How can I Call Go Functions from My C Program Using GCCGO?

Calling Go Functions from a C Program Using GCCGO

Introduction

To integrate Go code within an existing C project, one must create an object file (.o) from the Go file and incorporate it into the C project's archive (.a) file. This article provides a detailed explanation on how to accomplish this using GCCGO.

Building a Go Object File

To create an object file from a Go file, use the gccgo command with the -c option to compile the file without linking it:

gccgo -c printString.go -o printString.o -fgo-prefix=print -Wall -Werror -march=native

The -fgo-prefix option specifies a prefix for the symbols exported from the Go file.

Linking the Go Function to the C Program

To link the Go object file to the C program, use extern declarations and the __asm__ keyword in the C code:

<code class="c">#include <stdio.h>

extern int PrintString(char*) __asm__ ("print.main.PrintString");</code>

This declares the PrintString function with the correct signature and symbol name for GCCGO.

Compilation

To compile the C program with the Go object file, use gccgo with the -o option to specify the output executable:

gccgo -o main c_caller.c printString.o -Wall -Werror -march=native

Addressing Compilation Errors

The initial compilation error mentioned in the original question indicates that GCCGO expects a main function in the Go file. However, this is not required when linking to a C program. To resolve this issue, ensure that:

• The Go file does not contain a main function.
• The C program defines its own main function.

Additional Options

To address the linker errors related to missing library references, try the following options:

• --static-libgo: Use the static version of the Go library.
• -static: Link against static libraries instead of shared libraries.
• -Wl,-R,/path/to/libgo.so's_folder: Specify the path to the folder containing libgo.so.

Example from Go 1.5 (August 2015)

Go 1.5 introduced the ability to build C-compatible libraries directly using the go tool. This method simplifies the process significantly:

_main.c

<code class="c">#include <stdio.h>

int main()
{
    char *string_to_pass = NULL;
    if (asprintf(&amp;string_to_pass, "This is a test.") < 0) {
        printf("asprintf fail");
        return -1;
    }

    PrintString(string_to_pass);
    return 0;
}</code>

main.go

<code class="go">package main

import "C"
import "fmt"

//export PrintString
func PrintString(cs *C.char) {
    s := C.GoString(cs)
    fmt.Println(s)
}

func main() {}</code>

Building as a Static Library:

go build -buildmode c-archive -o mygopkg.a
gcc -o main _main.c mygopkg.a -lpthread

Building as a Shared Library:

go build -buildmode c-shared -o mygopkg.so
LD_RUN_PATH=$(pwd) gcc -o main _main.c mygopkg.so -lpthread

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