Blocks are a non-standard extension added by Apple Inc. to Clang's implementations of the C, C++, and Objective-C programming languages that uses a lambda expression-like syntax to create closures within these languages. Blocks are supported for programs developed for Mac OS X 10.6+ and iOS 4.0+,[1] although third-party runtimes allow use on Mac OS X 10.5 and iOS 2.2+[2] and non-Apple systems.
Berkeley Electronic Press Selected Works. I have a problem compiling a program with '-std=c11 -stdlib=libc' under mac os x 10.8.3 using clang from xcode 4.6.2. When I try to use std::memfn or (deprecated) std::memfunref, I get. OSXCross uses clang as the default compiler for building its tools, and also as a cross-compiler to create macOS binaries. In clang there is no difference between cross-compilation and native compilation, so OSXCross can use a normal clang install for both. Cross-Building for Mac OS X¶. For cross-compiling for OS X you need the Clang/LLVM compiler, the cctools (ld, lipo, ), and the OSX SDK. Clang/LLVM is a cross compiler by default and is available on nearly every GNU/Linux distribution, so you just need a proper port of the cctools and the OS X SDK. QEMU on OS X (macOS) hosts. While QEMU's main host platform is Linux, it is also supported on operating systems for Apple's Mac computers (known as OS X or macOS).The official support policy covers the last two released versions of OSX; QEMU might work on older versions, but it is not guaranteed and it might not even compile on older versions.
Apple designed blocks with the explicit goal of making it easier to write programs for the Grand Central Dispatch threading architecture,[3][4] although it is independent of that architecture and can be used in much the same way as closures in other languages. Apple has implemented blocks both in their own branch of the GNU Compiler Collection[1] and in the upstream ClangLLVM compiler front end. Language runtime library support for blocks is also available as part of the LLVM project. The Khronos group uses blocks syntax to enqueue kernels from within kernels as of version 2.0 of OpenCL.[5]
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Like function definitions, blocks can take arguments, and declare their own variables internally. Unlike ordinary C function definitions, their value can capture state from their surrounding context. A block definition produces an opaque value which contains both a reference to the code within the block and a snapshot of the current state of local stack variables at the time of its definition. The block may be later invoked in the same manner as a function pointer. The block may be assigned to variables, passed to functions, and otherwise treated like a normal function pointer, although the application programmer (or the API) must mark the block with a special operator (Block_copy) if it's to be used outside the scope in which it was defined.
Given a block value, the code within the block can be executed at any later time by calling it, using the same syntax that would be used for calling a function.
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Examples[edit]
A simple example capturing mutable state in the surrounding scope is an integer range iterator:[6]
Compile and execute[edit]
The blocks runtime is not part of the C library(s) linked by default on some systems. If this is the case, it is required to explicitly link to this library:
The runtime is a part of clang's runtime, but is sometimes not installed with the clang package. A standalone runtime extracted from compiler-rt is available.[7]
Relation to GCC nested functions[edit]
Blocks bear a superficial resemblance to GCC's extension of C to support lexically scopednested functions.[8] However, GCC's nested functions, unlike blocks, must not be called after the containing scope has exited, as that would result in undefined behavior.
GCC-style nested functions currently use dynamic creation of executable thunks on most architectures when taking the address of the nested function. On most architectures (including X86), these thunks are created on the stack, which requires marking the stack executable. Executable stacks are generally considered to be a potential security hole. Blocks do not require the use of executable thunks, so they do not share this weakness. On the other hand, blocks introduces a completely new type for the pointer, while pointers to nested functions in GCC are regular function pointers and can be used directly with existing code.
See also[edit]
- C++11 (includes 'lambda expressions')
References[edit]
- ^ ab'Blocks Programming Topics'. Apple Developer. Apple Inc. Retrieved 2011-03-08.
- ^https://code.google.com/p/plblocks/
- ^'Grand Central Dispatch'(PDF) (technology brief). Apple. 2009-09-03. Archived from the original(PDF) on 2009-09-20. Retrieved June 9, 2009.
- ^Siracusa, John (September 1, 2009). 'Mac OS X 10.6 Snow Leopard: the Ars Technica review: Blocks'. Ars Technica.
- ^Munshi, Aaftab, ed. (2013-07-13). 'The OpenCL C Specification. Version 2.0. Document Revision 11'(PDF). Khronos OpenCL Working Group. p. 173. Archived from the original(PDF) on 2013-11-05. Retrieved 2013-07-23.
- ^Bengtsson, Joachim. 'Programming with C Blocks on Apple Devices'. Archived from the original on 2017-11-15. Retrieved 2009-09-17.
- ^'mackyle/blocksruntime: standalone blocks runtime'. Retrieved 15 January 2020.
- ^'Nested Functions: Using the GNU Compiler Collection (GCC)'.
External links[edit]
- 'Clang Language Extensions: Blocks'. LLVM Project. Retrieved 2013-01-20.
- ''compiler-rt' Runtime Library'. LLVM Project. Retrieved 2013-01-20.
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