Cygwin: Implement the GNU extension clearenv

winsup/cygwin/common.din

@@ -306,6 +306,7 @@ cimag NOSIGFE
 cimagf NOSIGFE
 cimagl NOSIGFE
 cleanup_glue NOSIGFE
+clearenv SIGFE
 clearerr SIGFE
 clearerr_unlocked SIGFE
 clock SIGFE

winsup/cygwin/environ.cc

@@ -720,6 +720,26 @@ unsetenv (const char *name)
   return -1;
 }
 
+/* Clear the environment.  */
+extern "C" int
+clearenv (void)
+{
+  __try
+    {
+      if (cur_environ () == lastenviron)
+	{
+	  free (lastenviron);
+	  lastenviron = NULL;
+	}
+      __cygwin_environ = NULL;
+      update_envptrs ();
+      return 0;
+    }
+  __except (EFAULT) {}
+  __endtry
+  return -1;
+}
+
 /* Minimal list of Windows vars which must be converted to uppercase.
    Either for POSIX compatibility of for backward compatibility with
    existing applications. */

winsup/cygwin/include/cygwin/stdlib.h

@@ -26,6 +26,9 @@ char *canonicalize_file_name (const char *);
 #if __BSD_VISIBLE || __POSIX_VISIBLE >= 200112
 int unsetenv (const char *);
 #endif
+#if __MISC_VISIBLE
+int clearenv (void);
+#endif
 #if __XSI_VISIBLE
 char *ptsname (int);
 int grantpt (int);