text: 
  #include <assert.h>
  #include <ctype.h>
  #include <limits.h>
  #include <string.h>
  
  
  #define LUA_COMPAT_ALL
  
  #include "lua.h"
  #include "lauxlib.h"
  
  /*
  ** {======================================================
  ** Library for packing/unpacking structures.
  ** $Id: struct.c,v 1.2 2008/04/18 20:06:01 roberto Exp $
  ** =======================================================
  */
  /*
  ** Valid formats:
  ** > - big endian
  ** < - little endian
  ** ![num] - alignment
  ** x - pading
  ** b/B - signed/unsigned byte
  ** h/H - signed/unsigned short
  ** l/L - signed/unsigned long
  ** i/In - signed/unsigned integer with size `n' (default is size of int)
  ** cn - sequence of `n' chars (from/to a string); when packing, n==0 means
          the whole string; when unpacking, n==0 means use the previous
          read number as the string length
  ** p - pointer: unpacks to a light_userdata, packs a light_userdata or userdata.
                  not affected by endiannes setting.
  ** s - zero-terminated string
  ** f - float
  ** d - double
  ** ' ' - ignored
  */
  
  
  /* is 'x' a power of 2? */
  #define isp2(x)         ((x) > 0 && ((x) & ((x) - 1)) == 0)
  
  /* dummy structure to get alignment requirements */
  struct cD {
    char c;
    double d;
  };
  
  
  #define PADDING         (sizeof(struct cD) - sizeof(double))
  #define MAXALIGN        (PADDING > sizeof(int) ? PADDING : sizeof(int))
  
  
  /* endian options */
  #define BIG     0
  #define LITTLE  1
  
  
  static union {
    int dummy;
    char endian;
  } const native = {1};
  
  
  typedef struct Header {
    int endian;
    int align;
  } Header;
  
  
  static size_t getnum (const char **fmt, size_t df) {
    if (!isdigit(**fmt))  /* no number? */
      return df;  /* return default value */
    else {
      size_t a = 0;
      do {
        a = a*10 + *((*fmt)++) - '0';
      } while (isdigit(**fmt));
      return a;
    }
  }
  
  
  #define defaultoptions(h)       ((h)->endian = native.endian, (h)->align = 1)
  
  
  
  static size_t optsize (lua_State *L, char opt, const char **fmt) {
    switch (opt) {
      case 'B': case 'b': return sizeof(char);
      case 'H': case 'h': return sizeof(short);
      case 'L': case 'l': return sizeof(long);
      case 'f':  return sizeof(float);
      case 'd':  return sizeof(double);
      case 'x': return 1;
      case 'c': return getnum(fmt, 1);
      case 's': case ' ': case '<': case '>': case '!': return 0;
      case 'i': case 'I': {
        int sz = getnum(fmt, sizeof(int));
        if (!isp2(sz))
          luaL_error(L, "integral size %d is not a power of 2", sz);
        return sz;
      }
      case 'p': return sizeof(void*);
      default: {
        const char *msg = lua_pushfstring(L, "invalid format option [%c]", opt);
        return luaL_argerror(L, 1, msg);
      }
    }
  }
  
  
  static int gettoalign (size_t len, Header *h, int opt, size_t size) {
    if (size == 0 || opt == 'c') return 0;
    if (size > (size_t)h->align) size = h->align;  /* respect max. alignment */
    return  (size - (len & (size - 1))) & (size - 1);
  }
  
  
  static void commoncases (lua_State *L, int opt, const char **fmt, Header *h) {
    switch (opt) {
      case  ' ': return;  /* ignore white spaces */
      case '>': h->endian = BIG; return;
      case '<': h->endian = LITTLE; return;
      case '!': {
        int a = getnum(fmt, MAXALIGN);
        if (!isp2(a))
          luaL_error(L, "alignment %d is not a power of 2", a);
        h->align = a;
        return;
      }
      default: assert(0);
    }
  }
  
  
  static void putinteger (lua_State *L, luaL_Buffer *b, int arg, int endian,
                          int size) {
    lua_Number n = luaL_checknumber(L, arg);
    unsigned long value;
    if (n < (lua_Number)LONG_MAX)
      value = (long)n;
    else
      value = (unsigned long)n;
    if (endian == LITTLE) {
      int i;
      for (i = 0; i < size; i++)
        luaL_addchar(b, (value >> 8*i) & 0xff);
    }
    else {
      int i;
      for (i = size - 1; i >= 0; i--)
        luaL_addchar(b, (value >> 8*i) & 0xff);
    }
  }
  
  
  static void correctbytes (char *b, int size, int endian) {
    if (endian != native.endian) {
      int i = 0;
      while (i < --size) {
        char temp = b[i];
        b[i++] = b[size];
        b[size] = temp;
      }
    }
  }
  
  
  static int b_pack (lua_State *L) {
    luaL_Buffer b;
    const char *fmt = luaL_checkstring(L, 1);
    Header h;
    int arg = 2;
    size_t totalsize = 0;
    defaultoptions(&h);
    lua_pushnil(L);  /* mark to separate arguments from string buffer */
    luaL_buffinit(L, &b);
    while (*fmt != '\0') {
      int opt = *fmt++;
      size_t size = optsize(L, opt, &fmt);
      int toalign = gettoalign(totalsize, &h, opt, size);
      totalsize += toalign;
      while (toalign-- > 0) luaL_addchar(&b, '\0');
      switch (opt) {
        case 'b': case 'B': case 'h': case 'H':
        case 'l': case 'L': case 'i': case 'I': {  /* integer types */
          putinteger(L, &b, arg++, h.endian, size);
          break;
        }
        case 'x': {
          luaL_addchar(&b, '\0');
          break;
        }
        case 'f': {
          float f = (float)luaL_checknumber(L, arg++);
          correctbytes((char *)&f, size, h.endian);
          luaL_addlstring(&b, (char *)&f, size);
          break;
        }
        case 'd': {
          double d = luaL_checknumber(L, arg++);
          correctbytes((char *)&d, size, h.endian);
          luaL_addlstring(&b, (char *)&d, size);
          break;
        }
        case 'c': case 's': {
          size_t l;
          const char *s = luaL_checklstring(L, arg++, &l);
          if (size == 0) size = l;
          luaL_argcheck(L, l >= (size_t)size, arg, "string too short");
          luaL_addlstring(&b, s, size);
          if (opt == 's') {
            luaL_addchar(&b, '\0');  /* add zero at the end */
            size++;
          }
          break;
        }
        case 'p': {
          void *p;
          luaL_argcheck(L, lua_isuserdata(L, arg) || lua_isnil(L, arg), arg, "userdata or nil required");
          p = alien_touserdata(L, arg++);
          luaL_addlstring(&b, (char*)&p, size);
          break;
        }
        default: commoncases(L, opt, &fmt, &h);
      }
      totalsize += size;
    }
    luaL_pushresult(&b);
    return 1;
  }
  
  
  static lua_Number getinteger (const char *buff, int endian,
                          int issigned, int size) {
    unsigned long l = 0;
    if (endian == BIG) {
      int i;
      for (i = 0; i < size; i++)
        l |= (unsigned long)(unsigned char)buff[size - i - 1] << (i*8);
    }
    else {
      int i;
      for (i = 0; i < size; i++)
        l |= (unsigned long)(unsigned char)buff[i] << (i*8);
    }
    if (!issigned)
      return (lua_Number)l;
    else {  /* signed format */
      unsigned long mask = ~(0UL) << (size*8 - 1);
      if (l & mask)  /* negative value? */
        l |= mask;  /* signal extension */
      return (lua_Number)(long)l;
    }
  }
  
  static int b_size (lua_State *L) {
    Header h;
    const char *fmt = luaL_checkstring(L, 1);
    size_t totalsize = 0;
    defaultoptions(&h);
    while (*fmt) {
      int opt = *fmt++;
      switch(opt) {
        case  ' ': break;  /* ignore white spaces */
        case '>': h.endian = BIG; break;
        case '<': h.endian = LITTLE; break;
        case '!': {
          int a = getnum(&fmt, MAXALIGN);
          if (!isp2(a))
            luaL_error(L, "alignment %d is not a power of 2", a);
          h.align = a;
          break;
        }
        default: {
          size_t size = optsize(L, opt, &fmt);
          totalsize += gettoalign(totalsize, &h, opt, size);
          if (size == 0)
            luaL_error(L, "options `c0' - `s' have undefined sizes");
          totalsize += size;
        }
      }
    }
    lua_pushnumber(L, totalsize);
    return 1;
  }
  
  static int b_offset (lua_State *L) {
    Header h;
    const char *fmt = luaL_checkstring(L, 1);
    int offset = luaL_optint(L, 2, 1);
    int optn = 1;
    size_t totalsize = 0;
    defaultoptions(&h);
    while (*fmt && optn < offset) {
      int opt = *fmt++;
      switch(opt) {
        case  ' ': break;  /* ignore white spaces */
        case '>': h.endian = BIG; break;
        case '<': h.endian = LITTLE; break;
        case '!': {
          int a = getnum(&fmt, MAXALIGN);
          if (!isp2(a))
            luaL_error(L, "alignment %d is not a power of 2", a);
          h.align = a;
          break;
        }
        default: {
          size_t size = optsize(L, opt, &fmt);
          totalsize += gettoalign(totalsize, &h, opt, size);
          if (size == 0)
            luaL_error(L, "options `c0' - `s' have undefined sizes");
          totalsize += size;
          optn += 1;
        }
      }
    }
    lua_pushnumber(L, totalsize + 1);
    return 1;
  }
  
  
  static int b_unpack (lua_State *L) {
    Header h;
    const char *fmt = luaL_checkstring(L, 1);
    size_t ld, pos;
    const char *data;
    if(lua_isuserdata(L, 2)) {
      data = alien_touserdata(L, 2);
      ld = (size_t)luaL_checkinteger(L, 3);
      pos = luaL_optint(L, 4, 1) - 1;
    } else {
      data = luaL_checklstring(L, 2, &ld);
      pos = luaL_optint(L, 3, 1) - 1;
    }
    defaultoptions(&h);
    lua_settop(L, 2);
    while (*fmt) {
      int opt = *fmt++;
      size_t size = optsize(L, opt, &fmt);
      pos += gettoalign(pos, &h, opt, size);
      luaL_argcheck(L, pos+size <= ld, 2, "data string too short");
      switch (opt) {
        case 'b': case 'B': case 'h': case 'H':
        case 'l': case 'L': case 'i':  case 'I': {  /* integer types */
          int issigned = islower(opt);
          lua_Number res = getinteger(data+pos, h.endian, issigned, size);
          lua_pushnumber(L, res);
          break;
        }
        case 'x': {
          break;
        }
        case 'f': {
          float f;
          memcpy(&f, data+pos, size);
          correctbytes((char *)&f, sizeof(f), h.endian);
          lua_pushnumber(L, f);
          break;
        }
        case 'd': {
          double d;
          memcpy(&d, data+pos, size);
          correctbytes((char *)&d, sizeof(d), h.endian);
          lua_pushnumber(L, d);
          break;
        }
        case 'c': {
          if (size == 0) {
            if (!lua_isnumber(L, -1))
              luaL_error(L, "format `c0' needs a previous size");
            size = lua_tonumber(L, -1);
            lua_pop(L, 1);
            luaL_argcheck(L, pos+size <= ld, 2, "data string too short");
          }
          lua_pushlstring(L, data+pos, size);
          break;
        }
        case 's': {
          const char *e = (const char *)memchr(data+pos, '\0', ld - pos);
          if (e == NULL)
            luaL_error(L, "unfinished string in data");
          size = (e - (data+pos)) + 1;
          lua_pushlstring(L, data+pos, size - 1);
          break;
        }
        case 'p': {
          void* p;
          memcpy(&p, data+pos, size);
          lua_pushlightuserdata(L, p);
          break;
        }
        default: commoncases(L, opt, &fmt, &h);
      }
      pos += size;
    }
    lua_pushinteger(L, pos + 1);
    return lua_gettop(L) - 2;
  }
  
  /* }====================================================== */
  