Golang C (.so) imports segmentation violation on call - c

Question

Hello I'm going to work with thirdparty library (.so file) with golang in linux environment. So I tried to practice a bit with something trivial, like importing functions from linux native libs. And got stuck on importing and calling sqrt function. Here is my code:
package main
// #cgo LDFLAGS: -ldl
// #include <dlfcn.h>
// #include <stdio.h>
import "C"
import "fmt"
func main() {
export_name := "sqrt"
lib_path := "/lib/libm.so.6"
//Loading .so
handle := C.dlopen(C.CString(lib_path), C.RTLD_LAZY)
if handle == nil {
fmt.Println(lib_path+":\tNOT FOUND")
return
} else {
fmt.Println(lib_path+":\tSUCCESS")
}
//looking for function address
func_pointer := C.dlsym(handle, C.CString(export_name ))
if func_pointer == nil {
fmt.Println(export_name+":\tNOT FOUND")
return
} else {
fmt.Println(export_name+":\t", func_pointer)
}
//negotiating datatypes
//From c lib description: double sqrt(double x);
sqrt := *(*(func(float64)float64))(func_pointer)
//Calling function
sqrt(4)
}
when I run it, I always get segmentation violation:
/lib/libm.so.6: SUCCESS
sqrt: 0x7f37117ea270
unexpected fault address 0x0
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x80 addr=0x0 pc=0x4019fa]
goroutine 1 [running]:
runtime.throw(0x4a6643, 0x5)
/usr/lib/go/src/runtime/panic.go:566 +0x95 fp=0xc42004be00 sp=0xc42004bde0
runtime.sigpanic()
/usr/lib/go/src/runtime/sigpanic_unix.go:27 +0x288 fp=0xc42004be58 sp=0xc42004be00
main.main()
/home/afx/goc/so.go:37 +0x2ba fp=0xc42004bf48 sp=0xc42004be58
runtime.main()
/usr/lib/go/src/runtime/proc.go:183 +0x1f4 fp=0xc42004bfa0 sp=0xc42004bf48
runtime.goexit()
/usr/lib/go/src/runtime/asm_amd64.s:2086 +0x1 fp=0xc42004bfa8 sp=0xc42004bfa0
goroutine 17 [syscall, locked to thread]:
runtime.goexit()
/usr/lib/go/src/runtime/asm_amd64.s:2086 +0x1
exit status 2
What is the problem ?
Thank you in advance.
P.S.
When I'm redefining function pointers of native Go functions (like here Go: convert unsafe.Pointer to function pointer and vice versa) everything works fine. But import fails.

Answer 1

Here is solution. I had to use bridge C function:
package main
// #cgo LDFLAGS: -ldl
// #include <dlfcn.h>
// #include <stdio.h>
//
// double
// my_sqrt_bridge(void *f, double x)
// {
// //description: ((return_data_type (*)(input_data_type))bridge_input_function_pointer) (bridge_input_value)
// return ((double (*)(double))f)(x);
// }
import "C"
import "fmt"
func main() {
export_name := "sqrt"
lib_path := "/lib/libm.so.6"
//Loading .so
handle := C.dlopen(C.CString(lib_path), C.RTLD_LAZY)
if handle == nil {
fmt.Println(lib_path + ":\tNOT FOUND")
return
} else {
fmt.Println(lib_path + ":\tSUCCESS")
}
//looking for function address
func_pointer := C.dlsym(handle, C.CString(export_name))
if func_pointer == nil {
fmt.Println(export_name + ":\tNOT FOUND")
return
} else {
fmt.Println(export_name+":\t", func_pointer)
}
fmt.Printf("%f", C.my_sqrt_bridge(func_pointer, 2))
}

Answer 2

Would it be possible in your case to let cgo link the library for you? Eg:
package main
/*
#cgo LDFLAGS: -lm
#include <math.h>
*/
import "C"
import "fmt"
func main() {
fmt.Printf("%g\n", C.sqrt(2)) //prints 1.4142135623730951
}
For a third party library /some/lib/dir/libxxx.so:
LDFLAGS: -L/some/lib/dir -lxxx