# Writing a dynamic library in Crystal, and calling it from C

* Download all files.
* Build the Crystal part:

```
crystal build --release --cross-compile --prelude ./no_main library.cr
              ^ You can leave this out
                        ^ We want to link ourselves!
                                        ^ Use our custom prelude!
```

* Link the Crystal part into a shared library.  The previous command gives you the line, though we have to modify it a bit:

```
gcc library.o -o libfoo.so -rdynamic -shared -lpcre -lgc -lpthread /usr/lib/crystal/ext/libcrystal.a -levent -lrt -ldl -L/usr/lib -L/usr/local/lib
                                     ^ Add this to link into a shared lib
              ^ We want a decent name for the shared lib too.
```

* Build and link the C program:

```
gcc -o program program.c -L$PWD -lfoo
                         ^ We stored our lib right 'here'
                                ^ And we want to link to it
```

* Run the program:

```
./program
```

* Verify (Linux only):

Using `ldd program`, we see the dynamic dependencies of our program:

```
        linux-vdso.so.1 (0x00007ffc56d8c000)
        libfoo.so (0x00007f80846e9000)         <---- Here it is!
        libc.so.6 => /usr/lib/libc.so.6 (0x00007f8084343000)
        libpcre.so.1 => /usr/lib/libpcre.so.1 (0x00007f80840d0000)
        libgc.so.1 => /usr/lib/libgc.so.1 (0x00007f8083e66000)
        libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f8083c48000)
        libevent-2.1.so.6 => /usr/lib/libevent-2.1.so.6 (0x00007f80839f2000)
        librt.so.1 => /usr/lib/librt.so.1 (0x00007f80837ea000)
        libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f80835e6000)
        libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007f80833cf000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f8084909000)
        libatomic_ops.so.1 => /usr/lib/libatomic_ops.so.1 (0x00007f80831cc000)
        libcrypto.so.1.1 => /usr/lib/libcrypto.so.1.1 (0x00007f8082d4c000)
```

## Enjoy!