[SR-397] Linux - module using <dispatch/dispatch.h> does not compile cleanly with "swift build"

[swift-ci]

Previous ID	SR-397
Radar	None
Original Reporter	tomsheffler (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Attachment: Download

Environment

Ubuntu 14.04

Additional Detail from JIRA

Votes	3
Component/s	Compiler, Package Manager
Labels	Bug, Dispatch, Linux
Assignee	@aciidb0mb3r
Priority	Medium

md5: 3ce10e8ea99c16ef2da7e8631d663144

is duplicated by:

• SR-415 blocks support in external c libraries disabled
• SR-577 Compiler should pass -Xcc -fblocks when processing .h files

Issue Description:

While libdispatch is well-supported on Linux, using "Swift Build" for a module wrapping <dispatch/dispatch.h> does not compile cleanly. The problem is that to get all of the features of dispatch, "-fblocks" needs to be specified to Clang, and currently, "swift build" does not enable "-fblocks" for modules that wrap C libraries.

• using "swift build" does not include the -fblocks directive

• compiling directly with swiftc and including the -fblocks directive works fine

• creating a module map that specifies "requires blocks" causes a complaint that Objective-C is not supported

Here is my original report, and a Github repo that illustrates the problem.

I made a module called "CDispatch" with a module.modulemap like this

=======
module CDispatch [system] {
header "/usr/include/dispatch/dispatch.h"
export *
link "dispatch"
}

Then I created a little demo project called gcd4 whose Source/main.swift prints some things and then uses a dispatch queue and a block to print a message after 2 seconds delay.

=========
CDispatch.dispatch_after(time, queue, {
print("Delayed!")
})

The entire project is checked in at https://github.com/sheffler/gcd4 <https://github.com/sheffler/gcd4\>
and the CDispatch module is checked in at https://github.com/sheffler/CDispatch <https://github.com/sheffler/CDispatch\>

If I try to "swift build" the project, it almost works but reports that dispatch_after is not found. It seems that this function is not defined if the "blocks" feature is not provided at compilation time.

========
Compiling Swift Module 'gcd4' (1 sources)
/home/sheffler/swift/gcd4/Sources/main.swift:42:1: error: module 'CDispatch' has no member named 'dispatch_after'
CDispatch.dispatch_after(time, queue, {
^~~~~~~~~ ~~~~~~~~~~~~~~
<unknown>:0: error: build had 1 command failures
swift-build: exit(1): ["/home/sheffler/src/swift-2.2-SNAPSHOT-2015-12-01-b-ubuntu14.04/usr/bin/swift-build-tool", "-f", "/home/sheffler/swift/gcd4/.build/debug/gcd4.o/llbuild.yaml"]

I got the demo program to work by first using "swift build" to retrieve the CDispatch module, and then manually running the compiler like this (and including the "-Xcc -fblocks" arguments)

swiftc -v -o gcd4 Sources/main.swift -I .build/debug -j8 -Onone -g -Xcc -fblocks -Xcc -F-module-map=Packages/CDispatch-1.0.0/module.modulemap -I Packages/CDispatch-1.0.0 -I /usr/local/include

[drewcrawford]

cc: @mxcl

Would it be an acceptable resolution here to allow Package.swift to have an extraSwiftCArgs: ["-Xcc","-fblocks"] specifier? That is a simple solution that I can PR quickly.

[SR-415] is possibly a dupe of this bug.

[drewcrawford]

I'm going to PR the solution above.

[drewcrawford]

It looks like this has been PRed and closed before under #75.

I'm going to resurrect this discussion on the list; clearing assignment for now.

[drewcrawford]

I have resurrected this idea under PR #107. If that PR is merged, we can say

otherCompilerOptions: ["-Xcc", "-fblocks"] 

to work around this issue.

[belkadan]

We should just add this to the compiler. However, I think we were hoping to establish a different blocks ABI on Linux, so that we can at least use Swift's retain-counting on them instead of Objective-C's.

[belkadan]

cc @jckarter, @parkera

[mxcl]

So, to be clear, `-fblocks` is required for anything that links to `CDispatch`?

[parkera]

It may be possible to use dispatch without blocks (the _f functions), but it's not something I really care about supporting in Swift.

[swift-ci]

Comment by Tom Sheffler (JIRA)

This issue is actually pretty subtle. At the present time, there is probably only one C module anywhere that provides a header file with blocks – and that is libdispatch. Expanding the module mechanism to handle this one case may be overkill.

On the other hand, it blocked me. So already it's a small issue.

Perhaps there could be support for a "good" way to make modules, and a "messy" way ... that is more akin to using clang to create a library, but requires the programmer to create a Makefile or recipe. Even a well-documented workaround would be ok.

In the future there will be a more Swift-y libDispatch, and from the comments here I'm surmising that it will use runtime support different from that of ObjC.

[swift-ci]

Comment by Daniel Eggert (JIRA)

In my opinion the _f variants are perfectly fine. I added workaround code to SR-577 that shows how to use them with blocks — the block variants of libdispatch’s functions also call into these after wrapping the blocks. That approach would allow for a a different blocks ABI on Linux.

[mxcl]

In case you missed it you can now specify `-Xcc` and `-Xlinker` flags to `swift build`.

So here you could do `swift build -Xcc -fblocks`

[swift-ci]

Comment by Tom Sheffler (JIRA)

I verified this against swift-DEVELOPMENT-SNAPSHOT-2016-01-25-a/swift-DEVELOPMENT-SNAPSHOT-2016-01-25-a-ubuntu14.04.tar.gz

Using

swift build -Xcc -fblocks

this example compiles and runs just fine now.

[swift-ci]

Comment by Joel Saltzman (JIRA)

It mostly works for me. I can't seem to get dispatch_get_specific to compile. I updated the working example project to show my compile errors:

root@dec0c641b9ec:/gcd# swift build -Xcc -fblocks
Compiling Swift Module 'gcd' (1 sources)
/gcd/Sources/main.swift:45:20: error: use of unresolved identifier 'dispatch_get_specific'
print("name: (dispatch_get_specific(&name))")

gcd4-master.zip

[swift-ci]

Comment by Joel Saltzman (JIRA)

Old version of libdispatch on Linux 😛

[abertelrud]

In general, most use of libdispatch on Linux should use https://github.com/apple/swift-corelibs-libdispatch. But if I read this correctly, this issue here is about instead using the C API directly. And from s00p3rj03l (JIRA User)'s comment above, it sounds as if this may now work, if the version of `libdispatch` was old?