Thankyou Ian for your responses. Thanks, Nitish
On Fri, 6 Mar 2020, 21:01 Ian Lance Taylor, <i...@golang.org> wrote: > 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/CALjMrq7WUzRhU%3Dj2O-SxpYGNhPBm2-8oSfVcOtmVUDP8%2BEw3BQ%40mail.gmail.com.
