I found a related-but-different problem when investigating issue #13830.

Making the _cgoCheckPointer call explicitly in the function parameters of a deferred call causes the pointer check to occur at the wrong time. We end up checking whether the argument had a Go pointer at the point where the call was deferred, not whether the argument had a Go pointer at the point where the C call was actually made.

Consider this program:

package main

/*
#include <stdint.h>

struct sneaky {
  int *p;
};

void sneak(struct sneaky *s, uintptr_t *u) {
  *u = (uintptr_t)(s->p);
}
*/
import "C"

import "fmt"

func main() {
    var s C.struct_sneaky
    var u C.uintptr_t

    defer func() { fmt.Println(u) }()
    defer C.sneak(&s, &u)

    var escapee C.int
    s.p = &escapee
}

With go version go1.6.1 linux/amd64, the program (incorrectly) prints a leaked address and exits with code 0.

Changing the call to C.sneak to an explicit call at the end of main causes the program to (correctly) panic with a cgo pointer violation.

The problem is that the _cgoCheckPointer calls wrap the function's arguments, not the function call itself:

  defer _Cfunc_sneak(_cgoCheckPointer((*_Ctype_struct_sneaky)(&s), true).(*_Ctype_struct_sneaky), &u)