On Fri, Mar 6, 2020 at 7:18 AM Nitish Saboo <nitish.sabo...@gmail.com> wrote: > > > 3) Why is 'fmt.Println(InitStruct)' printing the struct fields even after > > freeing the memory ? [b] > > Because that's how C works. C is not a memory safe language, and it > doesn't become memory safe just because you make the calls from Go. > > 1)You mean to say the dangling pointer will return the struct fields unless > we make it 'nil' explicitly ..am i correct ?
What you get by dereferencing a dangling pointer is unpredictable. If you dereference it immediately after the C.free, as you are doing, you will typically get the same values as before the C.free. Even that can vary depending on the behavior of the C memory allocator. If you set the pointer to nil, then you no longer have a dangling pointer. https://en.wikipedia.org/wiki/Dangling_pointer > 2) Here, C.free() will completely free the memory allocation ...right ? Yes, it will completely free the memory allocated by C.calloc. Ian > On Fri, Mar 6, 2020 at 7:55 PM Ian Lance Taylor <i...@golang.org> wrote: >> >> On Fri, Mar 6, 2020 at 4:25 AM Nitish Saboo <nitish.sabo...@gmail.com> wrote: >> > >> > So what did I do : >> > >> > main.go >> > ======== >> > >> > var InitStruct *C.struct_Foo; >> > >> > func InitializeEngine(pattern string, path string) { >> > pattern_db := C.CString(pattern) >> > module_path := C.CString(path) >> > if InitStruct != nil{ >> > C.free(unsafe.Pointer(InitStruct)) >> > } >> > InitStruct := (*C.Foo)(C.calloc(1, C.sizeof_struct_Foo)) >> > InitStruct.cb = (C.key_value_cb)(C.callOnMeGo_cgo) >> > InitStruct.data = C.int(5) >> > fmt.Println(InitStruct) <<<<<<<<<<<<<<<<<<<<'&{0x4b7cb0 5 [0 0 0 0]}' got >> > printed >> > fmt.Printf("%p\n", InitStruct) >> > <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<'0x1905630' got printed >> > C.initialize_engine(pattern_db, module_path, InitStruct) >> > >> > } >> > >> > func ReloadPatternDB(patterndb string) { >> > >> > if InitStruct != nil{ >> > fmt.Println(InitStruct) <<<<<<<<<<<<<<<'&{0x4b7cb0 5 [0 0 0 0]}' got >> > printed >> > fmt.Printf("%p\n", InitStruct) <<<<<<<<<<<<<'0x1905630' got printed >> > C.free(unsafe.Pointer(InitStruct)) >> > fmt.Printf("%p\n", InitStruct). <<<<<<<<<<<'0x1905630' got printed >> > ...Older memory address getting printed even after freeing the memory [a] >> > fmt.Println(InitStruct). <<<<<<<<<<<<<<'&{0x863b470 5 [0 0 0 0]}' got >> > printed. Why is struct fields getting printed? [b] >> > } >> > path := C.CString(patterndb) >> > InitStruct = (*C.Foo)(C.calloc(1, C.sizeof_struct_Foo)) >> > InitStruct.cb = (C.key_value_cb)(C.callOnMeGo_cgo) >> > InitStruct.data = C.int(5) >> > fmt.Println(InitStruct) <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< >> > '&{0x4b7cb0 5 [0 0 0 0]}' got printed >> > C.reload_pattern_db(path, InitStruct) <<<<<<<<<. '0x9031d40' got printed >> > >> > } >> > >> > >> > InitStruct is a global variable.I am calling C functions >> > 'initialize_engine' and 'reload_pattern_db' respectively. >> > >> > 1) Even after freeing the memory in 'reload_pattern_db', the older memory >> > address is getting printed.Why? [a] >> >> Here you are using calls to C.calloc and C.free, so you are using C >> pointers and C memory allocation. That is fine. But in C, calling >> C.free doesn't somehow zero out the pointer. It leaves the pointer >> unchanged, and it becomes a dangling pointer that is unsafe to use. >> Basically, if you use C calls, you get C behavior. That is true even >> when calling the C functions from Go. >> >> > 2) Do I have to explicitly point 'InitStruct=nil' after freeing the memory. >> >> Yes, that will remove the dangling pointer. >> >> > 3) Why is 'fmt.Println(InitStruct)' printing the struct fields even after >> > freeing the memory ? [b] >> >> Because that's how C works. C is not a memory safe language, and it >> doesn't become memory safe just because you make the calls from Go. >> >> Ian -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CAOyqgcUk0RRCN9YRbnR_iiA9QMHgpY-3X19nZJ9smN3sYEYwnw%40mail.gmail.com.
