libbb: extend "errno pointer" trick to other than __GLIBC__

Savings on musl:

function                                             old     new   delta
resume_main                                          582     614     +32
lbb_prepare                                            -      20     +20
seq_main                                             432     449     +17
fgetsetversion                                        74      88     +14
...
script_main                                         1207    1180     -27
close_silently                                        28       -     -28
shell_builtin_ulimit                                 655     626     -29
lineedit_read_key                                    280     247     -33
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 25/123 up/down: 182/-882)      Total: -700 bytes
   text	   data	    bss	    dec	    hex	filename
1005858	    551	   5676	1012085	  f7175	busybox_old
1005136	    551	   5680	1011367	  f6ea7	busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

3 files changed, 7 insertions, 6 deletions

diff --git a/include/libbb.h b/include/libbb.h
index 83ecca47f..df7e45404 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -339,12 +339,13 @@ struct BUG_off_t_size_is_misdetected {
 #endif
 #endif
 
-#if defined(__GLIBC__)
-/* glibc uses __errno_location() to get a ptr to errno */
-/* We can just memorize it once - no multithreading in busybox :) */
+#if defined(errno)
+/* If errno is a define, assume it's "define errno (*__errno_location())"
+ * and we will cache it's result in this variable */
 extern int *const bb_errno;
 #undef errno
 #define errno (*bb_errno)
+#define bb_cached_errno_ptr 1
 #endif
 
 #if !(ULONG_MAX > 0xffffffff)
diff --git a/libbb/appletlib.c b/libbb/appletlib.c
index a515c3fe3..717c63649 100644
--- a/libbb/appletlib.c
+++ b/libbb/appletlib.c
@@ -303,7 +303,7 @@ void lbb_prepare(const char *applet
 void lbb_prepare(const char *applet
                 IF_FEATURE_INDIVIDUAL(, char **argv))
 {
-#ifdef __GLIBC__
+#ifdef bb_cached_errno_ptr
         (*(int **)not_const_pp(&bb_errno)) = __errno_location();
         barrier();
 #endif
diff --git a/libbb/ptr_to_globals.c b/libbb/ptr_to_globals.c
index 8ba9cd154..2232c6864 100644
--- a/libbb/ptr_to_globals.c
+++ b/libbb/ptr_to_globals.c
@@ -14,7 +14,7 @@ struct globals;
  * but here we make it live in R/W memory */
 struct globals *ptr_to_globals;
 
-#ifdef __GLIBC__
+#ifdef errno
 int *bb_errno;
 #endif
 
@@ -27,7 +27,7 @@ int *bb_errno;
  * on weird architectures, compilers, linkers and so on */
 struct globals *const ptr_to_globals __attribute__ ((section (".data")));
 
-#ifdef __GLIBC__
+#ifdef errno
 int *const bb_errno __attribute__ ((section (".data")));
 #endif