cs: more portable approach to errno

Have the main Racket executable export a function to get errno, which
avoids portability problems trying to access it via the FFI.

Related to #2344

racket/src/cs/c/boot.c

@@ -5,6 +5,7 @@
 #include <fcntl.h>
 #include <string.h>
 #include <stdlib.h>
+#include <errno.h>
 #include "scheme.h"
 #include "rktio.h"
 
@@ -78,10 +79,16 @@ static void racket_exit(int v)
   exit(v);
 }
 
+static int racket_errno()
+{
+  return errno;
+}
+
 static void init_foreign()
 {
 # include "rktio.inc"
   Sforeign_symbol("racket_exit", (void *)racket_exit);
+  Sforeign_symbol("racket_errno", (void *)racket_errno);
 }
 
 void racket_boot(int argc, char **argv, char *self,

racket/src/cs/rumble/foreign.ss

@@ -1814,24 +1814,36 @@
 
 ;; function is called with interrupts disabled
 (define get-errno
-  (let ([get-&errno-name
-         (case (machine-type)
-           [(a6nt ta6nt i3nt ti3nt)
-            (load-shared-object "msvcrt.dll")
-            "_errno"]
-           [(a6osx ta6osx i3osx ti3osx)
-            (load-shared-object "libc.dylib")
-            "__error"]
-           [(a6le ta6le i3le ti3le)
-            (load-shared-object "libc.so.6")
-            "__errno_location"]
-           [else
-            ;; FIXME for more platforms
-            (load-shared-object "libc.so")
-            "__error"])])
-    (let ([get-&errno (foreign-procedure get-&errno-name () void*)])
-      (lambda ()
-        (foreign-ref 'int (get-&errno) 0)))))
+  (cond
+   [(foreign-entry? "racket_errno")
+    (foreign-procedure "racket_errno" () int)]
+   [else
+    ;; We get here only during a bootstrapping process or in a
+    ;; development mode that is not running in a Racket executable
+    (let ([get-&errno-name
+           (case (machine-type)
+             [(a6nt ta6nt i3nt ti3nt)
+              (load-shared-object "msvcrt.dll")
+              "_errno"]
+             [(a6osx ta6osx i3osx ti3osx)
+              (load-shared-object "libc.dylib")
+              "__error"]
+             [(a6le ta6le i3le ti3le)
+              (load-shared-object "libc.so.6")
+              "__errno_location"]
+             [else #f])])
+      (cond
+       [get-&errno-name
+        (let ([get-&errno (foreign-procedure get-&errno-name () void*)])
+          (lambda ()
+            (foreign-ref 'int (get-&errno) 0)))]
+       [else
+        (let ([warned? #f])
+          (lambda ()
+            (unless warned?
+              (set! warned? #t)
+              (#%printf "Warning: not recording actual errno value\n"))
+            0))]))]))
 
 ;; function is called with interrupts disabled
 (define get-last-error