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cmd/link: Go referencing C + C referencing Go = duplicate symbols #11263

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shazow opened this issue Jun 17, 2015 · 4 comments
Closed

cmd/link: Go referencing C + C referencing Go = duplicate symbols #11263

shazow opened this issue Jun 17, 2015 · 4 comments
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@shazow
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shazow commented Jun 17, 2015

In a recent master, combining the two examples here will yield a duplicate symbol error on build. Either example alone works fine.

$ go version
go version devel +a2aaede Wed Jun 17 14:55:39 2015 +0000 darwin/amd64

package main

// typedef int (*intFunc) ();
//
// int
// bridge_int_func(intFunc f)
// {
//      return f();
// }
//
// int fortytwo()
// {
//      return 42;
// }
import "C"
import "fmt"

//export Hello
func Hello() {}

func main() {
    f := C.intFunc(C.fortytwo)
    fmt.Println(int(C.bridge_int_func(f)))
    // Output: 42
}

Including the //export Hello line yields this error:

$ go build -buildmode=c-archive
# _/Users/shazow/projects/gohttplib
duplicate symbol _bridge_int_func in:
    $WORK/_/Users/shazow/projects/gohttplib/_obj/_cgo_export.o
    $WORK/_/Users/shazow/projects/gohttplib/_obj/gohttplib.cgo2.o
duplicate symbol _fortytwo in:
    $WORK/_/Users/shazow/projects/gohttplib/_obj/_cgo_export.o
    $WORK/_/Users/shazow/projects/gohttplib/_obj/gohttplib.cgo2.o
ld: 2 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Doesn't matter if the -buildmode=c-archive flag is included. If the //export Hello line is removed, it works fine. If the inline C code is removed, it works fine.

Scenario: I'm trying to pass a callback (function pointer) from C into a Go function and I was trying to use an inline bridge function for calling it.
@ianlancetaylor
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This is documented at http://golang.org/cmd/cgo: "Using //export in a file places a restriction on the preamble: since it is copied into two different C output files, it must not contain any definitions, only declarations. Definitions must be placed in preambles in other files, or in C source files."

@shazow
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shazow commented Jun 18, 2015

Aaah that's what that means, thank you. Let me know if you'd like a docs patch to mention the duplicate symbols consequence for clarity.

@ianlancetaylor
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I'm happy to look at a doc patch if you think you can make it more clear. Thanks.

@shazow
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shazow commented Jun 18, 2015

@ianlancetaylor Proposed an edit here: https://go-review.googlesource.com/#/c/11247/

ianlancetaylor pushed a commit that referenced this issue Jun 19, 2015
@shazow @ianlancetaylor
doc: clarify duplicate symbol condition in cgo
9f3a2d5 
Spell out what will happen if a declaration and definition is included
in the same file, should help people who run into duplicate symbol
errors and search for relevant keywords.

This edit is based on opening issue #11263 erroneously.

Change-Id: I0645a9433b8668d2ede9b9a3f6550d802c26388b
Reviewed-on: https://go-review.googlesource.com/11247
Reviewed-by: Ian Lance Taylor <iant@golang.org>
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