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NAME

       ei - routines for handling the erlang binary term format

DESCRIPTION

       The  library  ei contains macros and functions to encode and decode the erlang binary term
       format.

       With ei, you can convert atoms, lists, numbers and binaries to and from the binary format.
       This  is  useful  when  writing  port programs and drivers. ei uses a given buffer, and no
       dynamic memory (with the exception of ei_decode_fun()), and is often quite fast.

       It also handles C-nodes, C-programs that talks erlang distribution with erlang  nodes  (or
       other  C-nodes)  using  the  erlang  distribution  format.  The  difference between ei and
       erl_interface is that ei uses the binary format directly when sending and receiving terms.
       It  is  also  thread safe, and using threads, one process can handle multiple C-nodes. The
       erl_interface library is built on top of ei, but of legacy reasons, it doesn't  allow  for
       multiple C-nodes. In general, ei is the preferred way of doing C-nodes.

       The decode and encode functions use a buffer an index into the buffer, which points at the
       point where to encode and decode. The index is updated  to  point  right  after  the  term
       encoded/decoded.  No  checking  is  done  whether  the  term fits in the buffer or not. If
       encoding goes outside the buffer, the program may crash.

       All functions takes two parameter, buf is a pointer to the buffer where the binary data is
       /  will  be,  index  is  a  pointer  to  an  index into the buffer. This parameter will be
       incremented with the size of the term decoded / encoded. The data is thus  at  buf[*index]
       when an ei function is called.

       The  encode functions all assumes that the buf and index parameters points to a buffer big
       enough for the data. To get the size of an encoded term, without encoding  it,  pass  NULL
       instead  of a buffer pointer. The index parameter will be incremented, but nothing will be
       encoded. This is the way in ei to "preflight" term encoding.

       There are also encode-functions that uses a dynamic buffer. It is often more convenient to
       use  these to encode data. All encode functions comes in two versions: those starting with
       ei_x, uses a dynamic buffer.

       All functions return 0 if successful, and -1 if not. (For instance, if a term  is  not  of
       the expected type, or the data to decode is not a valid erlang term.)

       Some  of  the  decode-functions needs a preallocated buffer. This buffer must be allocated
       big enough, and for non  compound  types  the  ei_get_type()  function  returns  the  size
       required (note that for strings an extra byte is needed for the 0 string terminator).

DATA TYPES

         erlang_char_encoding:

         typedef enum {
             ERLANG_ASCII = 1,
             ERLANG_LATIN1 = 2,
             ERLANG_UTF8 = 4
         }erlang_char_encoding;

           The  character  encodings  used for atoms. ERLANG_ASCII represents 7-bit ASCII. Latin1
           and UTF8 are different extensions of 7-bit ASCII. All 7-bit ASCII characters are valid
           Latin1  and  UTF8  characters.  ASCII  and Latin1 both represent each character by one
           byte. A UTF8 character can consist of one to four bytes. Note that these constants are
           bit-flags and can be combined with bitwise-or.

EXPORTS

       void ei_set_compat_rel(release_number)

              Types:

                 unsigned release_number;

              By  default,  the  ei  library  is  only  guaranteed  to  be  compatible with other
              Erlang/OTP components from the same release as the ei library itself. For  example,
              ei  from the OTP R10 release is not compatible with an Erlang emulator from the OTP
              R9 release by default.

              A call to ei_set_compat_rel(release_number) sets the ei  library  in  compatibility
              mode  of  release  release_number.  Valid  range  of  release_number is [7, current
              release]. This makes it possible to communicate  with  Erlang/OTP  components  from
              earlier releases.

          Note:
              If  this  function  is called, it may only be called once and must be called before
              any other functions in the ei library is called.

          Warning:
              You may run into trouble if this feature is used carelessly. Always make sure  that
              all  communicating  components are either from the same Erlang/OTP release, or from
              release X and release Y where all components from release Y  are  in  compatibility
              mode of release X.

       int ei_encode_version(char *buf, int *index)
       int ei_x_encode_version(ei_x_buff* x)

              Encodes  a version magic number for the binary format. Must be the first token in a
              binary term.

       int ei_encode_long(char *buf, int *index, long p)
       int ei_x_encode_long(ei_x_buff* x, long p)

              Encodes a long integer in the binary format. Note that if the code is 64  bits  the
              function ei_encode_long() is exactly the same as ei_encode_longlong().

       int ei_encode_ulong(char *buf, int *index, unsigned long p)
       int ei_x_encode_ulong(ei_x_buff* x, unsigned long p)

              Encodes  an unsigned long integer in the binary format. Note that if the code is 64
              bits the function ei_encode_ulong() is exactly the same as ei_encode_ulonglong().

       int ei_encode_longlong(char *buf, int *index, long long p)
       int ei_x_encode_longlong(ei_x_buff* x, long long p)

              Encodes a GCC long long or Visual C++  __int64  (64  bit)  integer  in  the  binary
              format. Note that this function is missing in the VxWorks port.

       int ei_encode_ulonglong(char *buf, int *index, unsigned long long p)
       int ei_x_encode_ulonglong(ei_x_buff* x, unsigned long long p)

              Encodes a GCC unsigned long long or Visual C++ unsigned __int64 (64 bit) integer in
              the binary format. Note that this function is missing in the VxWorks port.

       int ei_encode_bignum(char *buf, int *index, mpz_t obj)
       int ei_x_encode_bignum(ei_x_buff *x, mpz_t obj)

              Encodes a GMP mpz_t integer to binary format. To use this function the  ei  library
              needs to be configured and compiled to use the GMP library.

       int ei_encode_double(char *buf, int *index, double p)
       int ei_x_encode_double(ei_x_buff* x, double p)

              Encodes a double-precision (64 bit) floating point number in the binary format.

       int ei_encode_boolean(char *buf, int *index, int p)
       int ei_x_encode_boolean(ei_x_buff* x, int p)

              Encodes a boolean value, as the atom true if p is not zero or false if p is zero.

       int ei_encode_char(char *buf, int *index, char p)
       int ei_x_encode_char(ei_x_buff* x, char p)

              Encodes  a char (8-bit) as an integer between 0-255 in the binary format. Note that
              for historical reasons the integer argument is of type char.  Your  C  code  should
              consider the given argument to be of type unsigned char even if the C compilers and
              system may define char to be signed.

       int ei_encode_string(char *buf, int *index, const char *p)
       int ei_encode_string_len(char *buf, int *index, const char *p, int len)
       int ei_x_encode_string(ei_x_buff* x, const char *p)
       int ei_x_encode_string_len(ei_x_buff* x, const char* s, int len)

              Encodes a string in the binary format. (A string  in  erlang  is  a  list,  but  is
              encoded  as  a  character  array  in the binary format.) The string should be zero-
              terminated, except for the ei_x_encode_string_len() function.

       int ei_encode_atom(char *buf, int *index, const char *p)
       int ei_encode_atom_len(char *buf, int *index, const char *p, int len)
       int ei_x_encode_atom(ei_x_buff* x, const char *p)
       int ei_x_encode_atom_len(ei_x_buff* x, const char *p, int len)

              Encodes an atom in the binary format. The p parameter is the name of  the  atom  in
              latin1 encoding. Only upto MAXATOMLEN-1 bytes are encoded. The name should be zero-
              terminated, except for the ei_x_encode_atom_len() function.

       int ei_encode_atom_as(char *buf, int *index, const char *p, erlang_char_encoding from_enc,
       erlang_char_encoding to_enc)
       int   ei_encode_atom_len_as(char   *buf,   int   *index,   const   char   *p,   int   len,
       erlang_char_encoding from_enc, erlang_char_encoding to_enc)
       int  ei_x_encode_atom_as(ei_x_buff*  x,  const  char  *p,  erlang_char_encoding  from_enc,
       erlang_char_encoding to_enc)
       int  ei_x_encode_atom_len_as(ei_x_buff*  x,  const  char *p, int len, erlang_char_encoding
       from_enc, erlang_char_encoding to_enc)

              Encodes an atom in the binary format with  character  encoding  to_enc  (latin1  or
              utf8).  The  p  parameter  is the name of the atom with character encoding from_enc
              (ascii, latin1 or utf8). The name must either  be  zero-terminated  or  a  function
              variant  with  a  len  parameter must be used. If to_enc is set to the bitwise-or'd
              combination (ERLANG_LATIN1|ERLANG_UTF8), utf8 encoding is only  used  if  the  atom
              string can not be represented in latin1 encoding.

              The  encoding  will  fail  if  p is not a valid string in encoding from_enc, if the
              string is too long or if it can not be represented with character encoding to_enc.

              These functions were introduced in R16 release of Erlang/OTP as  part  of  a  first
              step  to  support  UTF8 atoms. Atoms encoded with ERLANG_UTF8 can not be decoded by
              earlier releases than R16.

       int ei_encode_binary(char *buf, int *index, const void *p, long len)
       int ei_x_encode_binary(ei_x_buff* x, const void *p, long len)

              Encodes a binary in the binary format. The data is at p, of len bytes length.

       int ei_encode_pid(char *buf, int *index, const erlang_pid *p)
       int ei_x_encode_pid(ei_x_buff* x, const erlang_pid *p)

              Encodes an erlang process identifier, pid, in the binary format.  The  p  parameter
              points  to  an  erlang_pid  structure (which should have been obtained earlier with
              ei_decode_pid()).

       int ei_encode_fun(char *buf, int *index, const erlang_fun *p)
       int ei_x_encode_fun(ei_x_buff* x, const erlang_fun* fun)

              Encodes a fun in the binary  format.  The  p  parameter  points  to  an  erlang_fun
              structure. The erlang_fun is not freed automatically, the free_fun should be called
              if the fun is not needed after encoding.

       int ei_encode_port(char *buf, int *index, const erlang_port *p)
       int ei_x_encode_port(ei_x_buff* x, const erlang_port *p)

              Encodes an erlang  port  in  the  binary  format.  The  p  parameter  points  to  a
              erlang_port   structure   (which   should   have   been   obtained   earlier   with
              ei_decode_port().

       int ei_encode_ref(char *buf, int *index, const erlang_ref *p)
       int ei_x_encode_ref(ei_x_buff* x, const erlang_ref *p)

              Encodes an erlang reference in the binary format.  The  p  parameter  points  to  a
              erlang_ref structure (which should have been obtained earlier with ei_decode_ref().

       int ei_encode_term(char *buf, int *index, void *t)
       int ei_x_encode_term(ei_x_buff* x, void *t)

              This  function encodes an ETERM, as obtained from erl_interface. The t parameter is
              actually an ETERM pointer. This function doesn't free the ETERM.

       int ei_encode_trace(char *buf, int *index, const erlang_trace *p)
       int ei_x_encode_trace(ei_x_buff* x, const erlang_trace *p)

              This function encodes an erlang trace token in the binary format. The  p  parameter
              points  to  a  erlang_trace structure (which should have been obtained earlier with
              ei_decode_trace().

       int ei_encode_tuple_header(char *buf, int *index, int arity)
       int ei_x_encode_tuple_header(ei_x_buff* x, int arity)

              This function encodes a tuple header, with a specified arity. The next arity  terms
              encoded  will  be  the  elements  of  the  tuple.  Tuples  and  lists  are  encoded
              recursively, so that a tuple may contain another tuple or list.

              E.g. to encode the tuple {a, {b, {}}}:

              ei_encode_tuple_header(buf, &i, 2);
              ei_encode_atom(buf, &i, "a");
              ei_encode_tuple_header(buf, &i, 2);
              ei_encode_atom(buf, &i, "b");
              ei_encode_tuple_header(buf, &i, 0);

       int ei_encode_list_header(char *buf, int *index, int arity)
       int ei_x_encode_list_header(ei_x_buff* x, int arity)

              This function encodes a list header, with a specified arity. The next arity+1 terms
              are  the  elements  (actually its arity cons cells) and the tail of the list. Lists
              and tuples are encoded recursively, so that a list  may  contain  another  list  or
              tuple.

              E.g. to encode the list [c, d, [e | f]]:

              ei_encode_list_header(buf, &i, 3);
              ei_encode_atom(buf, &i, "c");
              ei_encode_atom(buf, &i, "d");
              ei_encode_list_header(buf, &i, 1);
              ei_encode_atom(buf, &i, "e");
              ei_encode_atom(buf, &i, "f");
              ei_encode_empty_list(buf, &i);

          Note:
              It  may  seem  that  there is no way to create a list without knowing the number of
              elements in advance. But indeed there is a way. Note that the list [a, b, c] can be
              written as [a | [b | [c]]]. Using this, a list can be written as conses.

              To encode a list, without knowing the arity in advance:

              while (something()) {
                  ei_x_encode_list_header(&x, 1);
                  ei_x_encode_ulong(&x, i); /* just an example */
              }
              ei_x_encode_empty_list(&x);

       int ei_encode_empty_list(char* buf, int* index)
       int ei_x_encode_empty_list(ei_x_buff* x)

              This function encodes an empty list. It's often used at the tail of a list.

       int ei_get_type(const char *buf, const int *index, int *type, int *size)

              This  function  returns  the type in type and size in size of the encoded term. For
              strings and atoms, size is the number of characters not including  the  terminating
              0.  For  binaries,  size  is the number of bytes. For lists and tuples, size is the
              arity of the object. For other types, size is  0.  In  all  cases,  index  is  left
              unchanged.

       int ei_decode_version(const char *buf, int *index, int *version)

              This  function  decodes the version magic number for the erlang binary term format.
              It must be the first token in a binary term.

       int ei_decode_long(const char *buf, int *index, long *p)

              This function decodes a long integer from the binary format. Note that if the  code
              is   64   bits   the   function   ei_decode_long()   is   exactly   the   same   as
              ei_decode_longlong().

       int ei_decode_ulong(const char *buf, int *index, unsigned long *p)

              This function decodes an unsigned long integer from the binary format. Note that if
              the  code  is  64  bits  the  function  ei_decode_ulong()  is  exactly  the same as
              ei_decode_ulonglong().

       int ei_decode_longlong(const char *buf, int *index, long long *p)

              This function decodes a GCC long long or Visual C++ __int64 (64 bit)  integer  from
              the binary format. Note that this function is missing in the VxWorks port.

       int ei_decode_ulonglong(const char *buf, int *index, unsigned long long *p)

              This  function  decodes a GCC unsigned long long or Visual C++ unsigned __int64 (64
              bit) integer from the binary format. Note that this  function  is  missing  in  the
              VxWorks port.

       int ei_decode_bignum(const char *buf, int *index, mpz_t obj)

              This  function  decodes  an integer in the binary format to a GMP mpz_t integer. To
              use this function the ei library needs to be configured and compiled to use the GMP
              library.

       int ei_decode_double(const char *buf, int *index, double *p)

              This  function  decodes an double-precision (64 bit) floating point number from the
              binary format.

       int ei_decode_boolean(const char *buf, int *index, int *p)

              This function decodes a boolean value from the binary format. A boolean is actually
              an atom, true decodes 1 and false decodes 0.

       int ei_decode_char(const char *buf, int *index, char *p)

              This  function decodes a char (8-bit) integer between 0-255 from the binary format.
              Note that for historical reasons the returned integer is of type char. Your C  code
              should  consider  the  returned  value  to  be  of type unsigned char even if the C
              compilers and system may define char to be signed.

       int ei_decode_string(const char *buf, int *index, char *p)

              This function decodes a string from the binary format. A string in erlang is a list
              of integers between 0 and 255. Note that since the string is just a list, sometimes
              lists are encoded as strings by term_to_binary/1, even if it was not intended.

              The string is copied to p, and enough space must be allocated. The returned  string
              is null terminated so you need to add an extra byte to the memory requirement.

       int ei_decode_atom(const char *buf, int *index, char *p)

              This  function  decodes an atom from the binary format. The null terminated name of
              the atom is placed at p. There can be  at  most  MAXATOMLEN  bytes  placed  in  the
              buffer.

       int ei_decode_atom_as(const char *buf, int *index, char *p, int plen, erlang_char_encoding
       want, erlang_char_encoding* was, erlang_char_encoding* result)

              This function decodes an atom from the binary format. The null terminated  name  of
              the atom is placed in buffer at p of length plen bytes.

              The wanted string encoding is specified by  want. The original encoding used in the
              binary format (latin1 or utf8) can be obtained from *was. The  actual  encoding  of
              the  resulting  string  (7-bit ascii, latin1 or utf8) can be obtained from *result.
              Both was and result can be NULL. *result may differ from want if want is a bitwise-
              or'd  combination  like ERLANG_LATIN1|ERLANG_UTF8 or if *result turn out to be pure
              7-bit ascii (compatible with both latin1 and utf8).

              This function fails if the atom is too long for the buffer or  if  it  can  not  be
              represented with encoding want.

              This  function  was introduced in R16 release of Erlang/OTP as part of a first step
              to support UTF8 atoms.

       int ei_decode_binary(const char *buf, int *index, void *p, long *len)

              This function decodes a binary from the binary format. The len parameter is set  to
              the  actual size of the binary. Note that ei_decode_binary() assumes that there are
              enough room for the binary. The size required can be fetched by ei_get_type().

       int ei_decode_fun(const char *buf, int *index, erlang_fun *p)
       void free_fun(erlang_fun* f)

              This function decodes a fun from the binary format. The p parameter should be  NULL
              or  point  to  an  erlang_fun  structure.  This  is  the  only decode function that
              allocates memory; when the erlang_fun is no longer needed, it should be freed  with
              free_fun. (This has to do with the arbitrary size of the environment for a fun.)

       int ei_decode_pid(const char *buf, int *index, erlang_pid *p)

              Decodes a pid, process identifier, from the binary format.

       int ei_decode_port(const char *buf, int *index, erlang_port *p)

              This function decodes a port identifier from the binary format.

       int ei_decode_ref(const char *buf, int *index, erlang_ref *p)

              This function decodes a reference from the binary format.

       int ei_decode_trace(const char *buf, int *index, erlang_trace *p)

              Decodes an erlang trace token from the binary format.

       int ei_decode_tuple_header(const char *buf, int *index, int *arity)

              This  function decodes a tuple header, the number of elements is returned in arity.
              The tuple elements follows in order in the buffer.

       int ei_decode_list_header(const char *buf, int *index, int *arity)

              This function decodes a list header from the binary format. The number of  elements
              is returned in arity. The arity+1 elements follows (the last one is the tail of the
              list, normally an empty list.) If arity is 0, it's an empty list.

              Note that lists are encoded as strings, if they consist entirely of integers in the
              range  0..255.  This  function will not decode such strings, use ei_decode_string()
              instead.

       int ei_decode_ei_term(const char* buf, int* index, ei_term* term)

              This function decodes any term, or at least tries to. If the  term  pointed  at  by
              *index  in  buf fits in the term union, it is decoded, and the appropriate field in
              term->value is set, and *index is incremented by the term size.

              The function returns 1 on successful decoding, -1 on error, and 0 if the term seems
              alright, but does not fit in the term structure. If it returns 1, the index will be
              incremented, and the term contains the decoded term.

              The term structure will contain the arity for a tuple or list, size for  a  binary,
              string  or  atom.  It  will  contains a term if it's any of the following: integer,
              float, atom, pid, port or ref.

       int ei_decode_term(const char *buf, int *index, void *t)

              This function decodes a term from the binary format. The term is return in t  as  a
              ETERM*, so t is actually an ETERM** (see erl_interface(3erl). The term should later
              be deallocated.

              Note that this function is located in the erl_interface library.

       int ei_print_term(FILE* fp, const char* buf, int* index)
       int ei_s_print_term(char** s, const char* buf, int* index)

              This function prints a term, in clear text, to the file given by fp, or the  buffer
              pointed to by s. It tries to resemble the term printing in the erlang shell.

              In  ei_s_print_term(),  the  parameter  s  should  point  to a dynamically (malloc)
              allocated string of BUFSIZ bytes or a NULL pointer. The string may  be  reallocated
              (and  *s  may  be  updated)  by  this  function  if  the result is more than BUFSIZ
              characters. The string returned is zero-terminated.

              The return value is the number of characters written to the file or string,  or  -1
              if  buf[index] doesn't contain a valid term. Unfortunately, I/O errors on fp is not
              checked.

              The argument index is updated, i.e. this  function  can  be  viewed  as  en  decode
              function that decodes a term into a human readable format.

       int ei_x_format(ei_x_buff* x, const char* fmt, ...)
       int ei_x_format_wo_ver(ei_x_buff* x, const char *fmt, ... )

              Format  a term, given as a string, to a buffer. This functions works like a sprintf
              for erlang terms. The fmt contains a format string,  with  arguments  like  ~d,  to
              insert  terms from variables. The following formats are supported (with the C types
              given):

              ~a - an atom, char*
              ~c - a character, char
              ~s - a string, char*
              ~i - an integer, int
              ~l - a long integer, long int
              ~u - a unsigned long integer, unsigned long int
              ~f - a float, float
              ~d - a double float, double float
              ~p - an Erlang PID, erlang_pid*

              For instance, to encode a tuple with some stuff:

              ei_x_format("{~a,~i,~d}", "numbers", 12, 3.14159)
              encodes the tuple {numbers,12,3.14159}

              The ei_x_format_wo_ver() formats into a buffer, without the initial version byte.

       int ei_x_new(ei_x_buff* x)
       int ei_x_new_with_version(ei_x_buff* x)

              This function allocates a new ei_x_buff buffer. The fields of the structure pointed
              to  by  x  parameter  is  filled  in,  and  a  default  buffer  is  allocated.  The
              ei_x_new_with_version() also puts an initial version byte,  that  is  used  in  the
              binary format. (So that ei_x_encode_version() won't be needed.)

       int ei_x_free(ei_x_buff* x)

              This  function frees an ei_x_buff buffer. The memory used by the buffer is returned
              to the OS.

       int ei_x_append(ei_x_buff* x, const ei_x_buff* x2)
       int ei_x_append_buf(ei_x_buff* x, const char* buf, int len)

              These functions appends data at the end of the buffer x.

       int ei_skip_term(const char* buf, int* index)

              This function skips a term in the given buffer, it recursively  skips  elements  of
              lists  and tuples, so that a full term is skipped. This is a way to get the size of
              an erlang term.

              buf is the buffer.

              index is updated to point right after the term in the buffer.

          Note:
              This can be useful when you want to hold arbitrary terms: just skip them  and  copy
              the binary term data to some buffer.

              The function returns 0 on success and -1 on failure.

DEBUG INFORMATION

       Some  tips  on  what to check when the emulator doesn't seem to receive the terms that you
       send.

         * be careful with the version header, use ei_x_new_with_version() when appropriate

         * turn on distribution tracing on the erlang node

         * check the result codes from ei_decode_-calls

SEE ALSO

       erl_interface(3erl)