Engine-folder preload helper for Linux: b2LoadNativeLibHere() (no sudo)

src/box2dxt.lcb

@@ -442,8 +442,8 @@ private foreign handler _query_normal_y(in pI as CInt) returns CDouble binds to
 private foreign handler _query_fraction(in pI as CInt) returns CDouble binds to "c:box2dxt>b2lc_query_fraction!cdecl"
 
 -- ---- native loader assist (Linux) ----------------------------------
--- The ONLY two bindings that do NOT target the box2dxt shim: they bind to
--- the platform's own dynamic loader (dlopen/dlerror), resolved from the
+-- The ONLY three bindings that do NOT target the box2dxt shim: they bind to
+-- the platform's own loader/libc (dlopen/dlerror/realpath), resolved from the
 -- host process's global symbol table. That is the whole point -- they must
 -- be callable BEFORE box2dxt.so is loadable, so a script can preload the
 -- shim by absolute path. On Linux the engine hands the bare name
@@ -459,6 +459,10 @@ private foreign handler _query_fraction(in pI as CInt) returns CDouble binds to
 -- dlopen still lives in libdl -> "c:libdl.so.2>dlopen!cdecl".
 private foreign handler _dlopen(in pPath as ZStringNative, in pFlags as CInt) returns optional Pointer binds to "c:>dlopen!cdecl"
 private foreign handler _dlerror() returns optional ZStringNative binds to "c:>dlerror!cdecl"
+-- realpath("/proc/self/exe", nil) -> the running engine's own executable path
+-- on Linux; used by the b2LoadNativeLibHere helper below to locate the engine
+-- folder. Returns nothing off Linux (no /proc). Only ever called on Linux.
+private foreign handler _realpath(in pPath as ZStringNative, in pResolved as optional Pointer) returns optional ZStringNative binds to "c:>realpath!cdecl"
 
 -- small bool -> int helper (no foreign call, so no unsafe needed)
 private handler bi(in pB as Boolean) returns Integer
@@ -468,6 +472,57 @@ private handler bi(in pB as Boolean) returns Integer
    return 0
 end handler
 
+-- ---- engine-folder preload (Linux helper) --------------------------
+-- Locate the running engine's OWN folder (realpath /proc/self/exe), strip to
+-- its directory, and preload "<dir>/box2dxt.so" by absolute path -- so a user
+-- can just drop the library beside the OXT engine (or bundle it in a
+-- standalone) and a later bare-name bind resolves to it (no /usr/lib, no sudo,
+-- no LD_LIBRARY_PATH). Returns 1 if a library was loaded, else 0.
+--
+-- LINUX ONLY, and deliberately NOT auto-fired: realpath/dlopen are POSIX, so
+-- call this ONLY when the platform is Linux (e.g. the Kit gates it with
+-- `the platform is "Linux"`). It is intentionally NOT invoked from
+-- b2Version/checkABI -- those run on every platform, and touching the POSIX
+-- bindings on Windows/macOS would error (those platforms already load the
+-- library from beside the stack/app anyway). Requires the library's soname to
+-- be "box2dxt.so" (-DBOX2DXT_BARE_SONAME) or the resident copy will not match
+-- the engine's bare-name request.
+public handler b2LoadNativeLibHere() returns Integer
+   variable tExe as optional String
+   variable tPath as String
+   variable tLen as Integer
+   variable tCut as Integer
+   variable i as Integer
+   variable tLib as String
+   variable tHandle as optional Pointer
+   unsafe
+      put _realpath("/proc/self/exe", nothing) into tExe
+   end unsafe
+   if tExe is nothing then
+      return 0
+   end if
+   put tExe into tPath
+   put the number of chars in tPath into tLen
+   put 0 into tCut
+   repeat with i from 1 up to tLen
+      if char i of tPath is "/" then
+         put i into tCut
+      end if
+   end repeat
+   if tCut is 0 then
+      return 0
+   end if
+   put char 1 to tCut of tPath into tLib
+   put tLib & "box2dxt.so" into tLib
+   unsafe
+      put _dlopen(tLib, 258) into tHandle
+   end unsafe
+   if tHandle is nothing then
+      return 0
+   end if
+   return 1
+end handler
+
 -- =====================================================================
 -- Public API (callable from xTalk / OpenXTalk Script)
 -- =====================================================================