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
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