runtime: runtime error: cgo argument has Go pointer to Go pointer

[alexbrainman]

I had a crash alexbrainman/odbc#65 after compiling my program with latest version of Go.

I simplified my crash down to this:

# go version
go version devel +e7524d5 Tue Feb 23 03:42:38 2016 +0000 linux/386
# cat -n main.go
     1  package main
     2
     3  import (
     4          "unsafe"
     5  )
     6
     7  /*
     8  typedef void* VOIDPTR;
     9
    10  void cfunc(VOIDPTR p) {
    11  }
    12  */
    13  import "C"
    14
    15  type stru struct {
    16          a   []byte
    17          buf [8]byte
    18  }
    19
    20  func main() {
    21          var s stru
    22          s.a = s.buf[:]
    23          C.cfunc(C.VOIDPTR(unsafe.Pointer(&s.buf[0])))
    24  }
# go run main.go
panic: runtime error: cgo argument has Go pointer to Go pointer

goroutine 1 [running]:
panic(0x809e300, 0x1830a040)
        /root/go/src/runtime/panic.go:483 +0x331
main.main()
        /root/src/t/main.go:23 +0x8c
exit status 2
#

I don't undestand what this program doing wrong that CGO function parmeter checker crashed it. Can someone explain it to me, please? Also, if I remove line 22, the program runs with no problem. Why?

Thank you.

I suspect @ianlancetaylor will be interested.

Alex

[bradfitz]

I believe this is because the 8 bytes are in the same memory allocation, contiguous with the pointer in the slice of the a field. The cgo checker doesn't know you're only sharing memory to the *byte, as it only sees the address and the address is within a range which contains Go pointers.

[ianlancetaylor]

Hmmm, it works fine if you remove the C.VOIDPTR type conversion. I'll look into this.

[alexbrainman]

it works fine if you remove the C.VOIDPTR type conversion

It does indeed.

Alex

[ianlancetaylor]

@bradfitz By the way, this kind of thing is supposed to work if the address operation is one of the direct arguments to the C call. In that specific case, cgo is supposed to know that only that particular field needs to be checked for pointers, not the entire object to which the pointer points.

[gopherbot]

CL https://golang.org/cl/19860 mentions this issue.

[gopherbot]

CL https://golang.org/cl/22041 mentions this issue.