Libmagic is a magic number recognition library, remember everytime you called file utility on a file to know its type?

➜  file /usr/bin/rm
/usr/bin/rm: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=cbae26b2a032b1ce3129d56aee2bcf70dd8deeb0, stripped
➜  nim-magic file /
/: directory
➜  file /usr/include/stdio.h
/usr/include/stdio.h: C source, ASCII text

import magic

echo magic.guessFile("/usr/bin/rm")

The output should be something like

ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=cbae26b2a032b1ce3129d56aee2bcf70dd8deeb0, stripped

FFI Chapter of Nim in Action is freely available.

from os import fileExists, expandFilename

Well, libmagic has libmagic.so in your library path /usr/lib/libmagic.so and a header file magic.h in /usr/include/magic.h. create a constant for the libmagic library name.

const libName* = "libmagic.so"

We should extract the constants from the header

#define MAGIC_NONE              0x0000000 /* No flags */
#define MAGIC_DEBUG             0x0000001 /* Turn on debugging */
#define MAGIC_SYMLINK           0x0000002 /* Follow symlinks */
#define MAGIC_COMPRESS          0x0000004 /* Check inside compressed files */
#define MAGIC_DEVICES           0x0000008 /* Look at the contents of devices */
#define MAGIC_MIME_TYPE         0x0000010 /* Return the MIME type */
#define MAGIC_CONTINUE          0x0000020 /* Return all matches */
#define MAGIC_CHECK             0x0000040 /* Print warnings to stderr */
....

So in nim It'd be something like this

const  MAGIC_NONE*  = 0x000000                 # No flags 
const  MAGIC_DEBUG* = 0x000001                 # Turn on debugging 
const  MAGIC_SYMLINK* = 0x000002                 # Follow symlinks 
const  MAGIC_COMPRESS* = 0x000004                # Check inside compressed files 
const  MAGIC_DEVICES* = 0x000008                 # Look at the contents of devices 
const  MAGIC_MIME_TYPE* = 0x000010            # Return only the MIME type 
const  MAGIC_CONTINUE* = 0x000020             # Return all matches 
const  MAGIC_CHECK* = 0x000040                 # Print warnings to stderr 
const  MAGIC_PRESERVE_ATIME* = 0x000080        # Restore access time on exit 
const  MAGIC_RAW* = 0x000100                    # Don't translate unprint chars 
const  MAGIC_ERROR* = 0x000200                 # Handle ENOENT etc as real errors 
const  MAGIC_MIME_ENCODING* = 0x000400         # Return only the MIME encoding 
const  MAGIC_NO_CHECK_COMPRESS* = 0x001000     # Don't check for compressed files 
const  MAGIC_NO_CHECK_TAR* = 0x002000         # Don't check for tar files 
const  MAGIC_NO_CHECK_SOFT* = 0x004000         # Don't check magic entries 
const  MAGIC_NO_CHECK_APPTYPE* = 0x008000        # Don't check application type 
const  MAGIC_NO_CHECK_ELF* = 0x010000            # Don't check for elf details 
const  MAGIC_NO_CHECK_ASCII* = 0x020000         # Don't check for ascii files 
const  MAGIC_NO_CHECK_TOKENS* = 0x100000         # Don't check ascii/tokens 

typedef struct magic_set *magic_t; so the only type we have is a pointer to some struct (object)

type Magic = object
type MagicPtr* = ptr Magic 

magic_t magic_open(int);
void magic_close(magic_t);

const char *magic_getpath(const char *, int);
const char *magic_file(magic_t, const char *);
const char *magic_descriptor(magic_t, int);
const char *magic_buffer(magic_t, const void *, size_t);

const char *magic_error(magic_t);
int magic_getflags(magic_t);
int magic_setflags(magic_t, int);

int magic_version(void);
int magic_load(magic_t, const char *);
int magic_load_buffers(magic_t, void **, size_t *, size_t);

int magic_compile(magic_t, const char *);
int magic_check(magic_t, const char *);
int magic_list(magic_t, const char *);
int magic_errno(magic_t);

we only care about magic_open, magic_load, magic_close, magic_file, magic_error

# magic_t magic_open(int);
proc magic_open(i:cint) : MagicPtr {.importc, dynlib:libName.}

magic_open is a proc declared in dynamic lib libmagic.so, that is takes a cint "compatible c int" i and returns a MagicPtr.

From the manpage

The function magic_open() creates a magic cookie pointer and returns it. It returns NULL if there was an error allocating the magic cookie. The flags argument specifies how the other magic functions should behave

# void magic_close(magic_t);
proc magic_close(p:MagicPtr): void {.importc,  dynlib:libName.}

magic_close is a proc declared in dynlib libmagic.so and takes an argumnet p of type MagicPtr and returns void

From the manpage

The magic_close() function closes the magic(5) database and deallocates any resources used.

#int magic_load(magic_t, const char *);
proc magic_load(p:MagicPtr, s:cstring) : cint {.importc, dynlib: libName.}

magic_load is a proc declared in dynlib libmagic.so takes argument p of type MagicPtr and a cstring "compatible c string" s and returns a cint

From manpage:

The magic_load() function must be used to load the colon separated list of database files passed in as filename, or NULL for the default database file before any magic queries can performed.

#int magic_errno(magic_t);
proc magic_error(p: MagicPtr) : cstring  {.importc, dynlib:libName.}

magic_errno is a proc declared in dynlib libmagic.so and takes argument p of type MagicPtr and returns a cstring

From manpage

The magic_error() function returns a textual explanation of the last error, or NULL if there was no error.

#const char *magic_file(magic_t, const char *);
proc magic_file(p:MagicPtr, filepath: cstring): cstring {.importc, dynlib: libName.} 

magic_file is proc declared in dynlib libmagic.so takes argument p of type MagicPtr and a filepath of type cstring and returns a cstring

From manpage:

The magic_file() function returns a textual description of the contents of the filename argument, or NULL if an error occurred. If the filename is NULL, then stdin is used.

It'd be annoying for people to write C code and take care of pointers and such in a higher level language like nim

So let's expose a proc guessFile takes a filepath and flags and internally use the functions we exposed through the FFI in the previous step.

proc guessFile*(filepath: string, flags: cint = MAGIC_NONE): string =
    var mt : MagicPtr
    mt = magic_open(flags)
    discard magic_load(mt, nil)

    if fileExists(expandFilename(filepath)):
        result = $magic_file(mt, cstring(filepath))
    magic_close(mt)

Only one note here to convert from cstring to string we use the toString operator $

        result = $magic_file(mt, cstring(filepath))