c++: distinguish errors from other logs

Errors and normal logs are normaly logged differently in js and also
displayed differently in browser consoles. For this to work for our
logs from WASM, we must output errors to stderr and otherwise stdout.

Author: dmitrylyzo

src/SubtitleOctopus.cpp

@@ -67,9 +67,13 @@ void buffer_free(buffer_t *buf) {
 void msg_callback(int level, const char *fmt, va_list va, void *data) {
     if (level > log_level) // 6 for verbose
         return;
-    printf("libass: ");
-    vprintf(fmt, va);
-    printf("\n");
+
+    const int ERR_LEVEL = 1;
+    FILE* stream = level <= ERR_LEVEL ? stderr : stdout;
+
+    fprintf(stream, "libass: ");
+    vfprintf(stream, fmt, va);
+    fprintf(stream, "\n");
 }
 
 const float MIN_UINT8_CAST = 0.9 / 255;
@@ -112,15 +116,15 @@ class SubtitleOctopus {
     void initLibrary(int frame_w, int frame_h) {
         ass_library = ass_library_init();
         if (!ass_library) {
-            printf("ass_library_init failed!\n");
+            fprintf(stderr, "ass_library_init failed!\n");
             exit(2);
         }
 
         ass_set_message_cb(ass_library, msg_callback, NULL);
 
         ass_renderer = ass_renderer_init(ass_library);
         if (!ass_renderer) {
-            printf("ass_renderer_init failed!\n");
+            fprintf(stderr, "ass_renderer_init failed!\n");
             exit(3);
         }
 
@@ -135,7 +139,7 @@ class SubtitleOctopus {
         removeTrack();
         track = ass_read_file(ass_library, subfile, NULL);
         if (!track) {
-            printf("Failed to start a track\n");
+            fprintf(stderr, "Failed to start a track\n");
             exit(4);
         }
     }
@@ -144,7 +148,7 @@ class SubtitleOctopus {
         removeTrack();
         track = ass_read_memory(ass_library, buf, (size_t)bufsize, NULL);
         if (!track) {
-            printf("Failed to start a track\n");
+            fprintf(stderr, "Failed to start a track\n");
             exit(4);
         }
     }
@@ -266,7 +270,7 @@ class SubtitleOctopus {
         // make float buffer for blending
         float* buf = (float*)buffer_resize(&m_blend, sizeof(float) * width * height * 4, 0);
         if (buf == NULL) {
-            printf("libass: error: cannot allocate buffer for blending\n");
+            fprintf(stderr, "libass: cannot allocate buffer for blending\n");
             return &m_blendResult;
         }
         memset(buf, 0, sizeof(float) * width * height * 4);