Question

I ve discovered (below) that I need to use -fblocks when compiling code which uses blocks.

What library do I need to link against to let the linker resolve _NSConcreteStackBlock? (On Ubuntu 9.10 AMD64.)

chris@chris-desktop:~$ clang ctest.c 

ctest.c:3:25: error: blocks support disabled - compile with -fblocks or pick a
      deployment target that supports them
void call_a_block(void (^blockptr)(int)) {
                        ^
ctest.c:11:19: error: blocks support disabled - compile with -fblocks or pick a
      deployment target that supports them
    call_a_block( ^(int y) { 
                  ^
2 diagnostics generated.
chris@chris-desktop:~$ clang ctest.c -fblocks
/tmp/cc-4sPSeO.o: In function `main :
ctest.c:(.text+0x79): undefined reference to `_NSConcreteStackBlock 
collect2: ld returned 1 exit status
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

Clang doesn t yet provide an easy way to use blocks on platforms that don t have built-in operating system support (e.g., SnowLeopard). You can find some more information on the libdispatch project here: http://libdispatch.macosforge.org/ and on the compiler-rt project (which provides the blocks runtime) here: http://compiler-rt.llvm.org/ but this is not yet well packaged for Clang end users.

If you want to dig in a bit, the compiler-rt project does have the blocks runtime in it, and you can use that to build a library which will provide the NSConcreteStackBlock.

Answer 2

Use the instructions at http://mackyle.github.com/blocksruntime/ to build a libBlocksRuntime.a library that you can link with.

Answer 3

Install the libBlocksRuntime on Ubuntu with:

sudo apt-get install llvm
sudo apt-get install clang
sudo apt-get install libblocksruntime-dev

To compile, include the library and -fblocks:

clang ctest.c -fblocks -lBlocksRuntime

This is also available on other operating systems. FreeBSD and MidnightBSD both include clang and libBlocksRuntime.so

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