Call a function in a shared library: Difference between revisions

=={{header|Go}}==

{{trans|C}}

{{works with|Ubuntu 18.04}}

<br>

Dynamically calling a function from a shared library can only be accomplished in Go using 'cgo' and, even then, the function pointer returned by 'dlsysm' can only be called via a C bridging function as calling C function pointers directly from Go is not currently supported.

This is the C code to produce fakeimglib.so:

<lang c>#include <stdio.h>

/* gcc -shared -fPIC -nostartfiles fakeimglib.c -o fakeimglib.so */

int openimage(const char *s)

{

static int handle = 100;

fprintf(stderr, "opening %s\n", s);

return handle++;

}</lang>

And this is the Go code to dynamically load the .so file and call the 'openimage' function - or if the .so file (or the function itself) is not available, to call the internal version of the function:

<lang go>package main

/*

#cgo LDFLAGS: -ldl

#include <stdlib.h>

#include <dlfcn.h>

typedef int (*someFunc) (const char *s);

int bridge_someFunc(someFunc f, const char *s) {

return f(s);

}

*/

import "C"

import (

"fmt"

"os"

"unsafe"

)

var handle = -1

func myOpenImage(s string) int {

fmt.Fprintf(os.Stderr, "internal openImage opens %s...\n", s)

handle++

return handle

}

func main() {

libpath := C.CString("./fakeimglib.so")

defer C.free(unsafe.Pointer(libpath))

imglib := C.dlopen(libpath, C.RTLD_LAZY)

var imghandle int

if imglib != nil {

openimage := C.CString("openimage")

defer C.free(unsafe.Pointer(openimage))

fp := C.dlsym(imglib, openimage)

if fp != nil {

fi := C.CString("fake.img")

defer C.free(unsafe.Pointer(fi))

imghandle = int(C.bridge_someFunc(C.someFunc(fp), fi))

} else {

imghandle = myOpenImage("fake.img")

}

C.dlclose(imglib)

} else {

imghandle = myOpenImage("fake.img")

}

fmt.Printf("opened with handle %d\n", imghandle)

}</lang>

{{output}}

<pre>

Same as C entry.

</pre>