Blocks (C language extension) - AbsoluteAstronomy.com

Blocks are a nonstandard extension added by Apple Inc. to the C

C (programming language)

C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....

, C++

C++

C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...

, and Objective-C

Objective-C

Objective-C is a reflective, object-oriented programming language that adds Smalltalk-style messaging to the C programming language.Today, it is used primarily on Apple's Mac OS X and iOS: two environments derived from the OpenStep standard, though not compliant with it...

programming languages that uses a lambda expression-like syntax to create closure

Closure (computer science)

In computer science, a closure is a function together with a referencing environment for the non-local variables of that function. A closure allows a function to access variables outside its typical scope. Such a function is said to be "closed over" its free variables...

s within these languages. Blocks are supported for programs developed for Mac OS X 10.6+ and iOS 4.0+.

Apple designed blocks with the explicit goal of making it easier to write programs for the Grand Central Dispatch threading architecture, 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

GNU Compiler Collection

The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...

and in the Clang

Clang

Clang is a compiler front end for the C, C++, Objective-C, and Objective-C++ programming languages. It uses the Low Level Virtual Machine as its back end, and Clang has been part of LLVM releases since LLVM 2.6....

LLVM compiler front end. Language runtime library

Runtime library

In computer programming, a runtime library is a special program library used by a compiler, to implement functions built into a programming language, during the execution of a computer program...

support for blocks is also available as part of the LLVM project.

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.

Examples

A simple example capturing mutable state in the surrounding scope is an integer range iterator

Iterator

In computer programming, an iterator is an object that enables a programmer to traverse a container. Various types of iterators are often provided via a container's interface...

include
include

typedef int (^IntBlock);

IntBlock MakeCounter(int start, int increment) {
__block int i = start;

return Block_copy( ^ {
int ret = i;
i += increment;
return ret;
});

}

int main(void) {
IntBlock mycounter = MakeCounter(5, 2);
printf("First call: %d\n", mycounter);
printf("Second call: %d\n", mycounter);
printf("Third call: %d\n", mycounter);

/* because it was copied, it must also be released */
Block_release(mycounter);

return 0;
}
/* Output:
First call: 5
Second call: 7
Third call: 9

Relation to GCC nested functions

Blocks bear a superficial resemblance to GCC

GNU Compiler Collection

The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...

's extension of C to support lexically scoped nested function

Nested function

In computer programming, a nested function is a function which is lexically encapsulated within another function. It can only be called by the enclosing function or by functions directly or indirectly nested within the same enclosing function. In other words, the scope of the nested function is...

s. However, GCC's nested functions, unlike blocks, cannot be called after the containing scope has exited.

GCC-style nested functions also require dynamic creation of executable thunk

Thunk

Thunk may refer to:* Thunk , a piece of code to perform a delayed computation * Thunk : a feature of some virtual function table implementations...

s 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

Executable space protection

In computer security, executable space protection is the marking of memory regions as non-executable, such that an attempt to execute machine code in these regions will cause an exception...

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.

Examples

Relation to GCC nested functions

See also