Hi Ian,
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.
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))
}
Thanks,
Nitish
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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