Use another language to call a function: Difference between revisions

Here am I

</pre>

=== Alternative Method ===

As of Go 1.5, this is now possible without modifying the C code thanks to the addition of buildmodes. Buildmodes allow Go code to be compiled to standard C libraries (both dynamic and static).

The Go code for this task is as follows:

<lang go>

// This buildmode requires the package to be main

package main

// Import C so we can export the function to C and use C types

//#include <stdlib.h> // for size_t

import "C"

// Import reflect and unsafe so we can wrap the C array in a Go slice

import "reflect"

import "unsafe"

// This buildmode also requires a main function, but it is never actually called

func main() {}

// The message to copy into the buffer

const msg = "Here am I"

// Here we declare the Query function using C types and export it to C

//export Query

func Query(buffer *C.char, length *C.size_t) C.int {

// Check there is enough space in the buffer

if int(*length) < len(msg) {

return 0

}

// Wrap the buffer in a slice to make it easier to copy into

sliceHeader := reflect.SliceHeader {

Data: uintptr(unsafe.Pointer(buffer)),

Len: len(msg),

Cap: len(msg),

}

bufferSlice := *(*[]byte)(unsafe.Pointer(&sliceHeader))

// Iterate through the message and copy it to the buffer, byte by byte

for i:=0;i<len(msg);i++ {

bufferSlice[i] = msg[i]

}

// Set length to the amount of bytes we copied

(*length) = C.size_t(len(msg))

return 1

}

</lang>

Assuming this is saved to query.go (and that the C code is saved as main.c) it can be compiled with:

<pre>

$ go build -buildmode=c-shared query.go

$ gcc main.c -L. -lquery -o main

</pre>

This creates a library file, a header file for the library (not used) and an executable dynamically linked to the library file.

The executable can be run with:

<pre>

$ LD_LIBRARY_PATH=. ./main