moved FPS indicator prefix trailing ' ' trim

moved to video_capture.cpp  - it is a bit technical and it is nicer to
have it there than in the huge main.cpp

src/main.cpp

@@ -318,10 +318,8 @@ static void *capture_thread(void *arg)
         struct wait_obj *wait_obj = wait_obj_init();
         steady_clock::time_point t0 = steady_clock::now();
         int frames = 0;
-        char *print_fps_prefix = vidcap_get_fps_print_prefix(uv->capture_device) ? strdupa(vidcap_get_fps_print_prefix(uv->capture_device)) : NULL;
-        if (print_fps_prefix && print_fps_prefix[strlen(print_fps_prefix) - 1] == ' ') { // trim trailing ' '
-                print_fps_prefix[strlen(print_fps_prefix) - 1] = '\0';
-        }
+        const char              *print_fps_prefix =
+            vidcap_get_fps_print_prefix(uv->capture_device);
 
         while (!uv->should_exit_capture) {
                 /* Capture and transmit video... */

src/utils/macros.h

@@ -128,6 +128,7 @@
 /// the following limits are used mostly for static array allocations
 enum {
         MAX_CPU_CORES = 256,  ///< maximal expected CPU core count
+        SHORT_STR     = 128,
         STR_LEN       = 2048, ///< "standard" string length placeholder
 };
 

src/video_capture.cpp

@@ -13,7 +13,7 @@
  * @ingroup vidcap
  */
 /*
- * Copyright (c) 2005-2023 CESNET, z. s. p. o.
+ * Copyright (c) 2005-2024 CESNET
  * Copyright (c) 2001-2004 University of Southern California
  *
  * Redistribution and use in source and binary forms, with or without
@@ -61,6 +61,7 @@
 #include "debug.h"
 #include "lib_common.h"
 #include "module.h"
+#include "utils/macros.h"
 #include "video_capture.h"
 #include "video_capture_params.h"
 
@@ -229,12 +230,26 @@ struct video_frame *vidcap_grab(struct vidcap *state, struct audio_frame **audio
 }
 
 /**
- * @brief If not-NULL returned, display doesn't hae own FPS indicator and wants
- * to use a generic one (prefixed with returned module name)
+ * @returns nullptr if display has own FPS indicator
+ * @returns otherwise the prefix (without trailing space, eg. "[GL]") to be used
+ *          in a generic fps indicator
  */
 const char *vidcap_get_fps_print_prefix(struct vidcap *state)
 {
         assert(state->magic == VIDCAP_MAGIC);
-        return state->funcs->generic_fps_indicator_prefix;
+        if (state->funcs->generic_fps_indicator_prefix == NULL) {
+                return NULL;
+        }
+        thread_local char buf[SHORT_STR];
+
+        unsigned len = snprintf(buf, sizeof buf, "%s",
+                                state->funcs->generic_fps_indicator_prefix);
+        if (len > sizeof buf - 1) { // truncated
+                len = sizeof buf - 1;
+        }
+        if (len > 0 && buf[len - 1] == ' ') { // trim trailing ' '
+                buf[len - 1] = '\0';
+        }
+        return buf;
 }