[cfe-dev] Is [extern_c] necessary in module map?

[191919]

Hi,  

The original post was a bugzilla report ( http://llvm.org/bugs/show_bug.cgi?id=21626 ), Richard Smith suggested that bugzilla wasn’t a good place to discuss about the development details, so I moved the post to here.

I wanted to accelerate the build process of one of my OS X projects and I tried to use clang’s module feature but had no luck.

This is a trivial source code:

```
#import <Cocoa/Cocoa.h>
int main() { }

```

I built it with clang svn trunk:

$ /opt/bin/clang++ -fmodules -c -o m m.mm

and I got a huge bunch of error messages:

```
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'CoreFoundation' imported from /System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:6:
While building module 'Dispatch' imported from /System/Library/Frameworks/CoreFoundation.framework/Headers/CFStream.h:15:
In file included from <module-includes>:1:
In file included from /usr/include/dispatch/dispatch.h:52:
/usr/include/dispatch/queue.h:368:1: error: import of C++ module 'Darwin.sys.qos' appears within extern "C" language linkage specification
#include <sys/qos.h>
^
/usr/include/dispatch/queue.h:69:1: note: extern "C" language linkage specification begins here
__BEGIN_DECLS
^
/usr/include/sys/cdefs.h:71:23: note: expanded from macro '__BEGIN_DECLS'
#define __BEGIN_DECLS extern "C" {
^



… tons of lines omitted …

```

I was told that this was because the system’s module map was not adapted to be built as C++ mode, and I should add [extern_c] to the module map files. So I modified module.map files in

/usr/include
/usr/include/mach-o
/usr/include/dispatch
/usr/include/objc

then cleaned ModuleCache in tmp directory and I compiled the source again, here was what I got from the compiler:

```
$ /opt/bin/clang++ -fmodules -c -o m m.mm
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'CoreGraphics' imported from /System/Library/Frameworks/Foundation.framework/Headers/NSGeometry.h:12:
While building module 'IOKit' imported from /System/Library/Frameworks/CoreGraphics.framework/Headers/CGDisplayConfiguration.h:8:
In file included from <module-includes>:7:
/System/Library/Frameworks/IOKit.framework/Headers/IOCFPlugIn.h:32:1: error: import of C++ module 'CoreFoundation.CFPlugIn' appears within extern "C" language linkage specification
#include <CoreFoundation/CFPlugIn.h>
^
/System/Library/Frameworks/IOKit.framework/Headers/IOCFPlugIn.h:30:1: note: extern "C" language linkage specification begins here
__BEGIN_DECLS
^
/usr/include/sys/cdefs.h:71:23: note: expanded from macro '__BEGIN_DECLS'
#define __BEGIN_DECLS extern "C" {
```



In short, I needed to add [extern_c] to every module map file in every framework.

I don't get the point of [extern_c], why it is required? In header files, all symbols that have C linkage can be recognized and exported correctly, clang does't have to depend on another marker to distinguish them against "C++" linkage.

Richard Smith kindly explained the point of [extern_c]:
> The point of [extern_c] is twofold: (1) it builds thetarget module in an implicit extern "C" context (which
> affects its semantics in lots of ways), and (2) it
> allows the module to be imported in an extern "C"
> context. You don't want (2) without (1), because that
> can lead to silent and hard-to-diagnose build breaks.
So here is my problem:

I don't know whether it is correct: `module` works like precompiled headers which works fine without [extern_c]. Since `extern "C"` is a linkage, i.e., it is used in link-time, and the linker works fine with current defines in headers, why another marker is needed at compile-time?

My another problem is:

I have modified all module maps in OS X SDK and recompile the code, this time the header-file-scattering problem. In OS X/iOS SDK, header files in one framework could be in another framework, for example: IORegistryEntry.h belongs to IOKit，but it is placed under Kernel.framework. I know this is Apple's problem, too, but the old question: without `-fmodules`, the compiler works fine with locating the file in both objective-C and C++ mode.

Below was what I got from the compiler:

```
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'CoreGraphics' imported from /System/Library/Frameworks/Foundation.framework/Headers/NSGeometry.h:12:
While building module 'IOKit' imported from /System/Library/Frameworks/CoreGraphics.framework/Headers/CGDisplayConfiguration.h:8:
In file included from <module-includes>:126:
/System/Library/Frameworks/IOKit.framework/Headers/video/IOVideoDevice.h:12:10: fatal error: 'IOKit/IOService.h' file not found
#include <IOKit/IOService.h>
^
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'CoreGraphics' imported from /System/Library/Frameworks/Foundation.framework/Headers/NSGeometry.h:12:
In file included from <module-includes>:1:
In file included from /System/Library/Frameworks/CoreGraphics.framework/Headers/CoreGraphics.h:41:
/System/Library/Frameworks/CoreGraphics.framework/Headers/CGDisplayConfiguration.h:8:10: fatal error: could not build module 'IOKit'
#include <IOKit/IOKitLib.h>
~~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'CoreGraphics' imported from /System/Library/Frameworks/Foundation.framework/Headers/NSGeometry.h:12:
While building module 'IOSurface' imported from /System/Library/Frameworks/CoreGraphics.framework/Headers/CGDisplayStream.h:9:
In file included from <module-includes>:1:
In file included from /System/Library/Frameworks/IOSurface.framework/Headers/IOSurface.h:13:
/System/Library/Frameworks/IOSurface.framework/Headers/IOSurfaceAPI.h:12:10: fatal error: could not build module 'IOKit'
#include <IOKit/IOKitLib.h>
~~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
In file included from <module-includes>:1:
In file included from /System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:42:
In file included from /System/Library/Frameworks/Foundation.framework/Headers/NSKeyedArchiver.h:8:
/System/Library/Frameworks/Foundation.framework/Headers/NSGeometry.h:12:9: fatal error: could not build module 'CoreGraphics'
#import <CoreGraphics/CGBase.h>
~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'CoreServices' imported from /System/Library/Frameworks/Foundation.framework/Headers/NSURLError.h:12:
While building module 'DiskArbitration' imported from /System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/Files.h:56:
In file included from <module-includes>:1:
In file included from /System/Library/Frameworks/DiskArbitration.framework/Headers/DiskArbitration.h:29:
/System/Library/Frameworks/DiskArbitration.framework/Headers/DADisk.h:28:10: fatal error: could not build module 'IOKit'
#include <IOKit/IOKitLib.h>
~~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'CoreServices' imported from /System/Library/Frameworks/Foundation.framework/Headers/NSURLError.h:12:
In file included from <module-includes>:1:
In file included from /System/Library/Frameworks/CoreServices.framework/Headers/CoreServices.h:23:
In file included from /System/Library/Frameworks/CoreServices.framework/Frameworks/AE.framework/Headers/AE.h:20:
In file included from /System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/CarbonCore.h:87:
In file included from /System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/Components.h:26:
/System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/Files.h:56:10: fatal error: could not build module 'DiskArbitration'
#include <DiskArbitration/DADisk.h>
~~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'ApplicationServices' imported from /System/Library/Frameworks/Foundation.framework/Headers/NSAppleEventDescriptor.h:7:
In file included from <module-includes>:1:
/System/Library/Frameworks/ApplicationServices.framework/Headers/ApplicationServices.h:23:10: fatal error: could not build module 'CoreServices'
#include <CoreServices/CoreServices.h>
~~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'ApplicationServices' imported from /System/Library/Frameworks/Foundation.framework/Headers/NSAppleEventDescriptor.h:7:
While building module 'CoreText' imported from /System/Library/Frameworks/ApplicationServices.framework/Frameworks/ATS.framework/Headers/SFNTLayoutTypes.h:16:
In file included from <module-includes>:1:
In file included from /System/Library/Frameworks/ApplicationServices.framework/Frameworks/CoreText.framework/Headers/CoreText.h:21:
In file included from /System/Library/Frameworks/CoreText.framework/Headers/CTFont.h:21:
/System/Library/Frameworks/CoreText.framework/Headers/CTFontDescriptor.h:28:10: fatal error: could not build module 'CoreGraphics'
#include <CoreGraphics/CGAffineTransform.h>
~~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'Foundation' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:
While building module 'ApplicationServices' imported from /System/Library/Frameworks/Foundation.framework/Headers/NSAppleEventDescriptor.h:7:
While building module 'ImageIO' imported from /System/Library/Frameworks/ApplicationServices.framework/Headers/ApplicationServices.h:47:
In file included from <module-includes>:1:
In file included from /System/Library/Frameworks/ApplicationServices.framework/Frameworks/ImageIO.framework/Headers/ImageIO.h:16:
/System/Library/Frameworks/ImageIO.framework/Headers/CGImageSource.h:14:10: fatal error: could not build module 'CoreGraphics'
#include <CoreGraphics/CoreGraphics.h>
~~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
In file included from <module-includes>:1:
/System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:12:9: fatal error: could not build module 'Foundation'
#import <Foundation/Foundation.h>
~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'AppKit' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:13:
In file included from <module-includes>:1:
/System/Library/Frameworks/AppKit.framework/Headers/AppKit.h:10:9: fatal error: could not build module 'Foundation'
#import <Foundation/Foundation.h>
~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'AppKit' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:13:
While building module 'QuartzCore' imported from /System/Library/Frameworks/AppKit.framework/Headers/NSColor.h:36:
In file included from <module-includes>:1:
In file included from /System/Library/Frameworks/QuartzCore.framework/Headers/CAAnimation.h:6:
In file included from /System/Library/Frameworks/QuartzCore.framework/Headers/CALayer.h:6:
In file included from /System/Library/Frameworks/QuartzCore.framework/Headers/CAMediaTiming.h:6:
/System/Library/Frameworks/QuartzCore.framework/Headers/CABase.h:17:10: fatal error: could not build module 'ApplicationServices'
#include <ApplicationServices/ApplicationServices.h>
~~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'AppKit' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:13:
While building module 'QuartzCore' imported from /System/Library/Frameworks/AppKit.framework/Headers/NSColor.h:36:
While building module 'CoreVideo' imported from /System/Library/Frameworks/QuartzCore.framework/Headers/CAOpenGLLayer.h:7:
In file included from <module-includes>:1:
In file included from /System/Library/Frameworks/CoreVideo.framework/Headers/CoreVideo.h:25:
/System/Library/Frameworks/CoreVideo.framework/Headers/CVDisplayLink.h:24:10: fatal error: could not build module 'ApplicationServices'
#include <ApplicationServices/ApplicationServices.h>
~~~~~~~~^
While building module 'Cocoa' imported from m.mm:1:
While building module 'AppKit' imported from /System/Library/Frameworks/Cocoa.framework/Headers/Cocoa.h:13:
While building module 'CoreData' imported from /System/Library/Frameworks/AppKit.framework/Headers/NSPredicateEditorRowTemplate.h:12:
In file included from <module-includes>:1:
/System/Library/Frameworks/CoreData.framework/Headers/CoreData.h:8:9: fatal error: could not build module 'Foundation'
#import <Foundation/Foundation.h>
~~~~~~~^
m.mm:1:9: fatal error: could not build module 'Cocoa'
#import <Cocoa/Cocoa.h>
~~~~~~~^
15 errors generated.
```



--  
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