fix Issue 17829 - core.stdc.errno does not work with -betterC

src/core/memory.d

@@ -814,6 +814,7 @@ struct GC
  * Pure variants of C's memory allocation functions `malloc`, `calloc`, and
  * `realloc` and deallocation function `free`.
  *
+ * UNIX 98 requires that errno be set to ENOMEM upon failure.
  * Purity is achieved by saving and restoring the value of `errno`, thus
  * behaving as if it were never changed.
  *
@@ -823,42 +824,33 @@ struct GC
  */
 void* pureMalloc(size_t size) @trusted pure @nogc nothrow
 {
-    const errno = fakePureGetErrno();
+    const errnosave = fakePureErrno();
     void* ret = fakePureMalloc(size);
-    if (!ret || errno != 0)
-    {
-        cast(void)fakePureSetErrno(errno);
-    }
+    fakePureErrno() = errnosave;
     return ret;
 }
 /// ditto
 void* pureCalloc(size_t nmemb, size_t size) @trusted pure @nogc nothrow
 {
-    const errno = fakePureGetErrno();
+    const errnosave = fakePureErrno();
     void* ret = fakePureCalloc(nmemb, size);
-    if (!ret || errno != 0)
-    {
-        cast(void)fakePureSetErrno(errno);
-    }
+    fakePureErrno() = errnosave;
     return ret;
 }
 /// ditto
 void* pureRealloc(void* ptr, size_t size) @system pure @nogc nothrow
 {
-    const errno = fakePureGetErrno();
+    const errnosave = fakePureErrno();
     void* ret = fakePureRealloc(ptr, size);
-    if (!ret || errno != 0)
-    {
-        cast(void)fakePureSetErrno(errno);
-    }
+    fakePureErrno() = errnosave;
     return ret;
 }
 /// ditto
 void pureFree(void* ptr) @system pure @nogc nothrow
 {
-    const errno = fakePureGetErrno();
+    const errnosave = fakePureErrno();
     fakePureFree(ptr);
-    cast(void)fakePureSetErrno(errno);
+    fakePureErrno() = errnosave;
 }
 
 ///
@@ -881,36 +873,45 @@ void pureFree(void* ptr) @system pure @nogc nothrow
 
 @system pure nothrow @nogc unittest
 {
-    const int errno = fakePureGetErrno();
+    const int errno = fakePureErrno();
 
     void* x = pureMalloc(10);            // normal allocation
-    assert(errno == fakePureGetErrno()); // errno shouldn't change
+    assert(errno == fakePureErrno()); // errno shouldn't change
     assert(x !is null);                   // allocation should succeed
 
     x = pureRealloc(x, 10);              // normal reallocation
-    assert(errno == fakePureGetErrno()); // errno shouldn't change
+    assert(errno == fakePureErrno()); // errno shouldn't change
     assert(x !is null);                   // allocation should succeed
 
     fakePureFree(x);
 
     void* y = pureCalloc(10, 1);         // normal zeroed allocation
-    assert(errno == fakePureGetErrno()); // errno shouldn't change
+    assert(errno == fakePureErrno()); // errno shouldn't change
     assert(y !is null);                   // allocation should succeed
 
     fakePureFree(y);
 
     // subtract 2 because snn.lib adds 2 unconditionally before passing
     //  the size to the Windows API
     void* z = pureMalloc(size_t.max - 2); // won't affect `errno`
-    assert(errno == fakePureGetErrno()); // errno shouldn't change
+    assert(errno == fakePureErrno()); // errno shouldn't change
     assert(z is null);
 }
 
 // locally purified for internal use here only
+
+extern (C) private @system @nogc nothrow
+{
+    ref int fakePureErrnoImpl()
+    {
+        import core.stdc.errno;
+        return errno();
+    }
+}
+
 extern (C) private pure @system @nogc nothrow
 {
-    pragma(mangle, "getErrno") int fakePureGetErrno();
-    pragma(mangle, "setErrno") int fakePureSetErrno(int);
+    pragma(mangle, "fakePureErrnoImpl") ref int fakePureErrno();
 
     pragma(mangle, "malloc") void* fakePureMalloc(size_t);
     pragma(mangle, "calloc") void* fakePureCalloc(size_t nmemb, size_t size);

src/core/stdc/errno.d

@@ -8,7 +8,7 @@
  *      $(LINK2 http://www.boost.org/LICENSE_1_0.txt, Boost Software License 1.0).
  *    (See accompanying file LICENSE)
  * Authors:   Sean Kelly, Alex Rønne Petersen
- * Source:    $(DRUNTIMESRC core/stdc/_errno.d)
+ * Source:    https://github.com/dlang/druntime/blob/master/src/core/stdc/errno.d
  * Standards: ISO/IEC 9899:1999 (E)
  */
 
@@ -27,15 +27,78 @@ else version (WatchOS)
 nothrow:
 @nogc:
 
-///
-@property int errno() { return getErrno(); }
-///
-@property int errno(int n) { return setErrno(n); }
+version (CRuntime_DigitalMars)
+{
+    extern (C)
+    {
+        ref int _errno();
+        alias errno = _errno;
+    }
+}
+else version (CRuntime_Microsoft)
+{
+    extern (C)
+    {
+        ref int _errno();
+        alias errno = _errno;
+    }
+}
+else version (CRuntime_Glibc)
+{
+    extern (C)
+    {
+        ref int __errno_location();
+        alias errno = __errno_location;
+    }
+}
+else version (FreeBSD)
+{
+    extern (C)
+    {
+        ref int __error();
+        alias errno = __error;
+    }
+}
+else version (linux)
+{
+    extern (C)
+    {
+        ref int __errno_location();
+        alias errno = __errno_location;
+    }
+}
+else version (Darwin)
+{
+    extern (C)
+    {
+        ref int __error();
+        alias errno = __error;
+    }
+}
+else version (OSX)
+{
+    extern (C)
+    {
+        ref int __error();
+        alias errno = __error;
+    }
+}
+else
+{
+    ///
+    @property int errno() { return getErrno(); }
+    ///
+    @property int errno(int n) { return setErrno(n); }
+
+    extern (C)
+    {
+        private int getErrno();      // for internal use
+        private int setErrno(int);   // for internal use
+    }
+}
 
 extern (C):
 
-private extern (C) int getErrno();      // for internal use
-private extern (C) int setErrno(int);   // for internal use
 
 version( Windows )
 {