Re: [Libguestfs] [PATCH 4/4] Go bindings: fix "C array of strings" -- char** -- allocation

On 09/20/21 14:33, Daniel P. Berrangé wrote: > On Mon, Sep 20, 2021 at 12:03:51PM +0100, Richard W.M. Jones wrote: >> On Mon, Sep 20, 2021 at 11:37:02AM +0100, Daniel P. Berrangé wrote: >>> What distro / go version do you see this on, as I can't reproduce >>> this pointer problem with a standalone demo app ? >> >> For me this started to happen after upgrading to >> golang-bin-1.17-2.fc36.x86_64 in Rawhide. It also caused this error: > > Hmm, I still cant reproduce the problem that Laszlo is fixing > > $ cat str.c > > #include <stdio.h> > > void foo(char **str) { > for (int i = 0; str[i] != NULL; i++) { > fprintf(stderr, "%d: %s (0x%p)\n", i, str[i], str[i]); > } > } > > $ cat str.go > package main > > /* > #cgo LDFLAGS: -L/home/berrange/t/lib -lstr > > #include <stdlib.h> > > extern void foo(char **str); > > */ > import "C" > > import ( > "fmt" > "unsafe" > ) > > func array_elem(arr **C.char, idx int) **C.char { > return (**C.char)(unsafe.Pointer(uintptr(unsafe.Pointer(arr)) + > (unsafe.Sizeof(arr) * uintptr(idx)))) > } > > func arg_string_list1(xs []string) **C.char { > r := make([]*C.char, 1+len(xs)) > for i, x := range xs { > r[i] = C.CString(x) > } > r[len(xs)] = nil > return &r[0] > } > > func arg_string_list2(xs []string) **C.char { > var r **C.char > r = (**C.char)(C.malloc(C.size_t(unsafe.Sizeof(*r) * (1 + uintptr(len(xs)))))) > for i, x := range xs { > str := array_elem(r, i) > *str = C.CString(x) > } > str := array_elem(r, len(xs)) > *str = nil > return r > } > > func free_string_list(argv **C.char) { > for i := 0; ; i++ { > str := (**C.char)(unsafe.Pointer(uintptr(unsafe.Pointer(argv)) + > (unsafe.Sizeof(*argv) * uintptr(i)))) > if *str == nil { > break > } > fmt.Printf("%x\n", *str) > C.free(unsafe.Pointer(*str)) > } > } > > func bar(str []string) { > cstr1 := arg_string_list1(str) > defer free_string_list(cstr1) > C.foo(cstr1) > cstr2 := arg_string_list2(str) > defer free_string_list(cstr2) > C.foo(cstr2) > } > > func main() { > bar([]string{"hello", "world"}) > } > > > My interpretation is that arg_string_list1 impl here should have > raised the error that Laszlo reports, but both impls work fine Can you create a new structure type, make the C function take the structure (or a pointer to the structure), and in the structure, make the field have this type: char * const * str; Because this is the scenario where the libguestfs test suite fails (panics). The libguestfs test suite has a *different* case that does match your example directly, and *that* case works in the libguestfs test suite flawlessly. The panic surfaces only in the "char*const* embedded in struct" case. (I assume "const" makes no difference, but who knows!)