Not able to compile with cgo

Question

What version of Go are you using (go version)?

$ go version
go version go1.20.2 linux/amd64

Project structure:

Directory structure --
example --> main.go
        -->lib
            lib.c
            lib.h

---

main.go

package main

// #include "lib/lib.h"
// #include <stdio.h>
// #include <stdlib.h>
import "C"
import (
    "fmt"
    "unsafe"
)

func main() {
    cstrin := C.CString("welcome")
    s1 := C.struct_s1{b: cstrin, a: 100}

    C.f32_123(&s1)
    cs := C.GoString(s1.b)
    fmt.Println(cs)
    fmt.Println(s1)
    C.free(unsafe.Pointer(cstrin))
}

lib/lib.c

#include <stdlib.h>
#include <stdio.h>

void printc(char *str, int *t)
{
     str = "Test";
     printf("%d\n", *t);
     *t = 30;
     printf("%s\n", str);
}

void f32_123(struct s1 *s)
{
     printf("%s\n", s->b);
     s->a = 10;
     s->b = "Hello123";
     printf("%d\n", s->a);
     printf("%s\n", s->b);
}

lib/lib.h

struct s1 {
    int a;
    char *b;
};

void printc(char *str, int *t);
void f32_123(struct s1 *s);

Error while compiling

/usr/local/go/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
/usr/bin/ld: /tmp/go-link-3024881602/000001.o: in function _cgo_cf24297edd23_Cfunc_f32_123': /tmp/go-build/cgo-gcc-prolog:49: undefined reference to f32_123'
collect2: error: ld returned 1 exit status

I am expecting the code to compile successfully, but somehow it is not. If I've read the documentation correctly, then I have to keep lib.c and lib.h in the same directory with the main.go file. But I am not sure if this can be achieved or I am doing something wrong.

• If I keep all the files into same directory example then compile is successful.

• If I keep lib.c and lib.h into subdir then compile fails

• If I remove one function f32_123 from main.go then also compile is successful and that is pretty strange and that is the reason opening this bug to get more understanding why compile has not issue with printc function when lib.h and lib.c is in subdir.

Answer 1

First, @JimB gave this accepted answer a while back: https://stackoverflow.com/a/28881331/5739452 which says that building an object or library in a sub directory isn't something go build can just do.

Given that, let's say you have this structure:

lib/
lib/lib.c
lib/lib.h
main.go

Here are some simpler files to keep things clear:

/* lib/lib.h */
struct foo {
    int x;
};
void show(struct foo *arg);

/* lib/lib.c */
#include <stdio.h>
#include "lib.h"
void show(struct foo *arg) {
    printf("[%d]\n", arg->x);
}

So, you can get this to build all from go build main.go if you have a main.go like this:

package main

// #cgo CFLAGS: -I${SRCDIR}/lib
// #include "lib.c"
import "C"

func main() {
    x := C.struct_foo{ x: 42 }
    C.show(&x)
}

which works because we actually #include the "C" source code for the library (which implicitly imports the lib/lib.h file).

However, for more complicated libraries, you will likely need to build them as a separate, more normal C tool chain, pre-work build step:

$ cd lib
$ cc -c lib.c
$ ar cr libx.a lib.o
$ cd ..

and then use a different Go file: main2.go:

package main

// #cgo CFLAGS: -I${SRCDIR}/lib
// #cgo LDFLAGS: -L${SRCDIR}/lib -lx
// #include "lib.h"
import "C"

func main() {
    x := C.struct_foo{ x: 42 }
    C.show(&x)
}