by nigel@vannevartech.com:

I had to use Swig to wrap some C++ code and had run into an issue with undefined symbols
while trying to run the Go executable in OS X Mountain Lion.

Background:

The C code generated by Swig is compiled into a .so using Clang c++, which has
crosscall2 as an undefined symbol. The Go executable is compiled separately using go
tools, which contains crosscall2().

I have a simplified example, illustrated here:

$ nm go/bin/main | grep crosscall2
0000000000026cc5 T _crosscall2
$ nm go/lib/example.so | grep crosscall2
                 U _crosscall2

And the main executable is linked to example.so by "go build" (presumably by
6l under the hood) like so:

$ DYLD_LIBRARY_PATH=go/lib otool -L go/bin/main 
go/bin/main:
    example.so (compatibility version 0.0.0, current version 0.0.0)
    /usr/lib/libSystem.B.dylib (compatibility version 0.0.0, current version 0.0.0)

In the example, I'm wrapping a simple function in C that returns a const char *, which
is then returned to Go as a string.

const char *message() {
  return "A Message";
}

I use this function in Go like:

func main() {
        fmt.Printf("Message is: %v\n", example.Message())
}

Once everything has been compiled, I than run
$ DYLD_LIBRARY_PATH=go/lib go/bin/main 

This operation triggers a call to crosscall2() in the generated C Swig code to create a
Go string. That forces a lazy resolution for crosscall2. In Mountain Lion, it failed to
resolve _crosscall2.

See attached package for complete source code.

What is the expected output?

The expected output is a successful execution with the output:

"Message is: A Message"

What do you see instead?

$ DYLD_LIBRARY_PATH=go/lib go/bin/main 
dyld: lazy symbol binding failed: Symbol not found: _crosscall2
  Referenced from: go/lib/example.so
  Expected in: flat namespace

dyld: Symbol not found: _crosscall2
  Referenced from: go/lib/example.so
  Expected in: flat namespace

SIGTRAP: trace trap
pc: 0x7fff6d36109d


goroutine 1 [syscall]:
example.Message(0xf84002b0d0, 0xf84002b0f0)
    /Users/nigelchoi/Code/crosscall2/go/src/example/example_gc.c:33 +0x32
main.main()
    /Users/nigelchoi/Code/crosscall2/go/src/main/main.go:9 +0x1c

goroutine 2 [syscall]:
created by runtime.main
    /usr/local/go/src/pkg/runtime/proc.c:221
rax     0x0
rbx     0x2400000
rcx     0x0
rdx     0x0
rdi     0x7fff6d3d0060
rsi     0x0
rbp     0xb0080c80
rsp     0xb0080c68
r8      0x7fff6d37ca14
r9      0xf
r10     0xb0080b38
r11     0x7fff6d3d0060
r12     0x1203
r13     0x0
r14     0x7fff6d3d0060
r15     0x269df
rip     0x7fff6d36109d
rflags  0x246
cs      0x2b
fs      0x0
gs      0x0

Which compiler are you using (5g, 6g, 8g, gccgo)?

I'm using the standard Go tools to compile the Go portion (go 1.0.3)

Clang c++ to compile the C portion (Apple clang version 4.1 (tags/Apple/clang-421.11.65)
(based on LLVM 3.1svn))

Which operating system are you using?

OS X Mountain Lion 10.8.2 (12C60)

Which version are you using?  (run 'go version')

go 1.0.3

Please provide any additional information below.

Curiously, another undefined symbol in example.so can be resolved back to the Go
executable, the __cgo_allocate symbol. Could it be bad formatting of the _crosscall2
function in the Mach-O?

$ DYLD_PRINT_BINDINGS=1 DYLD_LIBRARY_PATH=go/lib go/bin/main 
dyld: bind: example.so:0x020FC010 = main:__cgo_allocate, *0x020FC010 = 0x00026801
...
dyld: lazy symbol binding failed: Symbol not found: _crosscall2

Attachments:

1. crosscall2.tgz (879 bytes)