focal (3) ei.3erl.gz
NAME
ei - Routines for handling the Erlang binary term format.
DESCRIPTION
Note:
The support for VxWorks is deprecated as of OTP 22, and will be removed in OTP 23.
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 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 does not allow for multiple C-nodes. In general, ei is the preferred way of doing 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:
* buf is a pointer to the buffer where the binary data is or will be.
* index is a pointer to an index into the buffer. This parameter is incremented with the size of the
term decoded/encoded.
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.
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
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.
EXPORTS
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)
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)
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)
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_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. This
function is missing in the VxWorks port.
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)
Decodes a process identifier (pid) from the binary format.
int ei_decode_port(const char *buf, int *index, erlang_port *p)
Decodes a port identifier from the binary format.
int ei_decode_ref(const char *buf, int *index, erlang_ref *p)
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_term(const char *buf, int *index, void *t)
Decodes a term from the binary format. The term is return in t as a ETERM*, so t is actually an
ETERM** (see erl_eterm). The term is later to be deallocated.
Note:
This function is deprecated as of OTP 22 and will be removed in OTP 23 together with the old
legacy erl_interface library (functions with prefix erl_).
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)
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. This function is missing in the VxWorks port.
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)
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)
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)
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)
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)
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)
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. 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)
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)
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)
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)
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)
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)
Encodes a GCC long long or Visual C++ __int64 (64-bit) integer in the binary format. This function
is missing in the VxWorks port.
int ei_encode_map_header(char *buf, int *index, int arity)
int ei_x_encode_map_header(ei_x_buff* x, int arity)
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)
Encodes an Erlang process identifier (pid) in the binary format. Parameter p points to an
erlang_pid structure (which should have been obtained earlier with ei_decode_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)
Encodes an Erlang port in the binary format. Parameter p 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. Parameter p points to a erlang_ref structure
(which should have been obtained earlier with ei_decode_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)
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_term(char *buf, int *index, void *t)
int ei_x_encode_term(ei_x_buff* x, void *t)
Encodes an ETERM, as obtained from erl_interface. Parameter t is actually an ETERM pointer. This
function does not free the ETERM.
Note:
These functions are deprecated as of OTP 22 and will be removed in OTP 23 together with the old
legacy erl_interface library (functions with prefix erl_).
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)
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)
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)
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)
Encodes a GCC unsigned long long or Visual C++ unsigned __int64 (64-bit) integer in the binary
format. This function is missing in the VxWorks port.
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_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.
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. However, note the exception below.
If the ei library is used together with the erl_interface library, this function should not be
called directly. It will be called by the erl_init() function which should be used to initialize
the combination of the two libraries instead.
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(release_number)
Types:
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)
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, ... )
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)
Frees an ei_x_buff buffer. The memory used by the buffer is returned to the OS.
int ei_x_new(ei_x_buff* x)
int ei_x_new_with_version(ei_x_buff* x)
Allocates a new ei_x_buff buffer. 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:
* 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_eterm