added macro to fetch the address of errno

macros/README.md

@@ -185,10 +185,21 @@ library, use this macro in this way:
 
 `CRT     strlen`
 
-An underscore is prepended.
+An underscore is prepended on the Mac.
 
 ## Declaring `main()`
 
 Put `MAIN` on a line by itself. Notice there is no colon.
 
-An underscore is prepended.
+An underscore is prepended on the Mac.
+
+## `errno`
+
+The externally defined `errno` is accessed via a CRT function which
+isn't seen when coding in C and C++. The function is named differently
+on Mac versus Linux. To get the address of `errno` use:
+
+`ERRNO_ADDR`
+
+This macro makes the correct CRT call and leaves the address of `errno`
+in `x0`.

macros/apple-linux-convergence.S

@@ -75,6 +75,18 @@ main:
 #endif
 .endm
 
+/*  Fetching the address of the externally defined errno is quite
+    different on Apple and Linux. This macro leaves the address of
+    errno in x0.
+*/
+.macro  ERRNO_ADDR
+#if defined(__APPLE__)
+        bl      ___error
+#else
+        bl      __errno_location
+#endif
+.endm
+
 .macro  CRT label
 #if defined(__APPLE__)
         bl  _\label

more/apple_silicon/apple-linux-convergence.S

@@ -75,6 +75,18 @@ main:
 #endif
 .endm
 
+/*  Fetching the address of the externally defined errno is quite
+    different on Apple and Linux. This macro leaves the address of
+    errno in x0.
+*/
+.macro  ERRNO_ADDR
+#if defined(__APPLE__)
+        bl      ___error
+#else
+        bl      __errno_location
+#endif
+.endm
+
 .macro  CRT label
 #if defined(__APPLE__)
         bl  _\label

python/apple-linux-convergence.S

@@ -75,6 +75,18 @@ main:
 #endif
 .endm
 
+/*  Fetching the address of the externally defined errno is quite
+    different on Apple and Linux. This macro leaves the address of
+    errno in x0.
+*/
+.macro  ERRNO_ADDR
+#if defined(__APPLE__)
+        bl      ___error
+#else
+        bl      __errno_location
+#endif
+.endm
+
 .macro  CRT label
 #if defined(__APPLE__)
         bl  _\label

section_1/hello_world/apple-linux-convergence.S

@@ -75,6 +75,18 @@ main:
 #endif
 .endm
 
+/*  Fetching the address of the externally defined errno is quite
+    different on Apple and Linux. This macro leaves the address of
+    errno in x0.
+*/
+.macro  ERRNO_ADDR
+#if defined(__APPLE__)
+        bl      ___error
+#else
+        bl      __errno_location
+#endif
+.endm
+
 .macro  CRT label
 #if defined(__APPLE__)
         bl  _\label

section_2/float/apple-linux-convergence.S

@@ -75,6 +75,18 @@ main:
 #endif
 .endm
 
+/*  Fetching the address of the externally defined errno is quite
+    different on Apple and Linux. This macro leaves the address of
+    errno in x0.
+*/
+.macro  ERRNO_ADDR
+#if defined(__APPLE__)
+        bl      ___error
+#else
+        bl      __errno_location
+#endif
+.endm
+
 .macro  CRT label
 #if defined(__APPLE__)
         bl  _\label