Allow use of ^blocks, even with the 10.5 SDK, and even with a 10.5 runtime

chrome/browser/mac/closure_leopard_compat/definitions.S

+# Copyright (c) 2011 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Definitions of symbols that may be needed at runtime but aren't necessarily
+# present in the SDK chosen for compilation.
+#
+# This file provides a symbol for _NSConcreteGlobalBlock, normally present in
+# libSystem.dylib and provided by libclosure-38/data.c in Mac OS X 10.6 and
+# later. When using the 10.5 SDK, the symbol is not present. This file's
+# definition can be used with extreme care in an application that needs to use
+# the 10.5 SDK in conjunction with blocks.
+#
+# This file cooperates with the build system (closure_leopard_compat.gyp) to
+# produce a dynamic library that, when linked against, causes dependents to
+# look in libSystem for the symbols provided here. It also cooperates with a
+# header (block.h) that causes dependents to treat the symbols provided here
+# as weak imports, critical for the resultant output to be loadable on 10.5.
+
+# To simplify things, this file assumes it's being built with the 10.5 SDK,
+# a deployment target of 10.5, and is producing 32-bit x86 code. Other
+# combinations are possible, but not interesting for the time being. See
+# <sys/cdefs.h> for interesting ways that names might be mangled in other
+# configurations.

[TVL, 2011/08/08 18:50:20]

I'd probably throw in here what we talked about on chat, that this also requires
building on 10.6 so the compiler has blocks support and the current toolchains
don't check the target sdk before doing the blocks codegen.

+
+#include <AvailabilityMacros.h>
+
+#if MAC_OS_X_VERSION_MIN_REQUIRED != MAC_OS_X_VERSION_10_5 || \
+    MAC_OS_X_VERSION_MAX_ALLOWED != MAC_OS_X_VERSION_10_5 || \
+    !defined(__i386__)
+#error This file only supports 32-bit x86 code with both SDK and DT set to 10.5
+#endif
+
+#define DEFINE_GLOBAL_SYMBOL(name) \
+    .globl name ## ;\
+    name ## :
+
+.section __DATA,__data
+
+DEFINE_GLOBAL_SYMBOL(__NSConcreteGlobalBlock)
+
+# When this file is in use, the linker is expected to link things against both
+# this file and the real copy of libSystem present in the SDK. When doing so,
+# the linker is smart enough to produce only one LC_LOAD_DYLIB load command.
+# However, it's not smart enough to notice that while this file's dylib only
+# provides weak-imported symbols, the real libSystem's dylib does not.
+# Consequently, it may produce an LC_LOAD_WEAK_DYLIB load command for
+# libSystem instead of an ordinary LC_LOAD_DYLIB command. LC_LOAD_WEAK_DYLIB
+# declares that any symbol offered by the library, and in fact the entire
+# library, is permitted to be missing at runtime. This is entirely
+# inappropriate for libSystem. To counteract this problem, this file also
+# defines some other symbols that libSystem provides. Dependents of this
+# library are not expected to treat these other symbols as weak imports.

[TVL, 2011/08/08 18:50:20]

so the goal is provide atleast one api that stays "strong" to the whole library
isn't a weak import?  (not completely spelled out, so just confirming)

+
+.text
+
+# |exit| is a good one: because it's referenced by crt1.o, ordinary executables
+# are guaranteed to need this symbol. Unfortunately, there's no such symbol in
+# dylib1.o that libSystem is expected to provide, so a few other common libc
+# symbols are thrown into the mix.
+DEFINE_GLOBAL_SYMBOL(_exit)
+
+# Include |close| because well-written programs that use the standard library
+# are likely to refer to it. Include |open| for good measure because it goes
+# pretty well with this. Include the stdio abstractions for these functions
+# as well.
+DEFINE_GLOBAL_SYMBOL(_close$UNIX2003)
+DEFINE_GLOBAL_SYMBOL(_open$UNIX2003)
+DEFINE_GLOBAL_SYMBOL(_fclose)
+DEFINE_GLOBAL_SYMBOL(_fopen)
+DEFINE_GLOBAL_SYMBOL(_fdopen)
+DEFINE_GLOBAL_SYMBOL(_freopen$UNIX2003)
+
+# Commonly-used allocation functions.
+DEFINE_GLOBAL_SYMBOL(_malloc)
+DEFINE_GLOBAL_SYMBOL(_calloc)
+DEFINE_GLOBAL_SYMBOL(_realloc)
+DEFINE_GLOBAL_SYMBOL(_reallocf)
+DEFINE_GLOBAL_SYMBOL(_valloc)
+DEFINE_GLOBAL_SYMBOL(_free)
+
+# Include |printf|, |fprintf|, |sprintf|, |snprintf|, and |puts|, because
+# small test programs are likely to refer to one of these. puts is rarely
+# invoked directly, but the compiler may optimize simple printf calls into
+# puts calls.
+DEFINE_GLOBAL_SYMBOL(_printf)
+DEFINE_GLOBAL_SYMBOL(_fprintf)
+DEFINE_GLOBAL_SYMBOL(_sprintf)
+DEFINE_GLOBAL_SYMBOL(_snprintf)
+DEFINE_GLOBAL_SYMBOL(_puts)
+
+# Some <string.h> functions that are commonly used.
+DEFINE_GLOBAL_SYMBOL(_memcmp)
+DEFINE_GLOBAL_SYMBOL(_memcpy)
+DEFINE_GLOBAL_SYMBOL(_memmove)
+DEFINE_GLOBAL_SYMBOL(_memset)
+DEFINE_GLOBAL_SYMBOL(_strcasecmp)
+DEFINE_GLOBAL_SYMBOL(_strcat)
+DEFINE_GLOBAL_SYMBOL(_strchr)
+DEFINE_GLOBAL_SYMBOL(_strcmp)
+DEFINE_GLOBAL_SYMBOL(_strcpy)
+DEFINE_GLOBAL_SYMBOL(_strdup)
+DEFINE_GLOBAL_SYMBOL(_strlcat)
+DEFINE_GLOBAL_SYMBOL(_strlcpy)
+DEFINE_GLOBAL_SYMBOL(_strlen)
+DEFINE_GLOBAL_SYMBOL(_strncasecmp)
+DEFINE_GLOBAL_SYMBOL(_strncat)
+DEFINE_GLOBAL_SYMBOL(_strncmp)
+DEFINE_GLOBAL_SYMBOL(_strncpy)
+DEFINE_GLOBAL_SYMBOL(_strnstr)
+DEFINE_GLOBAL_SYMBOL(_strstr)
+
+# Some data-section symbols that might be referenced.
+
+.section __DATA,__data
+
+DEFINE_GLOBAL_SYMBOL(___stdinp)
+DEFINE_GLOBAL_SYMBOL(___stdoutp)
+DEFINE_GLOBAL_SYMBOL(___stderrp)
+
+#undef DEFINE_GLOBAL_SYMBOL