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Linux 2.6 - man page for ei (linux section 3erl)

ei(3erl)			       C Library Functions				 ei(3erl)

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  for-
       mat. 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 encod-
       ing 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 incre-
       mented 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).

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 for-
	      mat. 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-ter-
	      minated, 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. Only
	      upto MAXATOMLEN bytes are encoded. The name should be zero-terminated,  except  for
	      the ei_x_encode_atom_len() function.

       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 struc-
	      ture. 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 recur-
	      sively, 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_long-
	      long().

       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 com-
	      pilers 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 name of the atom is
	      placed at p . There can be at most MAXATOMLEN bytes placed in the buffer.

       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 allo-
	      cates 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 0 on successful decoding, -1 on error, and 1 if the term seems
	      alright, but does not fit in the term structure. If it returns 0, 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) allo-
	      cated 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  func-
	      tion 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)

Ericsson AB			       erl_interface 3.7.3				 ei(3erl)


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