Add console:print_err/1 and route io standard_error to stderr

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 - Added `filename:dirname/1`, `filename:basename/1,2`, `filename:extension/1`, `filename:rootname/1,2` and `filename:join/2`
 - Added `init:get_arguments/0`
+- Added `console:print_err/1` to write to standard error
 - Added Erlang distribution over serial (uart)
 - Added WASM32 JIT backend for Emscripten platform
 - Added `network:wifi_scan/0,1` to ESP32 network driver to scan available APs when in sta or sta+ap mode.
@@ -54,6 +55,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `header_continuation` / `trailer_header_continuation` response events are no longer emitted
 
 ### Fixed
+- Route `io:put_chars(standard_error, ...)` and `io:format(standard_error, ...)` to stderr instead
+  of aliasing them to standard_io (diagnostics no longer pollute an escript's stdout)
 - Stop using deprecated `term_from_int32` on STM32 platform
 - Stop using deprecated `term_from_int32` on RP2 platform
 - Stop using deprecated `term_from_int32` on ESP32 platform

UPDATING.md

@@ -42,6 +42,11 @@ were using the `0x210000` offset.
   * Instantiate with `const module = await AtomVM({ arguments: [...] })`
   * The old implicit global-script pattern is no longer supported
   * `AtomVM.worker.js` is no longer emitted; only `AtomVM.mjs` and `AtomVM.wasm` are needed
+- `io:put_chars(standard_error, ...)` (and `io:format/3`, `io:fwrite/3` targeting
+  `standard_error`) now write directly to the OS standard error stream via `console:print_err/1`,
+  instead of being routed through the process group leader as an alias for `standard_io`. Code or
+  tests that captured `standard_error` output by swapping the group leader must instead redirect
+  or intercept the underlying stderr stream, or pass an explicit pid/device.
 
 ## v0.6.4 -> v0.6.5
 

libs/eavmlib/src/console.erl

@@ -24,7 +24,7 @@
 %%-----------------------------------------------------------------------------
 -module(console).
 
--export([start/0, puts/1, puts/2, flush/0, flush/1, print/1]).
+-export([start/0, puts/1, puts/2, flush/0, flush/1, print/1, print_err/1]).
 
 %%-----------------------------------------------------------------------------
 %% @param   Text the string data to write to the console
@@ -74,6 +74,17 @@ flush(Console) ->
 print(_Text) ->
     erlang:nif_error(undefined).
 
+%%-----------------------------------------------------------------------------
+%% @param   Text the data to write to the standard error
+%% @returns ok if the data was written, or {error, Reason}, if there was
+%%          an error.
+%% @doc     Write a string to the console's standard error.
+%% @end
+%%-----------------------------------------------------------------------------
+-spec print_err(Text :: iodata()) -> ok | {error, term()}.
+print_err(_Text) ->
+    erlang:nif_error(undefined).
+
 %% Internal operations
 
 %% @private

libs/estdlib/src/io.erl

@@ -257,7 +257,7 @@ put_chars(standard_io, Chars) ->
             execute_request(Leader, {put_chars, unicode, Chars})
     end;
 put_chars(standard_error, Chars) ->
-    put_chars(standard_io, Chars).
+    console:print_err(Chars).
 
 %% @private
 convert_request({requests, Requests}) ->

src/libAtomVM/nifs.c

@@ -257,6 +257,7 @@ static term nif_atomvm_get_start_beam(Context *ctx, int argc, term argv[]);
 static term nif_atomvm_read_priv(Context *ctx, int argc, term argv[]);
 static term nif_atomvm_get_creation(Context *ctx, int argc, term argv[]);
 static term nif_console_print(Context *ctx, int argc, term argv[]);
+static term nif_console_print_err(Context *ctx, int argc, term argv[]);
 static term nif_base64_encode(Context *ctx, int argc, term argv[]);
 static term nif_base64_decode(Context *ctx, int argc, term argv[]);
 static term nif_base64_encode_to_string(Context *ctx, int argc, term argv[]);
@@ -827,6 +828,10 @@ static const struct Nif console_print_nif = {
     .base.type = NIFFunctionType,
     .nif_ptr = nif_console_print
 };
+static const struct Nif console_print_err_nif = {
+    .base.type = NIFFunctionType,
+    .nif_ptr = nif_console_print_err
+};
 static const struct Nif base64_encode_nif = {
     .base.type = NIFFunctionType,
     .nif_ptr = nif_base64_encode
@@ -5635,16 +5640,14 @@ static term nif_atomvm_get_creation(Context *ctx, int argc, term argv[])
     return term_make_maybe_boxed_int64(ctx->global->creation, &ctx->heap);
 }
 
-static term nif_console_print(Context *ctx, int argc, term argv[])
+static term console_print_to(FILE *stream, Context *ctx, term argv[])
 {
-    UNUSED(argc);
-
     term t = argv[0];
     if (term_is_binary(t)) {
         const char *data = term_binary_data(t);
-        unsigned long n = term_binary_size(t);
-        fprintf(stdout, "%.*s", (int) n, data);
-        fflush(stdout);
+        size_t n = term_binary_size(t);
+        fwrite(data, 1, n, stream);
+        fflush(stream);
     } else {
         size_t size;
         switch (interop_iolist_size(t, &size)) {
@@ -5655,6 +5658,10 @@ static term nif_console_print(Context *ctx, int argc, term argv[])
             case InteropBadArg:
                 RAISE_ERROR(BADARG_ATOM);
         }
+        if (size == 0) {
+            fflush(stream);
+            return OK_ATOM;
+        }
         char *buf = malloc(size);
         if (IS_NULL_PTR(buf)) {
             RAISE_ERROR(OUT_OF_MEMORY_ATOM);
@@ -5669,13 +5676,25 @@ static term nif_console_print(Context *ctx, int argc, term argv[])
                 free(buf);
                 RAISE_ERROR(BADARG_ATOM);
         }
-        fprintf(stdout, "%.*s", (int) size, buf);
-        fflush(stdout);
+        fwrite(buf, 1, size, stream);
+        fflush(stream);
         free(buf);
     }
     return OK_ATOM;
 }
 
+static term nif_console_print(Context *ctx, int argc, term argv[])
+{
+    UNUSED(argc);
+    return console_print_to(stdout, ctx, argv);
+}
+
+static term nif_console_print_err(Context *ctx, int argc, term argv[])
+{
+    UNUSED(argc);
+    return console_print_to(stderr, ctx, argv);
+}
+
 // clang-format off
 static const char b64_table[64] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
                                    'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',

src/libAtomVM/nifs.gperf

@@ -225,6 +225,7 @@ code_server:type_resolver/2, IF_HAVE_JIT(&code_server_type_resolver_nif)
 code_server:import_resolver/2, IF_HAVE_JIT(&code_server_import_resolver_nif)
 code_server:set_native_code/3, IF_HAVE_JIT(&code_server_set_native_code_nif)
 console:print/1, &console_print_nif
+console:print_err/1, &console_print_err_nif
 base64:encode/1, &base64_encode_nif
 base64:encode/2, &base64_encode_nif
 base64:decode/1, &base64_decode_nif