Hi Ian,
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]*
2) Do I have to explicitly point 'InitStruct=nil' after freeing the memory.
3) Why is 'fmt.Println(InitStruct)' printing the struct fields even after
freeing the memory ? *[b]*
Please clarify what am I missing here ?
Thanks,
Nitish
On Fri, Mar 6, 2020 at 12:19 AM Ian Lance Taylor <i...@golang.org> wrote:
> On Wed, Mar 4, 2020 at 10:06 PM Nitish Saboo <nitish.sabo...@gmail.com>
> wrote:
> >
> > Thank you for the response.Got it.
> > Basically, fmt.Println(InitStruct) is printing the values of fields, cb
> and data, for the struct Foo pointed by InitStruct.
> > fmt.Printf("%p\n", InitStruct) is printing the memory address.
> > Btw..can you please clarify the following :
> >
> > 1)Is this the correct way to pass a C struct from a Go code to C code.
>
> Sure, this is fine. There are other ways also, but this approach is fine.
>
> > 2) Can I free the struct memory assigned to Initstruct using calloc on
> Go side in the following manner ?
> >
> > if InitStruct != nil{
> > C.free(unsafe.Pointer(InitStruct))
> > }
>
> Yes.
>
> Ian
>
> > On Wed, Mar 4, 2020 at 11:19 PM Ian Lance Taylor <i...@golang.org>
> wrote:
> >>
> >> On Wed, Mar 4, 2020 at 3:57 AM Nitish Saboo <nitish.sabo...@gmail.com>
> wrote:
> >> >
> >> > I have CGO project.
> >> >
> >> > Following is my C header file and Go code
> >> >
> >> > syslog-node.h
> >> > ===============
> >> >
> >> > #ifndef _TEST_H_
> >> > #define _TEST_H_
> >> >
> >> > #include <stdlib.h>
> >> >
> >> > typedef void (*key_value_cb)(const char* key, const char* value,
> size_t value_len, int work);
> >> > typedef struct Foo{
> >> > key_value_cb cb;
> >> > int data;
> >> > }Foo;
> >> > int initialize_engine(const char* filename, const char* module_path,
> Foo *cb);
> >> > int reload_pattern_db(const char* filename, Foo *cb);
> >> >
> >> >
> >> > cfunc.go
> >> > ========
> >> > /*
> >> >
> >> > #include <stdio.h>
> >> >
> >> > // The gateway function
> >> > void callOnMeGo_cgo(char *key, char *value, size_t value_len, int
> work)
> >> > {
> >> > void ParsedData(const char *key, const char *value, size_t value_len,
> int work);
> >> > ParsedData(key, value, value_len, work);
> >> > }
> >> > */
> >> > import "C"
> >> >
> >> > 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(0)
> >> > fmt.Println(InitStruct) <<<<<<<<<<<<<<<<<<<<<<<<<<<'&{0x4b79d0 0 [0
> 0 0 0]}' got printed
> >> > C.initialize_engine(pattern_db, module_path, InitStruct)
> >> >
> >> > }
> >> >
> >> > func ReloadPatternDB(patterndb string) {
> >> >
> >> > path := C.CString(patterndb)
> >> > InitStruct = (*C.Foo)(C.calloc(1, C.sizeof_struct_Foo))
> <<<<<<<Allocating a new memory
> >> > InitStruct.cb = (C.key_value_cb)(C.callOnMeGo_cgo)
> >> > InitStruct.data = C.int(0)
> >> > fmt.Println(InitStruct) <<<<<<<<<<<<<<<<<<<<<<<<<<< '&{0x4b79d0 0
> [0 0 0 0]}' got printed
> >> > C.reload_pattern_db(path, InitStruct)
> >> >
> >> > }
> >> >
> >> >
> >> > I have a Go code where I have to call C functions 'initialize_engine'
> and 'reload_pattern_db' respectively(one after the other ) and pass the C
> struct from Go code.
> >> >
> >> >
> >> > The issue is after allocating memory to 'InitStruct' using calloc in
> 'InitializeEngine' method, the same memory location is getting printed in
> ReloadPatternDB method even though I am allocating a new memory to
> 'InitStruct' using calloc in 'ReloadPatternDB' method.
> >> >
> >> > 1) Is this the correct way to pass a C struct from a Go code to C
> code.
> >> > 2) Is this a correct way to free the struct memory assigned to
> InitStruct using calloc on Go side ?
> >>
> >> You haven't showed us where the same memory location is getting
> >> printed. Note that fmt.Println is not printing the address of
> >> InitStruct; it is printing the values of the fields. If you want to
> >> print the address, use fmt.Printf("%p\n", InitStruct).
> >>
> >> Also note that InitStruct is a global variable.
> >>
> >> Ian
>
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