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Erlang Interface
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Version 3.7.13

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# ei

### C LIBRARY

ei

### LIBRARY SUMMARY

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(3). 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(3)