Passing Struct from Go to C

[Nitish Saboo]

Hi,

I am trying to pass a Struct from Go to C (this is cgo).I m passing a callback function in that struct that will be called from C code to Go.

main.go

=======

type Callback struct {

Func     func(*C.char , *C.char , C.size_t, C.int)

UserData int32

}

Calling C code in the following manner

=========================================

C.initialize_engine(pattern_db, module_path, (C.Foo)(unsafe.Pointer(&Callback{

Func:     C.callOnMeGo_cgo,

UserData: C.int(1),

})))

//export ParsedData

func ParsedData(key *C.char, value *C.char, value_len C.size_t, work C.int) {

fmt.Println("WORK:")

fmt.Print()

//performing some operation here

}

cfunc.go

=========

package main

/*

#include <stdio.h>

// The gateway function

void callOnMeGo_cgo(char *key, char *value, size_t value_len, int work)

{

//printf("C.callOnMeGo_cgo(): called");

void ParsedData(const char *key, const char *value, size_t value_len, int work);

ParsedData(key, value, value_len, work);

}

*/

import "C"

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 cbnitish;

    int data;

}Foo;

int initialize_engine(const char* filename, const char* module_path, Foo *cb);

int reload_pattern_db(const char* filename, Foo *cb);

#endif

syslog-node.c

===============

gboolean pdbtool_accumulate_fields(NVHandle handle, const gchar *name, const gchar *value, gssize length, gpointer user_data)

{

  user_data->cbnitish(name, value, length, user_data->data);// Callback happening here

  return FALSE;

}

Eventually user_data will get the instance of the struct.

Getting following error during make command:

==============================================

nsaboo@ubuntu:~/Documents/goworkspace/src/poc$ make

gcc -L/usr/local/lib -lsyslog-ng -o syslog-node.so -L/usr/local/lib/syslog-ng -ldbparser -c `pkg-config --libs --cflags glib-2.0` -I/usr/local/include/syslog-ng/ -I./syslog-ng-3.6.2/ -I/usr/local/include/eventlog/ syslog-node.c

syslog-node.c: In function ‘pdbtool_accumulate_fields’:

syslog-node.c:33:12: warning: dereferencing ‘void *’ pointer

   user_data->cbnitish(name, value, length, user_data->data);

            ^~

syslog-node.c:33:12: error: request for member ‘cbnitish’ in something not a structure or union

syslog-node.c:33:53: warning: dereferencing ‘void *’ pointer

   user_data->cbnitish(name, value, length, user_data->data);

                                                     ^~

syslog-node.c:33:53: error: request for member ‘data’ in something not a structure or union

makefile:12: recipe for target 'syslog-node.so' failed

make: *** [syslog-node.so] Error 1

Can someone please guide me here ?

Thanks,

Nitish

[Nitish Saboo]

Hi,

Looks like I was able to resolve the C error.I am getting the following Go error while running 'make' command:

gcc -L/usr/local/lib -lsyslog-ng -o syslog-node.so -L/usr/local/lib/syslog-ng -ldbparser -c `pkg-config --libs --cflags glib-2.0` -I/usr/local/include/syslog-ng/ -I./syslog-ng-3.6.2/ -I/usr/local/include/eventlog/ syslog-node.c

go build -v -x main.go cfuncs.go
WORK=/tmp/go-build873967443
command-line-arguments
mkdir -p $WORK/b001/
cd /home/nsaboo/Documents/goworkspace/src/poc
pkg-config --cflags -- syslog-ng eventlog
pkg-config --libs -- syslog-ng eventlog
CGO_LDFLAGS='"-g" "-O2" "syslog-node.so" "-L/usr/local/lib/" "-lsyslog-ng" "-L/usr/local/lib/syslog-ng" "-ldbparser" "-L/usr/local/lib" "-Wl,--export-dynamic" "-lgmodule-2.0" "-pthread" "-lgthread-2.0" "-pthread" "-lglib-2.0" "-lsyslog-ng" "-lglib-2.0" "-levtlog"' /usr/local/go/pkg/tool/linux_amd64/cgo -objdir $WORK/b001/ -importpath command-line-arguments -- -I/usr/local/include/syslog-ng -I/usr/local/include/syslog-ng/ivykis -I/usr/local/include/eventlog -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I $WORK/b001/ -g -O2 -I/usr/local/include/ ./main.go ./cfuncs.go
cd $WORK
gcc -fno-caret-diagnostics -c -x c - || true
gcc -Qunused-arguments -c -x c - || true
gcc -fdebug-prefix-map=a=b -c -x c - || true
gcc -gno-record-gcc-switches -c -x c - || true
cd $WORK/b001
TERM='dumb' gcc -I /home/nsaboo/Documents/goworkspace/src/poc -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -I/usr/local/include/syslog-ng -I/usr/local/include/syslog-ng/ivykis -I/usr/local/include/eventlog -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I ./ -g -O2 -I/usr/local/include/ -o ./_x001.o -c _cgo_export.c
TERM='dumb' gcc -I /home/nsaboo/Documents/goworkspace/src/poc -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -I/usr/local/include/syslog-ng -I/usr/local/include/syslog-ng/ivykis -I/usr/local/include/eventlog -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I ./ -g -O2 -I/usr/local/include/ -o ./_x002.o -c main.cgo2.c
TERM='dumb' gcc -I /home/nsaboo/Documents/goworkspace/src/poc -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -I/usr/local/include/syslog-ng -I/usr/local/include/syslog-ng/ivykis -I/usr/local/include/eventlog -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I ./ -g -O2 -I/usr/local/include/ -o ./_x003.o -c cfuncs.cgo2.c
TERM='dumb' gcc -I /home/nsaboo/Documents/goworkspace/src/poc -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -I/usr/local/include/syslog-ng -I/usr/local/include/syslog-ng/ivykis -I/usr/local/include/eventlog -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I ./ -g -O2 -I/usr/local/include/ -o ./_cgo_main.o -c _cgo_main.c
cd /home/nsaboo/Documents/goworkspace/src/poc
TERM='dumb' gcc -I . -fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=$WORK/b001=/tmp/go-build -gno-record-gcc-switches -o $WORK/b001/_cgo_.o $WORK/b001/_cgo_main.o $WORK/b001/_x001.o $WORK/b001/_x002.o $WORK/b001/_x003.o -g -O2 syslog-node.so -L/usr/local/lib/ -lsyslog-ng -L/usr/local/lib/syslog-ng -ldbparser -L/usr/local/lib -Wl,--export-dynamic -lgmodule-2.0 -pthread -lgthread-2.0 -pthread -lglib-2.0 -lsyslog-ng -lglib-2.0 -levtlog
TERM='dumb' /usr/local/go/pkg/tool/linux_amd64/cgo -dynpackage main -dynimport $WORK/b001/_cgo_.o -dynout $WORK/b001/_cgo_import.go
cat >$WORK/b001/importcfg << 'EOF' # internal
# import config
packagefile bufio=/usr/local/go/pkg/linux_amd64/bufio.a
packagefile encoding/json=/usr/local/go/pkg/linux_amd64/encoding/json.a
packagefile fmt=/usr/local/go/pkg/linux_amd64/fmt.a
packagefile os=/usr/local/go/pkg/linux_amd64/os.a
packagefile path=/usr/local/go/pkg/linux_amd64/path.a
packagefile runtime=/usr/local/go/pkg/linux_amd64/runtime.a
packagefile time=/usr/local/go/pkg/linux_amd64/time.a
packagefile runtime/cgo=/usr/local/go/pkg/linux_amd64/runtime/cgo.a
packagefile syscall=/usr/local/go/pkg/linux_amd64/syscall.a
EOF
/usr/local/go/pkg/tool/linux_amd64/compile -o $WORK/b001/_pkg_.a -trimpath $WORK/b001 -p main -buildid hIEYVdT7igzjOGrFONpW/hIEYVdT7igzjOGrFONpW -goversion go1.12.4 -D _/home/nsaboo/Documents/goworkspace/src/poc -importcfg $WORK/b001/importcfg -pack -c=2 $WORK/b001/_cgo_gotypes.go $WORK/b001/main.cgo1.go $WORK/b001/cfuncs.cgo1.go $WORK/b001/_cgo_import.go
# command-line-arguments
./main.go:91:57: cannot convert unsafe.Pointer(*Callback literal) (type unsafe.Pointer) to type _Ctype_struct_Foo
./main.go:91:84: cannot use _Cgo_ptr(_Cfpvar_fp_callOnMeGo_cgo) (type unsafe.Pointer) as type func(*_Ctype_char, *_Ctype_char, _Ctype_ulong, _Ctype_int) in field value
./main.go:91:128: cannot use _Ctype_int(1) (type _Ctype_int) as type int32 in field value
./main.go:166:66: cannot convert unsafe.Pointer(*Callback literal) (type unsafe.Pointer) to type _Ctype_struct_Foo
./main.go:166:93: cannot use _Cgo_ptr(_Cfpvar_fp_callOnMeGo_cgo) (type unsafe.Pointer) as type func(*_Ctype_char, *_Ctype_char, _Ctype_ulong, _Ctype_int) in field value
./main.go:166:137: cannot use _Ctype_int(1) (type _Ctype_int) as type int32 in field value
makefile:6: recipe for target 'main' failed
make: *** [main] Error 2

Can someone please guide me here ?

Thanks,

Nitish

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[Tamás Gulácsi]

*C.foo

[Nitish Saboo]

Hi Tamas,

I made it *C.Foo but still getting the error.

main.go

=======

type Callback struct {

Func     func(*C.char , *C.char , C.size_t, C.int)

UserData int32

}

Calling C code in the following manner

=========================================

C.initialize_engine(pattern_db, module_path, (*C.Foo)(unsafe.Pointer(&Callback{

Func:     C.callOnMeGo_cgo

Looks like there is some issue with my  struct declaration in Go.Can some please guide me  as in what is the issue ?

Error:

====

go build -v -x main.go cfuncs.go

WORK=/tmp/go-build037176041

/usr/local/go/pkg/tool/linux_amd64/compile -o $WORK/b001/_pkg_.a -trimpath $WORK/b001 -p main -buildid ZmTjWDq-3vSs8dJXUcwn/ZmTjWDq-3vSs8dJXUcwn -goversion go1.12.4 -D _/home/nsaboo/Documents/goworkspace/src/poc -importcfg $WORK/b001/importcfg -pack -c=2 $WORK/b001/_cgo_gotypes.go $WORK/b001/main.cgo1.go $WORK/b001/cfuncs.cgo1.go $WORK/b001/_cgo_import.go
# command-line-arguments
./main.go:91:85: cannot use _Cgo_ptr(_Cfpvar_fp_callOnMeGo_cgo) (type unsafe.Pointer) as type func(*_Ctype_char, *_Ctype_char, _Ctype_ulong, _Ctype_int) in field value
./main.go:91:129: cannot use _Ctype_int(1) (type _Ctype_int) as type int in field value
./main.go:166:94: cannot use _Cgo_ptr(_Cfpvar_fp_callOnMeGo_cgo) (type unsafe.Pointer) as type func(*_Ctype_char, *_Ctype_char, _Ctype_ulong, _Ctype_int) in field value
./main.go:166:148: type _Ctype_int is not an expression

makefile:6: recipe for target 'main' failed
make: *** [main] Error 2

On Wed, Jul 10, 2019 at 2:28 PM Tamás Gulácsi <tgula...@gmail.com> wrote:

*C.foo

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[andrey mirtchovski]

i don't think you can use a go function directly in the callback. you
need a correctly-typed C helper to do it. at least that used to be the
case. see examples:

https://github.com/mirtchovski/clamav/blob/master/cfuncs.go
https://github.com/mirtchovski/clamav/blob/master/callback.go

[Nitish Saboo]

Hi Andrey,

I understand the issue here but how can I pass a callback function in a struct to C code.I need a struct because I want to pass and an extra parameter 'userdata' as well that I would require back as part of callback.

Thanks,

Nitish

[andrey mirtchovski]

what i do is have a similar struct in Go, and the original C one. here
'callbacks' is the go one, and
"type Engine C.struct_cl_engine" is the opaque C one:

https://github.com/mirtchovski/clamav/blob/master/callback.go#L63

[Nitish Saboo]

Hi Andrey,

I am new to Go and cgo therefore did not exactly understand your point.

Can you explain me in the context of the code that I had posted.It will really help me to get hold of the things much better.

Thanks,

Nitish

[andrey mirtchovski]

the Go "Callback" struct may have completely different size and
alignment than the C.Foo one expected by C.initialize_engine.
therefore you want to construct the C version when calling
C.initialize_engine, and the Go version for everything else. Your call
may look something like this:

initStruct := C.calloc(1, sizeof(C.Foo))
initStruct.key_value_cb = C.callOnMeGo_cgo
initStrudct.data: C.int(1)

C.initialize_engine(pattern_db, module_path,
(*C.Foo)(unsafe.Pointer(initStruct)))

the Go Callback struct is not required unless you expect the callbacks
to change or be settable by library clients, in which case you'll need
to construct a more elaborate struct that implements locking and (at
least what I do) a map between callback names and interface{} pointing
to the callbacks sent by the library clients.

[Nitish Saboo]

Hi Andrey,

I made the following changes as suggested by you:

 InitStruct := C.calloc(1, C.sizeof_struct_Foo)

InitStruct.key_value_cb = C.callOnMeGo_cgo
InitStruct.data = C.int(1)
C.initialize_engine(pattern_db, module_path, (*C.Foo)(unsafe.Pointer(InitStruct)))

Still Getting following error:

Am i missing something?

go build -v -x main.go cfuncs.go

WORK=/tmp/go-build822337807

/usr/local/go/pkg/tool/linux_amd64/compile -o $WORK/b001/_pkg_.a -trimpath $WORK/b001 -p main -buildid 6VU9NC3t5EyTg_0nt_wU/6VU9NC3t5EyTg_0nt_wU -goversion go1.12.4 -D _/home/nsaboo/Documents/goworkspace/src/poc -importcfg $WORK/b001/importcfg -pack -c=2 $WORK/b001/_cgo_gotypes.go $WORK/b001/main.cgo1.go $WORK/b001/cfuncs.cgo1.go $WORK/b001/_cgo_import.go
# command-line-arguments
./main.go:92:12: InitStruct.key_value_cb undefined (type unsafe.Pointer has no field or method key_value_cb)
./main.go:93:12: InitStruct.data undefined (type unsafe.Pointer has no field or method data)
./main.go:170:12: InitStruct.key_value_cb undefined (type unsafe.Pointer has no field or method key_value_cb)
./main.go:171:12: InitStruct.data undefined (type unsafe.Pointer has no field or method data)

makefile:6: recipe for target 'main' failed

make: *** [main] Error 2

Thanks,

Nitish