High-level programming languages are popular, but there are areas where you will need to use unmanaged library implementations. This can be a call to specific OS functions, low-level access to devices, the need for performance in algorithms, and others. Under the cut, I will tell you what you can encounter while traveling in unmanaged code and what you should take with you.
You are standing on the doorstep of your cozy IDE, and you are not at all tempted to go to the world of source code, where it is dark and nothing is clear. For the success of the enterprise, first of all, you need to get hold of a map - a description of the headers of the library will come down, and it is better to have complete documentation. It usually looks like this:
...
#include <linux/netfilter_ipv4/ip_tables.h>
#include <libiptc/xtcshared.h>
#ifdef __cplusplus
extern "C" {
#endif
#define iptc_handle xtc_handle
#define ipt_chainlabel xt_chainlabel
#define IPTC_LABEL_ACCEPT "ACCEPT"
#define IPTC_LABEL_DROP "DROP"
#define IPTC_LABEL_QUEUE "QUEUE"
#define IPTC_LABEL_RETURN "RETURN"
/* Does this chain exist? */
int iptc_is_chain(const char *chain, struct xtc_handle *const handle);
/* Take a snapshot of the rules. Returns NULL on error. */
struct xtc_handle *iptc_init(const char *tablename);
/* Cleanup after iptc_init(). */
void iptc_free(struct xtc_handle *h);
...
Let's say you're in luck and the documentation is there. It describes the function signatures, structures used, aliases, and references to other used headers. The first quest is to find the library in the OS. Its name may differ from what is expected:
~$ find /usr/lib/x86_64-linux-gnu/ -maxdepth 1 -name 'libip*'
/usr/lib/x86_64-linux-gnu/libip6tc.so.0.1.0
/usr/lib/x86_64-linux-gnu/libip4tc.so
/usr/lib/x86_64-linux-gnu/libiptc.so.0
/usr/lib/x86_64-linux-gnu/libip4tc.so.0.1.0
/usr/lib/x86_64-linux-gnu/libip6tc.so.0
/usr/lib/x86_64-linux-gnu/libiptc.so.0.0.0
/usr/lib/x86_64-linux-gnu/libip4tc.so.0
/usr/lib/x86_64-linux-gnu/libiptc.so
/usr/lib/x86_64-linux-gnu/libip6tc.so
Numerical suffix means different versions of libraries. In general, we need the original libip4tc.so. You can look inside with one eye and make sure that it's worthwhile:
~$ nm -D /usr/lib/x86_64-linux-gnu/libip4tc.so ... 0000000000206230 D _edata 0000000000206240 B _end U __errno_location U fcntl 000000000000464c T _fini U __fprintf_chk U free U getsockopt w __gmon_start__ 0000000000001440 T _init 0000000000003c80 T iptc_append_entry 0000000000003700 T iptc_builtin 0000000000004640 T iptc_check_entry 0000000000003100 T iptc_commit 0000000000002ff0 T iptc_create_chain 00000000000043f0 T iptc_delete_chain ...
, , , . :
public static class Libiptc4
{
/* Prototype: iptc_handle_t iptc_init(const char *tablename) */
[DllImport("libip4tc.so")]
public static extern IntPtr iptc_init(string tablename);
}
/* Prototype: iptc_handle_t iptc_init(const char *tablename) */
[DllImport("libip4tc.so")]
public static extern IntPtr iptc_init(IntPtr tblPtr);
...
var tblPtr = Marshal.StringToHGlobalAnsi("filter");
var _handle = Libiptc4.iptc_init_ptr(tblPtr);
Marshal.FreeHGlobal(tblPtr);
.
, , , , .
, : . :
struct ipt_entry {
struct ipt_ip ip;
/* Mark with fields that we care about. */
unsigned int nfcache;
/* Size of ipt_entry + matches */
__u16 target_offset;
/* Size of ipt_entry + matches + target */
__u16 next_offset;
/* Back pointer */
unsigned int comefrom;
/* Packet and byte counters. */
struct xt_counters counters;
/* The matches (if any), then the target. */
unsigned char elems[0];
};
unsigned char elems[0]
. , , . :
*******************************************
* ip_entry *
* 112 bytes *
*******************************************
* matches *
* target_offset - 112 bytes *
*******************************************
* target *
* next_offset - target_offset - 112 bytes *
*******************************************
(matches
target
) ip_entry
. :
.
.
, , . ipt_entry
:
[StructLayout(LayoutKind.Sequential)]
public struct IptEntry
{
public IptIp ip;
public uint nfcache;
public ushort target_offset;
public ushort next_offset;
public uint comefrom;
public IptCounters counters;
};
Marshal.SizeOf<IptEntry>()
112 . matches
target
( ). : libiptc
8 ( long
), . , . :
static readonly int _WORDLEN = Marshal.SizeOf<long>();
public static int Align(int size)
{
return ((size + (_WORDLEN - 1)) & ~(_WORDLEN - 1));
}
, entry.target_offset
entry.next_offset
, :
IntPtr entryPtr = Marshal.AllocHGlobal(size);
Marshal.StructureToPtr<IptEntry>(entryPtr, entry, false);
Marshal.StructureToPtr<Match>(entryPtr + 112, match, false);
: , , :
var entry = Marshal.PtrToStructure<IptEntry>(point);
var match = Marshal.PtrToStructure<Match>(point + 112)
, union:
struct xt_entry_match {
union {
struct {
__u16 match_size;
/* Used by userspace */
char name[XT_EXTENSION_MAXNAMELEN];
__u8 revision;
} user;
struct {
__u16 match_size;
/* Used inside the kernel */
struct xt_match *match;
} kernel;
/* Total length */
__u16 match_size;
} u;
unsigned char data[0];
};
:
#define XT_EXTENSION_MAXNAMELEN 29
...
char name [XT_EXTENSION_MAXNAMELEN]
. header .
, . ushort, uint long , . . : , . . . :
byte [] convArray = BitConverter.GetBytes(value);
Array.Reverse(convArray);
ushort reverseEndian = BitConverter.ToUInt16(convArray,0);
ushort reverseEndian = (ushort)((value << 8) | (value >> 8));
. unmanaged code . /, errno. . , :
[DllImport("libip4tc.so", SetLastError = true)]
, , :
int errno = Marshal.GetLastWin32Error();
var errPtr = Libiptc4.iptc_strerror(errno);
string errStr = Marshal.PtrToStringAnsi(errPtr);
Linux c net.core (, /). : -, 32/64 , Windows . .
This concludes our journey. I will add that using low-level libraries seems to be a difficult and unpredictable approach in terms of success. Lack of documentation and complex interactions can be frightening. However, sometimes this is the only way to achieve the desired result.