Golang Works with Shared Object Through cgo (.so Not Found Issue)

jason19970210

3 min readSep 10, 2023

Faced the problem with Golang and cgo for a couple days, so just take a quick note to record what had happened.

Shortcut: Jump to [Key Point], [Solution] section
A quick look in [Gist]

From the beginning, my goal was aim to make Golang works with a shared object file (.so) built from a C code.
Just like others in 2023, I went to ChatGPT for an answer which can help me have a quick look for the solution how to achieve the goal.

Key Point

Noticed from Ref[2]: (the reference is written in Chinese)

1. The `.so` file built from gcc should has the `lib` prefix
example: `libsum.so`, `libcalc.so`

2. The value after `LDFLAGS` should goes with name without `lib`
example:
a) `libsum.so` -> LDFLAGS=sum
a) `libcalc.so` -> LDFLAGS=calc

ChatGPT shows me a sum.c & main.go

// sum.c
#include "sum.h"

int sum(int a, int b) {
    return a + b;
}

package main

/*
#cgo LDFLAGS: -L. -lsum
#include "sum.h"
*/
import "C"
import "fmt"

func main() {
 // Call the sum function from the shared library
 result := C.sum(5, 7)

 // Print the result
 fmt.Printf("Sum: %d\n", result)
}

I had put the files into following struct:

$ tree <project_folder>
.
├── libs
│   ├── sum.c
│   └── sum.h
└── main.go

With the build script:

$ cd <project_folder>/libs
$ gcc -shared -o libsum.so sum.c

$ cd <project_folder>
$ go build -o go_sum main.go

And here comes the first error:

$ go build -o go_sum main.go

# command-line-arguments
./main.go:5:10: fatal error: sum.h: No such file or directory
    5 | #include "sum.h"
      |          ^~~~~~~
compilation terminated.

It’s not a big deal to us to create a sum.h header file.

// sum.h

#include <stdio.h>

int sum(int a, int b);

There should also put another compile flag: CFLAGS for cgo in main.go , then rebuild the Golang executable file

package main

/*
#cgo CFLAGS: -Ilibs
#cgo LDFLAGS: -Llibs -lsum
#include "sum.h"
*/
import "C"
import "fmt"
...

Once build successfully, just run the executable file with name go_sum

Unfortunately, another error pops !

$ ./go_sum
./go_sum: error while loading shared libraries: libsum.so: cannot open shared object file: No such file or directory

By searching and try-error for hours, I had found the solution which I should make the .so file been found by linker.

Here is the command where we can track the .so file information linked to a executable file:

$ ldd <executable file>

## Results for go_sum
$ ldd go_sum
    linux-vdso.so.1 (0x00007ffde1aab000)
    libsum.so => not found
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f6a85b55000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f6a85d86000)

As the result, the libsum.so has not been recognized, here is nothing consider with the .so itself, but the linker.

Solution

Updated: 2023/09/20

By config correctly with LDFLAGS
Thanks to the answer from Arkadiusz Rychliński

#cgo LDFLAGS: -L./lib -Wl, -rpath, $ORIGIN/libs -lsum

By adding the LD_LIBRARY_PATH

$ export LD_LIBRARY_PATH=<project_folder>/lib/

After the line execute, rebuild the Golang executable file, then check again the lib has been recognized with ldd command.

$ go build -o go_sum main.go

$ ldd go_sum
    linux-vdso.so.1 (0x00007ffde1aab000)
    libsum.so => <project_folder>/libs/libsum.so (0x00007ff959dbb000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f6a85b55000)
    /lib64/ld-linux-x86-64.so.2 (0x00007f6a85d86000)