary>Make a badarg exception.</fsummary>
-      <desc><p>Make a badarg exception to be returned from a NIF, and set
-      an associated exception reason in <c>env</c>. If
-      <c>enif_make_badarg</c> is called, the term it returns <em>must</em>
-      be returned from the function that called it. No other return value
-      is allowed. Once a NIF or any function it calls invokes <c>enif_make_badarg</c>,
-      the runtime ensures that a <c>badarg</c> exception is raised when the NIF
-      returns, even if the NIF attempts to return a non-exception term instead.
-      Also, the term returned from <c>enif_make_badarg</c> may be passed only to
-      <seealso marker="#enif_is_exception">enif_is_exception</seealso> and
-      not to any other NIF API function.</p></desc>
+      <desc><p>Make a badarg exception to be returned from a NIF, and associate
+      it with the environment <c>env</c>. Once a NIF or any function
+      it calls invokes <c>enif_make_badarg</c>, the runtime ensures that a
+      <c>badarg</c> exception is raised when the NIF returns, even if the NIF
+      attempts to return a non-exception term instead.
+      The return value from <c>enif_make_badarg</c> may only be used as
+      return value from the NIF that invoked it (direct or indirectly)
+      or be passed to
+      <seealso marker="#enif_is_exception">enif_is_exception</seealso>, but
+      not to any other NIF API function.</p>
+      <note><p>In earlier versions (older than erts-7.0, OTP 18) the return value
+      from <c>enif_make_badarg</c> had to be returned from the NIF. This
+      requirement is now lifted as the return value from the NIF is ignored
+      if <c>enif_make_badarg</c> has been invoked.</p></note></desc>
     </func>
     <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_binary(ErlNifEnv* env, ErlNifBinary* bin)</nametext></name>
       <fsummary>Make a binary term.</fsummary>
@@ -936,8 +944,9 @@ typedef enum {
     <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_double(ErlNifEnv* env, double d)</nametext></name>
       <fsummary>Create a floating-point term</fsummary>
       <desc><p>Create a floating-point term from a <c>double</c>. If the <c>double</c> argument is
-      not finite or is NaN, <c>enif_make_double</c> returns a <c>badarg</c> exception.</p>
-      </desc>
+      not finite or is NaN, <c>enif_make_double</c> invokes
+      <seealso marker="#enif_make_badarg">enif_make_badarg</seealso>.
+      </p></desc>
     </func>
     <func><name><ret>int</ret><nametext>enif_make_existing_atom(ErlNifEnv* env, const char* name, ERL_NIF_TERM* atom, ErlNifCharEncoding encode)</nametext></name>
       <fsummary>Create an existing atom term</fsummary>
@@ -946,7 +955,8 @@ typedef enum {
       <seealso marker="#ErlNifCharEncoding">encode</seealso>. If the atom
       already exists store the term in <c>*atom</c> and return true, otherwise
       return false. If the length of <c>name</c> exceeds the maximum length
-      allowed for an atom, <c>enif_make_existing_atom</c> returns false.</p></desc>
+      allowed for an atom (255 characters), <c>enif_make_existing_atom</c>
+      returns false.</p></desc>
     </func>
     <func><name><ret>int</ret><nametext>enif_make_existing_atom_len(ErlNifEnv* env, const char* name, size_t len, ERL_NIF_TERM* atom, ErlNifCharEncoding encoding)</nametext></name>
       <fsummary>Create an existing atom term</fsummary>
@@ -955,8 +965,8 @@ typedef enum {
       <seealso marker="#ErlNifCharEncoding">encode</seealso>. Null-characters
       are treated as any other characters. If the atom already exists store the term
       in <c>*atom</c> and return true, otherwise return false. If <c>len</c> is greater
-      than the maximum length allowed for an atom, <c>enif_make_existing_atom_len</c>
-      returns false.</p></desc>
+      than the maximum length allowed for an atom (255 characters),
+      <c>enif_make_existing_atom_len</c> returns false.</p></desc>
     </func>
     <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_int(ErlNifEnv* env, int i)</nametext></name>
       <fsummary>Create an integer term</fsummary>