command: allow passing memory addresses to overlay_add

For the sake of libmpv. Might make things much easier for the user,
especially on Windows. On the other hand, it's a bit sketchy that a
command exists that makes the player access arbitrary memory regions.
(But do note that input commands are not meant to be "secure" and never
were - for example, there's the "run" command, which obviously allows
running random shell commands.)

DOCS/man/input.rst

@@ -456,6 +456,16 @@ Input Commands that are Possibly Subject to Change
     random points. Instead, call ``overlay_add`` again (preferably with a
     different memory region to prevent tearing).
 
+    It is also possible to pass a raw memory address for use as bitmap memory
+    by passing a memory address as integer prefixed with a ``&`` character.
+    Passing the wrong thing here will crash the player. The ``offset`` parameter
+    is not used and must be 0. This mode might be useful for use with libmpv.
+
+    On Windows, currently only raw memory addresses work. File mapping is not
+    implemented because a ``mmap`` compatibility layer is missing, and because
+    this kind of shared memory method would perhaps not be overly useful on
+    Windows.
+
     ``offset`` is the offset of the first pixel in the source file. It is
     passed directly to ``mmap`` and is subject to certain restrictions
     (see ``man mmap`` for details). In particular, this value has to be a

player/command.c

@@ -3326,6 +3326,12 @@ static int overlay_add(struct MPContext *mpctx, int id, int x, int y,
         if (!file[1] || end[0])
             fd = -1;
         close_fd = false;
+    } else if (file[0] == '&') {
+        char *end;
+        unsigned long long addr = strtoull(&file[1], &end, 0);
+        if (!file[1] || end[0])
+            addr = 0;
+        p = (void *)(uintptr_t)addr;
     } else {
         fd = open(file, O_RDONLY | O_BINARY | O_CLOEXEC);
     }