diff options
| -rw-r--r-- | lib/erl_interface/doc/src/ei.xml | 56 |
1 files changed, 37 insertions, 19 deletions
diff --git a/lib/erl_interface/doc/src/ei.xml b/lib/erl_interface/doc/src/ei.xml index 7808bfd94f..c22f3fb0ce 100644 --- a/lib/erl_interface/doc/src/ei.xml +++ b/lib/erl_interface/doc/src/ei.xml @@ -849,30 +849,48 @@ ei_encode_tuple_header(buf, &i, 0);</pre> </type> <desc> <marker id="ei_set_compat_rel"></marker> - <p>By default, the <c>ei</c> library is only guaranteed - to be compatible with other Erlang/OTP components from the same - release as the <c>ei</c> library itself. For example, - <c>ei</c> from - Erlang/OTP R10 is not compatible with an Erlang emulator - from Erlang/OTP R9 by default.</p> - <p>A call to <c>ei_set_compat_rel(release_number)</c> sets - the <c>ei</c> library in compatibility mode of release - <c>release_number</c>. Valid range of - <c>release_number</c> - is <c>[7, current release]</c>. This makes it possible to - communicate with Erlang/OTP components from earlier releases.</p> + <p>In general, the <c>ei</c> library is guaranteed + to be compatible with other Erlang/OTP components that are 2 major + releases older or newer than the <c>ei</c> library itself.</p> + <p>Sometimes an exception to the above rule has to be made to make new + features (or even bug fixes) possible. A call to + <c>ei_set_compat_rel(release_number)</c> sets + the <c>ei</c> library in compatibility mode of OTP release + <c>release_number</c>.</p> + <p>The only useful value for <c>release_number</c> is currently + <c>21</c>. This will only be useful and have an effect if <em>bit + strings</em> or <em>export funs</em> are received from a connected + node. Before OTP 22, bit strings and export funs were not supported by + <c>ei</c>. They were instead encoded using an undocumented fallback + tuple format when sent from the emulator to <c>ei</c>:</p> + <taglist> + <tag><c>Bit string</c></tag> + <item><p>The term <c><<42, 1:1>></c> was encoded as + <c>{<<42, 128>>, 1}</c>. The first element of the tuple is a + binary and the second element denotes how many bits of the last bytes + are part of the bit string. In this example only the most significant + bit of the last byte (128) is part of the bit string.</p> + </item> + <tag><c>Export fun</c></tag> + <item><p>The term <c>fun lists:map/2</c> was encoded as + <c>{lists,map}</c>. A tuple with the module, function and a missing + arity.</p> + </item> + </taglist> + <p>If <c>ei_set_compat_rel(21)</c> is <em>not</em> called then a connected + emulator will send bit strings and export funs correctly encoded. The + functions <seealso marker="#ei_decode_bitstring"><c>ei_decode_bitstring</c></seealso> + and <seealso marker="#ei_decode_fun"><c>ei_decode_fun</c></seealso> + has to be used to decode such terms. Calling + <c>ei_set_compat_rel(21)</c> should only be done as a workaround to + keep an old implementation alive, which expects to receive the + undocumented tuple formats for bit strings and/or export funs. + </p> <note> <p>If this function is called, it can only be called once and must be called before any other functions in the <c>ei</c> library are called.</p> </note> - <warning> - <p>You can run into trouble if this feature is used - carelessly. Always ensure that all communicating - components are either from the same Erlang/OTP release, or - from release X and release Y where all components - from release Y are in compatibility mode of release X.</p> - </warning> </desc> </func> |