Update embeding flags (#17454)

* Include threading flags. Fix #16868
* Add document about exporting symbols from main executable. Fix #17451

contrib/julia-config.jl

@@ -8,6 +8,8 @@ const options =
     "--ldlibs"
 ];
 
+threadingOn() = ccall(:jl_threading_enabled, Cint, ()) != 0
+
 function imagePath()
     opts = Base.JLOptions()
     unsafe_string(opts.image_file)
@@ -57,10 +59,11 @@ end
 function cflags()
     arg1 = replace(initDir(),"\\","\\\\\\\\")
     arg2 = replace(includeDir(),"\\","\\\\")
+    threading_def = threadingOn() ? "-DJULIA_ENABLE_THREADING=1 " : ""
     if is_unix()
-        return """-fPIC -DJULIA_INIT_DIR=\\"$arg1\\" -I$arg2"""
+        return """$(threading_def)-fPIC -DJULIA_INIT_DIR=\\"$arg1\\" -I$arg2"""
     else
-        return """-DJULIA_INIT_DIR=\\"$arg1\\" -I$arg2"""
+        return """$(threading_def)-DJULIA_INIT_DIR=\\"$arg1\\" -I$arg2"""
     end
 end
 

doc/manual/embedding.rst

@@ -56,6 +56,10 @@ Before the program terminates, it is strongly recommended to call ``jl_atexit_ho
    >>> julia.jl_init('.')
    250593296
 
+.. note::
+
+ If the julia program needs to access symbols from the main executable, it may be necessary to add ``-Wl,--export-dynamic`` linker flag at compile time on Linux in addition to the ones generated by ``julia-config.jl`` described below. This is not necessary when compiling a shared library.
+
 Using julia-config to automatically determine build parameters
 --------------------------------------------------------------
 

src/sys.c

@@ -742,6 +742,15 @@ JL_DLLEXPORT size_t jl_maxrss(void)
 #endif
 }
 
+JL_DLLEXPORT int jl_threading_enabled(void)
+{
+#ifdef JULIA_ENABLE_THREADING
+    return 1;
+#else
+    return 0;
+#endif
+}
+
 #ifdef __cplusplus
 }
 #endif

test/threads.jl

@@ -343,3 +343,12 @@ let async = Base.AsyncCondition(), t
     @test_throws EOFError wait(t)
     @test_throws EOFError wait(async)
 end
+
+# Compare the two ways of checking if threading is enabled.
+# `jl_tls_states` should only be defined on non-threading build.
+if ccall(:jl_threading_enabled, Cint, ()) == 0
+    @test nthreads() == 1
+    cglobal(:jl_tls_states) != C_NULL
+else
+    @test_throws ErrorException cglobal(:jl_tls_states)
+end