ei.3erl - Man Page

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.

ei allows you to 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, no dynamic memory (except ei_decode_fun()) and is often quite fast.

ei also handles C-nodes, C-programs that talks Erlang distribution with Erlang nodes (or other C-nodes) using the Erlang distribution format.The ei library is thread safe, and using threads, one process can handle multiple C-nodes.

The decode and encode functions use a buffer and 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 can crash.

All functions take two parameters:

The data is thus at buf[*index] when an ei function is called.

All encode functions assume that the buf and index parameters point to a buffer large enough for the data. To get the size of an encoded term, without encoding it, pass NULL instead of a buffer pointer. Parameter index is incremented, but nothing will be encoded. This is the way in ei to "preflight" term encoding.

There are also encode functions that use 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_ use a dynamic buffer of type ei_x_buff.

All functions return 0 if successful, otherwise -1 (for example, if a term is not of the expected type, or the data to decode is an invalid Erlang term).

Some of the decode functions need a pre-allocated buffer. This buffer must be allocated large enough, and for non-compound types the ei_get_type() function returns the size required (notice that for strings an extra byte is needed for the NULL-terminator).

Data Types

ei_term:

typedef struct {
    char ei_type;
    int arity;
    int size;
    union {
	long i_val;
	double d_val;
	char atom_name[MAXATOMLEN_UTF8];
	erlang_pid pid;
	erlang_port port;
	erlang_ref ref;
    } value;
} ei_term;

Structure written by ei_decode_ei_term(). The ei_type field is the type of the term which equals to what ei_get_type() sets *type to.

ei_x_buff:

A dynamically resized buffer. It is a struct with two fields of interest for the user:

char *buff:

Pointer to the dynamically allocated buffer.

int index:

Offset to the next byte to write which also equals the amount of bytes currently written.

An ei_x_buff is initialized by calling either ei_x_new() or ei_x_new_with_version(). The memory used by an initialized ei_x_buff is released by calling ei_x_free().

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. Latin-1 and UTF-8 are different extensions of 7-bit ASCII. All 7-bit ASCII characters are valid Latin-1 and UTF-8 characters. ASCII and Latin-1 both represent each character by one byte. An UTF-8 character can consist of 1-4 bytes. Notice that these constants are bit-flags and can be combined with bitwise OR.

erlang_fun:

Opaque data type representing an Erlang fun.

erlang_pid:

Opaque data type representing an Erlang process identifier.

erlang_port:

Opaque data type representing an Erlang port identifier.

erlang_ref:

Opaque data type representing an Erlang reference.

erlang_trace:

Opaque data type representing an Erlang sequential trace token.

Exports

int ei_cmp_pids(erlang_pid *a, erlang_pid *b)

Types:

erlang_pid

Compare two process identifiers. The comparison is done the same way as Erlang does.

Returns 0 if a and b are equal. Returns a value less than 0 if a compares as less than b. Returns a value larger than 0 if a compares as larger than b.

int ei_cmp_ports(erlang_port *a, erlang_port *b)

Types:

erlang_port

Compare two port identifiers. The comparison is done the same way as Erlang does.

Returns 0 if a and b are equal. Returns a value less than 0 if a compares as less than b. Returns a value larger than 0 if a compares as larger than b.

int ei_cmp_refs(erlang_ref *a, erlang_ref *b)

Types:

erlang_ref

Compare two references. The comparison is done the same way as Erlang does.

Returns 0 if a and b are equal. Returns a value less than 0 if a compares as less than b. Returns a value larger than 0 if a compares as larger than b.

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

Decodes an atom from the binary format. The NULL-terminated name of the atom is placed at p. At most MAXATOMLEN bytes can be 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)

Types:

erlang_char_encoding

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 (Latin-1 or UTF-8) can be obtained from *was. The encoding of the resulting string (7-bit ASCII, Latin-1, or UTF-8) can be obtained from *result. Both was and result can be NULL. *result can differ from want if want is a bitwise OR'd combination like ERLANG_LATIN1|ERLANG_UTF8 or if *result turns out to be pure 7-bit ASCII (compatible with both Latin-1 and UTF-8).

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

This function was introduced in Erlang/OTP R16 as part of a first step to support UTF-8 atoms.

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

Decodes an integer in the binary format to a GMP mpz_t integer. To use this function, the ei library must be configured and compiled to use the GMP library.

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

Decodes a binary from the binary format. Parameter len is set to the actual size of the binary. Notice that ei_decode_binary() assumes that there is enough room for the binary. The size required can be fetched by ei_get_type().

int ei_decode_bitstring(const char *buf, int *index, const char **pp, unsigned int *bitoffsp, size_t *nbitsp)

Decodes a bit string from the binary format.

pp:

Either NULL or *pp returns a pointer to the first byte of the bit string. The returned bit string is readable as long as the buffer pointed to by buf is readable and not written to.

bitoffsp:

Either NULL or *bitoffsp returns the number of unused bits in the first byte pointed to by *pp. The value of *bitoffsp is between 0 and 7. Unused bits in the first byte are the most significant bits.

nbitsp:

Either NULL or *nbitsp returns the length of the bit string in bits.

Returns 0 if it was a bit string term.

The number of bytes pointed to by *pp, which are part of the bit string, is (*bitoffsp + *nbitsp + 7)/8. If (*bitoffsp + *bitsp)%8 > 0 then only (*bitoffsp + *bitsp)%8 bits of the last byte are used. Unused bits in the last byte are the least significant bits.

The values of unused bits in the first and last byte are undefined and cannot be relied on.

Number of bits may be divisible by 8, which means a binary decodable by ei_decode_binary is also decodable by ei_decode_bitstring.

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

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)

Decodes a char (8-bit) integer between 0-255 from the binary format. For historical reasons the returned integer is of type char. Your C code is to consider the returned value to be of type unsigned char even if the C compilers and system can define char to be signed.

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

Decodes a double-precision (64-bit) floating point number from the binary format.

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

Types:

ei_term

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 1 is returned, the index is incremented, and term contains the decoded term.

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

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

Types:

erlang_fun

Decodes a fun from the binary format. Parameter p is to 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 is to be freed with free_fun. (This has to do with the arbitrary size of the environment for a fun.)

int ei_decode_iodata(const char *buf, int *index, int *size, char *outbuf)

Decodes a term of the type iodata(). The iodata() term will be flattened an written into the buffer pointed to by the outbuf argument. The byte size of the iodata is written into the integer variable pointed to by the size argument. Both size and outbuf can be set to NULL. The integer pointed to by the index argument is updated to refer to the term following after the iodata() term regardless of the the state of the size and the outbuf arguments.

Note that the buffer pointed to by the outbuf argument must be large enough if a non NULL value is passed as outbuf. You typically want to call ei_decode_iodata() twice. First with a non NULL size argument and a NULL outbuf argument in order to determine the size of the buffer needed, and then once again in order to do the actual decoding. Note that the integer pointed to by index will be updated by the call determining the size as well, so you need to reset it before the second call doing the actual decoding.

Returns 0 on success and -1 on failure. Failure might be either due to invalid encoding of the term or due to the term not being of the type iodata(). On failure, the integer pointed to by the index argument will be updated to refer to the sub term where the failure was detected.

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

Decodes a list header from the binary format. The number of elements is returned in arity. The arity+1 elements follow (the last one is the tail of the list, normally an empty list). If arity is 0, it is an empty list.

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

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

Decodes a long integer from the binary format. If the code is 64 bits, the function ei_decode_long() is the same as ei_decode_longlong().

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

Decodes a GCC long long or Visual C++ __int64 (64-bit) integer from the binary format.

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

Decodes a map header from the binary format. The number of key-value pairs is returned in *arity. Keys and values follow in this order: K1, V1, K2, V2, ..., Kn, Vn. This makes a total of arity*2 terms. If arity is zero, it is an empty map. A correctly encoded map does not have duplicate keys.

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

Types:

erlang_pid

Decodes a process identifier (pid) from the binary format.

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

Types:

erlang_port

Decodes a port identifier from the binary format.

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

Types:

erlang_ref

Decodes a reference from the binary format.

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

Decodes a string from the binary format. A string in Erlang is a list of integers between 0 and 255. Notice that as 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 must add an extra byte to the memory requirement.

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

Types:

erlang_trace

Decodes an Erlang trace token from the binary format.

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

Decodes a tuple header, the number of elements is returned in arity. The tuple elements follow in order in the buffer.

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

Decodes an unsigned long integer from the binary format. If the code is 64 bits, the function ei_decode_ulong() is the same as ei_decode_ulonglong().

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

Decodes a GCC unsigned long long or Visual C++ unsigned __int64 (64-bit) integer from the binary format.

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

Decodes the version magic number for the Erlang binary term format. It must be the first token in a binary term.

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)

Types:

ei_x_buff

Encodes an atom in the binary format. Parameter p is the name of the atom in Latin-1 encoding. Only up to MAXATOMLEN-1 bytes are encoded. The name is to be NULL-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)

Types:

ei_x_buff
erlang_char_encoding

Encodes an atom in the binary format. Parameter p is the name of the atom with character encoding from_enc (ASCII, Latin-1, or UTF-8). The name must either be NULL-terminated or a function variant with a len parameter must be used.

The encoding fails if p is not a valid string in encoding from_enc.

Argument to_enc is ignored. As from Erlang/OTP 20 the encoding is always done in UTF-8 which is readable by nodes as old as Erlang/OTP R16.

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

Types:

ei_x_buff

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

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)

Types:

ei_x_buff

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

int ei_encode_bitstring(char *buf, int *index, const char *p, size_t bitoffs, size_t nbits)
int ei_x_encode_bitstring(ei_x_buff* x, const char *p, size_t bitoffs, size_t nbits)

Types:

ei_x_buff

Encodes a bit string in the binary format.

The data is at p. The length of the bit string is nbits bits. The first bitoffs bits of the data at p are unused. The first byte which is part of the bit string is p[bitoffs/8]. The bitoffs%8 most significant bits of the first byte p[bitoffs/8] are unused.

The number of bytes which is part of the bit string is (bitoffs + nbits + 7)/8. If (bitoffs + nbits)%8 > 0 then only (bitoffs + nbits)%8 bits of the last byte are used. Unused bits in the last byte are the least significant bits.

The values of unused bits are disregarded and does not need to be cleared.

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

Types:

ei_x_buff

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)

Types:

ei_x_buff

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

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

Types:

ei_x_buff

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

Returns -1 if the floating point number is not finite.

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

Types:

ei_x_buff

Encodes an empty list. It is often used at the tail of a list.

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)

Types:

ei_x_buff
erlang_fun

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

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

Types:

ei_x_buff

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 can contain another list or tuple.

For example, 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. Notice 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_long(char *buf, int *index, long p)
int ei_x_encode_long(ei_x_buff* x, long p)

Types:

ei_x_buff

Encodes a long integer in the binary format. If the code is 64 bits, the function ei_encode_long() is the same as ei_encode_longlong().

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

Types:

ei_x_buff

Encodes a GCC long long or Visual C++ __int64 (64-bit) integer in the binary format.

int ei_encode_map_header(char *buf, int *index, int arity)
int ei_x_encode_map_header(ei_x_buff* x, int arity)

Types:

ei_x_buff

Encodes a map header, with a specified arity. The next arity*2 terms encoded will be the keys and values of the map encoded in the following order: K1, V1, K2, V2, ..., Kn, Vn.

For example, to encode the map #{a => "Apple", b => "Banana"}:

ei_x_encode_map_header(&x, 2);
ei_x_encode_atom(&x, "a");
ei_x_encode_string(&x, "Apple");
ei_x_encode_atom(&x, "b");
ei_x_encode_string(&x, "Banana");

A correctly encoded map cannot have duplicate keys.

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)

Types:

ei_x_buff
erlang_pid

Encodes an Erlang process identifier (pid) in the binary format. Parameter p points to an erlang_pid structure which should either have been obtained earlier with ei_decode_pid(), ei_self() or created by ei_make_pid().

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)

Types:

ei_x_buff
erlang_port

Encodes an Erlang port in the binary format. Parameter p points to an 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)

Types:

ei_x_buff
erlang_ref

Encodes an Erlang reference in the binary format. Parameter p points to an erlang_ref structure which either should have been obtained earlier with ei_decode_ref(), or created by ei_make_ref().

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)

Types:

ei_x_buff

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 is to be NULL-terminated, except for the ei_x_encode_string_len() function.

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)

Types:

ei_x_buff
erlang_trace

Encodes an Erlang trace token in the binary format. Parameter p 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)

Types:

ei_x_buff

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 can contain another tuple or list.

For example, 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_ulong(char *buf, int *index, unsigned long p)
int ei_x_encode_ulong(ei_x_buff* x, unsigned long p)

Types:

ei_x_buff

Encodes an unsigned long integer in the binary format. If the code is 64 bits, the function ei_encode_ulong() is the same as ei_encode_ulonglong().

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)

Types:

ei_x_buff

Encodes a GCC unsigned long long or Visual C++ unsigned __int64 (64-bit) integer in the binary format.

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

Types:

ei_x_buff

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

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

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 NULL. For binaries and bitstrings, *size is the number of bytes. For lists, tuples and maps, *size is the arity of the object. For other types, *size is 0. In all cases, index is left unchanged.

Currently *type is one of:

ERL_ATOM_EXT:

Decode using either ei_decode_atom(), ei_decode_atom_as(), or ei_decode_boolean().

ERL_BINARY_EXT:

Decode using either ei_decode_binary(), ei_decode_bitstring(), or ei_decode_iodata().

ERL_BIT_BINARY_EXT:

Decode using ei_decode_bitstring().

ERL_FLOAT_EXT:

Decode using ei_decode_double().

ERL_NEW_FUN_EXT

ERL_FUN_EXT
ERL_EXPORT_EXT: Decode using ei_decode_fun().

ERL_SMALL_INTEGER_EXT

ERL_INTEGER_EXT
ERL_SMALL_BIG_EXT
ERL_LARGE_BIG_EXT: Decode using either ei_decode_char(), ei_decode_long(), ei_decode_longlong(), ei_decode_ulong(), ei_decode_ulonglong(), or ei_decode_bignum().

ERL_LIST_EXT

ERL_NIL_EXT: Decode using either ei_decode_list_header(), or ei_decode_iodata().

ERL_STRING_EXT:

Decode using either ei_decode_string(), or ei_decode_iodata().

ERL_MAP_EXT:

Decode using ei_decode_map_header().

ERL_PID_EXT:

Decode using ei_decode_pid().

ERL_PORT_EXT:

Decode using ei_decode_port().

ERL_NEW_REFERENCE_EXT:

Decode using ei_decode_ref().

ERL_SMALL_TUPLE_EXT

ERL_LARGE_TUPLE_EXT: Decode using ei_decode_tuple_header().

Instead of decoding a term you can also skipped past it if you are not interested in the data by usage of ei_skip_term().

int ei_init(void)

Initialize the ei library. This function should be called once (and only once) before calling any other functionality in the ei library.

On success zero is returned. On failure a posix error code is returned.

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

Prints a term, in clear text, to the file specified 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(), parameter s is to point to a dynamically (malloc) allocated string of BUFSIZ bytes or a NULL pointer. The string can be reallocated (and *s can be updated) by this function if the result is more than BUFSIZ characters. The string returned is NULL-terminated.

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

Argument index is updated, that is, this function can be viewed as a decode function that decodes a term into a human-readable format.

void ei_set_compat_rel(unsigned release_number)

In general, the ei library is guaranteed to be compatible with other Erlang/OTP components that are 2 major releases older or newer than the ei library itself.

Sometimes an exception to the above rule has to be made to make new features (or even bug fixes) possible. A call to ei_set_compat_rel(release_number) sets the ei library in compatibility mode of OTP release release_number.

The only useful value for release_number is currently 21. This will only be useful and have an effect if bit strings or export funs are received from a connected node. Before OTP 22, bit strings and export funs were not supported by ei. They were instead encoded using an undocumented fallback tuple format when sent from the emulator to ei:

Bit string:

The term <<42, 1:1>> was encoded as {<<42, 128>>, 1}. The first element of the tuple is a binary and the second element denotes how many bits of the last bytes are part of the bit string. In this example only the most significant bit of the last byte (128) is part of the bit string.

Export fun:

The term fun lists:map/2 was encoded as {lists,map}. A tuple with the module, function and a missing arity.

If ei_set_compat_rel(21) is not called then a connected emulator will send bit strings and export funs correctly encoded. The functions ei_decode_bitstring and ei_decode_fun has to be used to decode such terms. Calling ei_set_compat_rel(21) should only be done as a workaround to keep an old implementation alive, which expects to receive the undocumented tuple formats for bit strings and/or export funs.

Note:

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

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

Skips a term in the specified buffer; 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: skip them and copy the binary term data to some buffer.

Returns 0 on success, otherwise -1.

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)

Types:

ei_x_buff

Appends data at the end of buffer x.

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

Types:

ei_x_buff
erlang_pid

Formats a term, given as a string, to a buffer. Works like a sprintf for Erlang terms. 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 example, to encode a tuple with some stuff:

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

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

int ei_x_free(ei_x_buff* x)

Types:

ei_x_buff

Deallocates the dynamically allocated content of the buffer referred by x. After deallocation, the buff field is set to NULL.

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

Types:

ei_x_buff

Initialize the dynamically realizable buffer referred to by x. The fields of the structure pointed to by parameter x is filled in, and a default buffer is allocated. ei_x_new_with_version() also puts an initial version byte, which is used in the binary format (so that ei_x_encode_version() will not be needed.)

Debug Information

Some tips on what to check when the emulator does not seem to receive the terms that you send:

Info

erl_interface 4.0 Ericsson AB C Library Functions