An Erlang node represented by an atom. This type is used to show that a function will never return a value; that is it will always throw an exception. A binary that contains some data. A bitstring that contains some data. A list ContentType that contains some items. A string that contains some characters.

An Erlang number. An Erlang process identifier. An Erlang port identifier. An Erlang reference. A timeout value that can be passed to a receive expression. An Erlang tuple. A term of type iovec , structured according to the Erlang external term format.

A list of binaries. Time unit expressed in parts per second. Symbolic representation of the time unit represented by the integer 1. Symbolic representation of the time unit represented by the integer The native time unit is determined at runtime system start, and remains the same until the runtime system terminates.

If a runtime system is stopped and then started again even on the same machine , the native time unit of the new runtime system instance can differ from the native time unit of the old runtime system instance. The result equals the number of whole native time units per second.

If the number of native time units per second does not add up to a whole number, the result is rounded downwards. The value of the native time unit gives you more or less no information about the quality of time values.

It sets a limit for the resolution and for the precision of time values, but it gives no information about the accuracy of time values. The resolution of the native time unit and the resolution of time values can differ significantly. Symbolic representation of the performance counter time unit used by the Erlang runtime system.

That is, it can differ between runtime restarts. Same as second. Same as millisecond. Same as microsecond. Same as nanosecond. An opaque handle identifying a NIF resource object. Process priority level. Process max heap size configuration. Process message queue data configuration. An Erlang stacktrace as described by Errors and Error Handling section in the Erlang Reference Manual.

The destination for a send operation, can be a remote or local process identifier, a local port, a reference denoting a process alias, a locally registered name, or a tuple {RegName, Node} for a registered name at another node. Returns an integer or float that is the arithmetical absolute value of Float or Int , for example:. Continues computing the adler32 checksum by combining the previous checksum, OldAdler , with the checksum of Data.

Combines two previously computed adler32 checksums. This computation requires the size of the data object for the second checksum to be known. Create an alias which can be used when sending messages to the process that created the alias. When the alias has been deactivated, messages sent using the alias will be dropped. The alias will be automatically deactivated when a reply message sent via the alias is received.

Note that both the server and the client in this example must be executing on at least OTP 24 systems in order for this to work. For more information on process aliases see the Process Aliases section of the Erlang Reference Manual. Returns a new tuple that has one element more than Tuple1 , and contains the elements in Tuple1 followed by Term as the last element. If the number of elements in the arguments are known at compile time, the call is better written as Fun Arg1, Arg2, Earlier, Fun could also be specified as {Module, Function} , equivalent to apply Module, Function, Args.

This use is deprecated and will stop working in a future release. Returns the result of applying Function in Module to Args. The applied function must be exported from Module. The arity of the function is the length of Args. If the number of arguments are known at compile time, the call is better written as Module:Function Arg1, Arg2, Returns a binary corresponding to the text representation of Atom. If Encoding is latin1 , one byte exists for each character in the text representation.

If Encoding is utf8 or unicode , the characters are encoded using UTF-8 where characters may require multiple bytes. Returns a list of unicode code points corresponding to the text representation of Atom , for example:. See unicode 3 for how to convert the resulting list to different formats. Failure: badarg if PosLen in any way references outside the binary. For details about the PosLen semantics, see binary 3. Returns the atom whose text representation is Binary.

If Encoding is utf8 or unicode , the binary must contain valid UTF-8 sequences. The number of characters that are permitted in an atom name is limited. The default limits can be found in the efficiency guide section Advanced. There is configurable limit on how many atoms that can exist and atoms are not garbage collected. The default limits can be found in efficiency guide section Advanced.

The Erlang system has a configurable limit for the total number of atoms that can exist, and atoms are not garbage collected. This function is thus the appropriate option when the input binary comes from an untrusted source.

See the next note for an example of when an atom exists in the source code for an Erlang module but not in the compiled version of the same module. Note that the compiler may optimize away atoms. Returns the float whose text representation is Binary , for example:.

The float string format is the same as the format for Erlang float literals except for that underscores are not permitted. Failure: badarg if Binary contains a bad representation of a float. Returns an integer whose text representation is Binary , for example:. Failure: badarg if Binary contains a bad representation of an integer. Returns an integer whose text representation in base Base is Binary , for example:.

Returns a list of integers corresponding to the bytes of Binary. The positions in the binary are numbered starting from 1. The one-based indexing for binaries used by this function is deprecated. All functions in module binary consistently use zero-based indexing. Returns an Erlang term that is the result of decoding binary object Binary , which must be encoded according to the Erlang external term format. When decoding binaries from untrusted sources, the untrusted source may submit data in a way to create resources, such as atoms and remote references, that cannot be garbage collected and lead to Denial of Service attack.

When enabled, it prevents decoding data that can be used to attack the Erlang runtime. In the event of receiving unsafe data, decoding fails with a badarg error.

This prevents creation of new atoms directly, creation of new atoms indirectly as they are embedded in certain structures, such as process identifiers, refs, and funs , and creation of new external function references. None of those resources are garbage collected, so unchecked creation of them can exhaust available memory. The safe option ensures the data is safely processed by the Erlang runtime but it does not guarantee the data is safe to your application. You must always validate data from untrusted sources.

If the binary is stored or transits through untrusted sources, you should also consider cryptographically signing it. Changes the return value to {Term, Used} where Used is the number of bytes actually read from Binary. Failure: badarg if safe is specified and unsafe data is decoded. Returns an integer that is the size in bits of Bitstring , for example:.

Returns a list of integers corresponding to the bytes of Bitstring. If the number of bits in the binary is not divisible by 8, the last element of the list is a bitstring containing the remaining bits. This implementation-dependent function increments the reduction counter for the calling process. In the Beam emulator, the reduction counter is normally incremented by one for each function and BIF call.

This BIF can be removed in a future version of the Beam machine without prior warning. It is unlikely to be implemented in other Erlang implementations. Returns an integer that is the number of bytes needed to contain Bitstring. That is, if the number of bits in Bitstring is not divisible by 8, the resulting number of bytes is rounded up.

Cancels a timer. TimerRef identifies the timer, and was returned by the BIF that created the timer. Asynchronous request for cancellation. Async defaults to false , which causes the cancellation to be performed synchronously. When Async is set to true , the cancel operation is performed asynchronously. Requests information about the Result of the cancellation. Info defaults to true , which means the Result is given. When Info is set to false , no information about the result of the cancellation is given.

otherwise ok is returned. If Result is an integer, it represents the time in milliseconds left until the canceled timer would have expired. If Result is false , a timer corresponding to TimerRef could not be found.

This can be either because the timer had expired, already had been canceled, or because TimerRef never corresponded to a timer. Even if the timer had expired, it does not tell you if the time-out message has arrived at its destination yet. The timer service that manages the timer can be co-located with another scheduler than the scheduler that the calling process is executing on.

If so, communication with the timer service takes much longer time than if it is located locally. If the calling process is in critical path, and can do other things while waiting for the result of this operation, or is not interested in the result of the operation, you want to use option {async, true}.

If using option {async, false} , the calling process blocks until the operation has been performed. Returns the smallest integer not less than Number.

For example:. Returns true if Module has old code , otherwise false. See also code 3. Checks if the node local process identified by Pid executes old code for Module. Determines if garbage collection is allowed when performing the operation.

If Pid equals self , and no async option has been passed, the operation is performed at once. Otherwise a request for the operation is sent to the process identified by Pid , and is handled when appropriate. If no async option has been passed, the caller blocks until CheckResult is available and can be returned. CheckResult informs about the result of the request as follows:. The process identified by Pid executes old code for Module. That is, the current call of the process executes old code for this module, or the process has references to old code for this module, or the process contains funs that references old code for this module.

The process identified by Pid does not execute old code for Module. Up until ERTS version 8. That is, direct references e. return addresses on the process stack , indirect references fun s in process context , and references to literals in the code. As of ERTS version 9. Indirect references via fun s will be ignored.

If such fun s exist and are used after a purge of the old code, an exception will be raised upon usage same as the case when the fun is received by the process after the purge. Literals will be taken care of copied at a later stage. This behavior can as of ERTS version 8. Converts the Time value of time unit FromUnit to the corresponding ConvertedTime value of time unit ToUnit. The result is rounded using the floor function. You can lose accuracy and precision when converting between time units.

To minimize such loss, collect all data at native time unit and do the conversion on the end result. Computes and returns the crc32 IEEE Continues computing the crc32 checksum by combining the previous checksum, OldCrc , with the checksum of Data. Combines two previously computed crc32 checksums. The time zone and Daylight Saving Time correction depend on the underlying OS.

The return value is based on the OS System Time. Decodes the binary Bin according to the packet protocol specified by Type. Similar to the packet handling done by sockets with option {packet,Type}. If an entire packet is contained in Bin , it is returned together with the remainder of the binary as {ok,Packet,Rest}.

If Bin does not contain the entire packet, {more,Length} is returned. Length is either the expected total size of the packet, or undefined if the expected packet size is unknown. If the packet does not conform to the protocol format, {error,Reason} is returned. Packets consist of a header specifying the number of bytes in the packet, followed by that number of bytes. The length of the header can be one, two, or four bytes; the order of the bytes is big-endian. The header is stripped off when the packet is returned.

A packet is a line-terminated by a delimiter byte, default is the latin-1 newline character. The Hypertext Transfer Protocol. The packets are returned with the format according to HttpPacket described earlier. A packet is either a request, a response, a header, or an end of header mark. Invalid lines are returned as HttpError.

Recognized request methods and header fields are returned as atoms. Others are returned as strings. Strings of unrecognized header fields are formatted with only capital letters first and after hyphen characters, for example, "Sec-Websocket-Key". Header field names are also returned in UnmodifiedField as strings, without any conversion or formatting. The protocol type http is only to be used for the first line when an HttpRequest or an HttpResponse is expected.

Sets the maximum allowed size of the packet body. If the packet header indicates that the length of the packet is longer than the maximum allowed length, the packet is considered invalid.

Defaults to 0, which means no size limit. For packet type line , lines longer than the indicated length are truncated. This use is only intended for backward compatibility. For packet type line , sets the delimiting byte. Returns a new tuple with element at Index removed from tuple Tuple1 , for example:. Makes the current code for Module become old code and deletes all references for this module from the export table.

Returns undefined if the module does not exist, otherwise true. This BIF is intended for the code server see code 3 and is not to be used elsewhere. Failure: badarg if there already is an old version of Module. If the monitoring is already turned off, nothing happens. It is therefore usually advisable to remove such a 'DOWN' message from the message queue after monitoring has been stopped. demonitor MonitorRef, [flush] can be used instead of demonitor MonitorRef if this cleanup is wanted.

This had one undesirable effect. You could never know when you were guaranteed not to receive a DOWN message because of the monitor. The current behavior can be viewed as two combined operations: asynchronously send a "demonitor signal" to the monitored entity and ignore any future results of the monitor. Failure: It is an error if MonitorRef refers to a monitoring started by another process.

Not all such cases are cheap to check. If checking is cheap, the call fails with badarg , for example if MonitorRef is a remote reference. The returned value is true unless info is part of OptionList. demonitor MonitorRef, [] is equivalent to demonitor MonitorRef.

Calling demonitor MonitorRef, [flush] is equivalent to the following, but more efficient:. The monitor was found and removed. In this case, no 'DOWN' message corresponding to this monitor has been delivered and will not be delivered.

The monitor was not found and could not be removed. This probably because someone already has placed a 'DOWN' message corresponding to this monitor in the caller message queue. If option info is combined with option flush , false is returned if a flush was needed, otherwise true. Forces the disconnection of a node. This appears to the node Node as if the local node has crashed.

This BIF is mainly used in the Erlang network authentication protocols. Returns true if disconnection succeeds, otherwise false. If the local node is not alive, ignored is returned. This function may return before nodedown messages have been delivered.

This BIF is intended for debugging only. The printed representation may contain internal details that do not match the high-level representation of the term in Erlang. Get distribution channel data from the local node that is to be passed to the remote node. The distribution channel is identified by DHandle. If no data is available, the atom none is returned. Only the process registered as distribution controller for the distribution channel identified by DHandle is allowed to call this function.

This function is used when implementing an alternative distribution carrier using processes as distribution controllers. Register an alternate input handler process for the distribution channel identified by DHandle. If an alternate input handler has been registered, only the registered input handler process is allowed to call this function. Returns the N th element numbering from 1 of Tuple , for example:. Returns the value Val associated with Key and deletes it from the process dictionary.

Returns undefined if no value is associated with Key. The average time complexity for the current implementation of this function is O 1 and the worst case time complexity is O N , where N is the number of items in the process dictionary.

Raises an exception of class error with the reason Reason. As evaluating this function causes an exception to be thrown, it has no return value. The intent of the exception class error is to signal that an unexpected error has happened for example, a function is called with a parameter that has an incorrect type.

See the guide about errors and error handling for additional information. Args is expected to be the list of arguments for the current function or the atom none. If it is a list, it is used to provide the arguments for the current function in the stack back-trace. If it is none , the arity of the calling function is used in the stacktrace. As evaluating this function causes an exception to be raised, it has no return value.

Raises an exception of class exit with exit reason Reason. The intent of the exception class exit is that the current process should be stopped for example when a message telling a process to stop is received. If a process calls exit kill and does not catch the exception, it will terminate with exit reason kill and also emit exit signals with exit reason kill not killed to all linked processes. Such exit signals with exit reason kill can be trapped by the linked processes.

Sends an exit signal with exit reason Reason to the process or port identified by Pid. The following behavior applies if Reason is any term, except normal or kill , and P is the process or port identified by Pid :. If P is not trapping exits , P exits with exit reason Reason. If P is trapping exits , the exit signal is transformed into a message {'EXIT', From, Reason} , where From is the process identifier of the process that sent the exit signal, and delivered to the message queue of P.

The following behavior applies if Reason is the term normal and Pid is the identifier of a process P which is not the same as the process that invoked erlang:exit Pid, normal the behavior when a process sends a signal with the normal reason to itself is described in the warning :.

If Reason is the atom kill , that is, if exit Pid, kill is called, an untrappable exit signal is sent to the process that is identified by Pid , which unconditionally exits with exit reason killed. The exit reason is changed from kill to killed to hint to linked processes that the killed process got killed by a call to exit Pid, kill. The only scenario that has not been covered by the description above is when a process P sends an exit signal with reason normal to itself, that is erlang:exit self , normal.

The behavior in this scenario is as follows:. Note that the behavior described above is different from when a process sends an exit signal with reason normal to another process. This is arguably strange but this behavior is kept for backward compatibility reasons.

Calculates, without doing the encoding, the maximum byte size for a term encoded in the Erlang external term format. The following condition applies always:. Returns a float by converting Number to a float, for example:. Returns a binary corresponding to the text representation of Float using fixed decimal point formatting.

Returns a string corresponding to the text representation of Float using fixed decimal point formatting. If option decimals is specified, the returned value contains at most Decimals number of digits past the decimal point.

If the number does not fit in the internal static buffer of bytes, the function throws badarg. If option compact is specified, the trailing zeros at the end of the list are truncated. This option is only meaningful together with option decimals. If option scientific is specified, the float is formatted using scientific notation with Decimals digits of precision.

The reason for this is explained in Representation of Floating Point Numbers. Returns the largest integer not greater than Number.

Returns a list with information about the fun Fun. Each list element is a tuple. The order of the tuples is undefined, and more tuples can be added in a future release.

This BIF is mainly intended for debugging, but it can sometimes be useful in library functions that need to verify, for example, the arity of a fun. Calling it will always call the function F with arity A in the latest code for module M. All other funs are called local. When a local fun is called, the same version of the code that created the fun is called even if a newer version of the module has been loaded.

The following elements are always present in the list for both local and external funs:. Type is local or external. If Fun is a local fun, Module is the module in which the fun is defined. If Fun is an external fun, Module is the module that the fun refers to.

If Fun is a local fun, Name is the name of the local function that implements the fun. This name was generated by the compiler, and is only of informational use. As it is a local function, it cannot be called directly. If no code is currently loaded for the fun, [] is returned instead of an atom.

If Fun is an external fun, Name is the name of the exported function that the fun refers to. Arity is the number of arguments that the fun is to be called with. Env a list is the environment or free variables for the fun. For external funs, the returned list is always empty. The following elements are only present in the list if Fun is local:. Pid is the process identifier of the process that originally created the fun.

It might point to the init process if the Fun was statically allocated when module was loaded this optimisation is performed for local functions that do not capture the environment.

Index an integer is an index into the module fun table. Uniq a binary is a unique value for this fun. It is calculated from the compiled code for the entire module. Uniq an integer is a unique value for this fun. Returns information about Fun as specified by Item , in the form {Item,Info}. For any fun, Item can be any of the atoms module , name , arity , env , or type. For an external fun, the value of any of these items is always the atom undefined. Returns String that represents the code that created Fun.

Explanation: test:add 1 and test:add 42 has the same string representation as the environment is not taken into account. Explanation: The string representations differ because the funs come from different fun expressions.

Forces an immediate garbage collection of the executing process. The function is not to be used unless it has been noticed or there are good reasons to suspect that the spontaneous garbage collection will occur too late or not at all. Otherwise a request for garbage collection is sent to the process identified by Pid , and will be handled when appropriate.

If no async option has been passed, the caller blocks until GCResult is available and can be returned. GCResult informs about the result of the garbage collection request as follows:. Returns the process dictionary as a list of {Key, Val} tuples. The items in the returned list can be in any order. Returns the value Val associated with Key in the process dictionary, or undefined if Key does not exist.

The expected time complexity for the current implementation of this function is O 1 and the worst case time complexity is O N , where N is the number of items in the process dictionary. Returns the magic cookie of the local node if the node is alive, otherwise the atom nocookie. Returns the magic cookie for node Node if the local node is alive, otherwise the atom nocookie.

Returns a list of all keys present in the process dictionary. Returns a list of keys that are associated with the value Val in the process dictionary. Returns the process identifier of the group leader for the process evaluating the function. Every process is a member of some process group and all groups have a group leader.

When a new process is spawned, it gets the same group leader as the spawning process. Initially, at system startup, init is both its own group leader and the group leader of all processes. Sets the group leader of Pid to GroupLeader. Typically, this is used when a process started from a certain shell is to have another group leader than init. The group leader should be rarely changed in applications with a supervision tree, because OTP assumes the group leader of their processes is their application master.

Setting the group leader follows the signal ordering guarantees described in the Processes Chapter in the Erlang Reference Manual. The same as halt 0, [].

The same as halt Status, []. Status must be a non-negative integer, a string, or the atom abort. Halts the Erlang runtime system. Has no return value. Depending on Status , the following occurs:. On many platforms, the OS supports only status codes A too large status code is truncated by clearing the high bits.

Before ERTS 9. Now any unicode string is valid. For integer Status , the Erlang runtime system closes all ports and allows async threads to finish their operations before exiting. To exit without such flushing, use Option as {flush,false}.

For statuses string and abort , option flush is ignored and flushing is not done. Returns the head of List , that is, the first element, for example:. Failure: badarg if List is the empty list [].

Puts the calling process into a wait state where its memory allocation has been reduced as much as possible. This is useful if the process does not expect to receive any messages soon. The process is awaken when a message is sent to it, and control resumes in Module:Function with the arguments specified by Args with the call stack emptied, meaning that the process terminates when that function returns.

If the process has any message in its message queue, the process is awakened immediately in the same way as described earlier. After this, all live data is in one continuous heap. The heap is then shrunken to the exact same size as the live data that it holds even if that size is less than the minimum heap size for the process. If the size of the live data in the process is less than the minimum heap size, the first garbage collection occurring after the process is awakened ensures that the heap size is changed to a size not smaller than the minimum heap size.

Notice that emptying the call stack means that any surrounding catch is removed and must be re-inserted after hibernation. Returns a new tuple with element Term inserted at position Index in tuple Tuple1. All elements from position Index and upwards are pushed one step higher in the new tuple Tuple2. Returns a binary corresponding to the text representation of Integer , for example:.

Returns a binary corresponding to the text representation of Integer in base Base , for example:. Returns a string corresponding to the text representation of Integer , for example:. Returns a string corresponding to the text representation of Integer in base Base , for example:.

Returns a binary that is made from the integers and binaries in IoListOrBinary , for example:. Returns an iovec that is made from the integers and binaries in IoListOrBinary. This function is useful when you want to flatten an iolist but you do not need a single binary. Returns true if the local node is alive that is, if the node can be part of a distributed system , otherwise false.

A node is alive if it is started with:. Returns true if Term is an atom, otherwise false. Returns true if Term is a binary, otherwise false. Returns true if Term is a bitstring including a binary , otherwise false.

Returns true if Term is the atom true or the atom false that is, a boolean. Otherwise returns false. Returns true if Term is a floating point number, otherwise false. Returns true if Term is a fun, otherwise false. Returns true if Term is a fun that can be applied with Arity number of arguments, otherwise false. Returns true if Term is an integer, otherwise false. Returns true if Term is a list with zero or more elements, otherwise false.

Returns true if Term is a map, otherwise false. Returns true if map Map contains Key and returns false if it does not contain the Key. The call fails with a {badmap,Map} exception if Map is not a map. Returns true if Term is an integer or a floating point number.

Returns true if Term is a process identifier, otherwise false. Returns true if Term is a port identifier, otherwise false. Returns true if the process exists and is alive, that is, is not exiting and has not exited.

Returns true if Term is a tuple and its first element is RecordTag. It emits code to verify that Term is a tuple, that its first element is RecordTag , and that the size is correct. Returns true if Term is a tuple, its first element is RecordTag , and its size is Size. Allowed in guard tests if RecordTag is a literal atom and Size is a literal integer. This BIF is documented for completeness. Returns true if Term is a reference, otherwise false.

Returns true if Term is a tuple, otherwise false. Sets up and activates a link between the calling process and another process or a port identified by PidOrPort. We will from here on call the identified process or port linkee. If the linkee is a port, it must reside on the same node as the caller. If one of the participants of a link terminates, it will send an exit signal to the other participant. The exit signal will contain the exit reason of the terminated participant.

Other cases when exit signals are triggered due to a link are when no linkee exist noproc exit reason and when the connection between linked processes on different nodes is lost or cannot be established noconnection exit reason. For more information on links and exit signals due to links, see the Processes chapter in the Erlang Reference Manual :. The noproc exception is not to be confused with an exit signal with exit reason noproc.

Currently it is "cheap" to check if the linkee exists when it is supposed to reside on the same node as the calling process. The link setup and activation is performed asynchronously. If the link already exists, or if the caller attempts to create a link to itself, nothing is done. A detailed description of the link protocol can be found in the Distribution Protocol chapter of the ERTS User's Guide. Earlier versions allowed only ISO-latin-1 characters as the implementation did not allow Unicode characters above Returns a binary that is made from the integers and binaries in IoList , for example:.

Returns a bitstring that is made from the integers and bitstrings in BitstringList. The last tail in BitstringList is allowed to be a bitstring. Returns the atom whose text representation is String , but only if there already exists such atom. An atom exists if it has been created by the run-time system by either loading code or creating a term in which the atom is part.

Failure: badarg if there does not already exist an atom whose text representation is String. Returns the float whose text representation is String , for example:. Failure: badarg if String contains a bad representation of a float. Returns an integer whose text representation is String , for example:.

Failure: badarg if String contains a bad representation of an integer. Returns an integer whose text representation in base Base is String , for example:. Returns a process identifier whose text representation is a String , for example:. Failure: badarg if String contains a bad representation of a process identifier.

This BIF is intended for debugging and is not to be used in application programs. Returns a port identifier whose text representation is a String , for example:.

Failure: badarg if String contains a bad representation of a port identifier. Returns a reference whose text representation is a String , for example:. Failure: badarg if String contains a bad representation of a reference. If Binary contains the object code for module Module , this BIF loads that object code. If the code for module Module already exists, all export references are replaced so they point to the newly loaded code.

The previously loaded code is kept in the system as old code, as there can still be processes executing that code. Returns either {module, Module} , or {error, Reason} if loading fails.

Reason is one of the following:. Loads and links a dynamic library containing native implemented functions NIFs for a module. so for Unix and. dll for Windows. Notice that on most OSs the library has to have a different name on disc when an upgrade of the nif is done.

If the name is the same, but the contents differ, the old library may be loaded instead. LoadInfo can be any term.

It is passed on to the library as part of the initialization. A good practice is to include a module version number to support future code upgrade scenarios. It returns either ok , or {error,{Reason,Text}} if loading fails. Reason is one of the following atoms while Text is a human readable string that can give more information about the failure:.

On the other hand, all functions declared with the -nifs attribute do not have to be implemented by the dynamic library. Returns a list of all loaded Erlang modules current and old code , including preloaded modules. Returns the current local date and time, {{Year, Month, Day}, {Hour, Minute, Second}} , for example:.

Converts local date and time to Universal Time Coordinated UTC , if supported by the underlying OS. Otherwise no conversion is done and Localtime is returned. Failure: badarg if Localtime denotes an invalid date and time. Returns a unique reference. The reference is unique among connected nodes.

Before OTP when a node is restarted multiple times with the same node name, references created on a newer node can be mistaken for a reference created on an older node with the same node name. Creates a new tuple of the specified Arity , where all elements are InitialValue , for example:. Creates a tuple of size Arity , where each element has value DefaultValue , and then fills in values from InitList.

Each list element in InitList must be a two-tuple, where the first element is a position in the newly created tuple and the second element is any term. If a position occurs more than once in the list, the term corresponding to the last occurrence is used.

Returns value Value associated with Key if Map contains Key. The call fails with a {badmap,Map} exception if Map is not a map, or with a {badkey,Key} exception if no value is associated with Key. Returns an integer, which is the number of key-value pairs in Map , for example:. The function tests both a match specification for "syntactic" correctness and runs the match specification against the object.

If the match specification contains errors, the tuple {error, Errors} is returned, where Errors is a list of natural language descriptions of what was wrong with the match specification. If Type is table , the object to match against is to be a tuple. If Type is trace , the object to match against is to be a list. The function returns {ok, Result, Flags, Warnings} , where Result is one of the following:.

This is a useful debugging and test tool, especially when writing complicated match specifications. Returns the largest of Term1 and Term2. Computes an MD5 message digest from Data , where the length of the digest is bits 16 bytes.

Data is a binary or a list of small integers and binaries. For more information about MD5, see RFC - The MD5 Message-Digest Algorithm. The MD5 Message-Digest Algorithm is not considered safe for code-signing or software-integrity purposes.

Finishes the update of an MD5 Context and returns the computed MD5 message digest. Update an MD5 Context with Data and returns a NewContext. Returns a list with information about memory dynamically allocated by the Erlang emulator. Each list element is a tuple {Type, Size}. The first element Type is an atom describing memory type.

The second element Size is the memory size in bytes. The total amount of memory currently allocated. This is the same as the sum of the memory size for processes and system. The total amount of memory currently used by the Erlang processes. This is part of the memory presented as processes memory. The total amount of memory currently allocated for the emulator that is not directly related to any Erlang process. Memory presented as processes is not included in this memory.

instrument 3 can be used to get a more detailed breakdown of what memory is part of this type. The total amount of memory currently allocated for atoms. This memory is part of the memory presented as system memory. The total amount of memory currently used for atoms. This memory is part of the memory presented as atom memory.

The total amount of memory currently allocated for binaries. The total amount of memory currently allocated for Erlang code. The total amount of memory currently allocated for ETS tables.

The maximum total amount of memory allocated since the emulator was started. This tuple is only present when the emulator is run with instrumentation.

The system value is not complete. Some allocated memory that is to be part of this value is not. When the emulator is run with instrumentation, the system value is more accurate, but memory directly allocated for malloc and friends is still not part of the system value.

Direct calls to malloc are only done from OS-specific runtime libraries and perhaps from user-implemented Erlang drivers that do not use the memory allocation functions in the driver interface. As the total value is the sum of processes and system , the error in system propagates to the total value. The different amounts of memory that are summed are not gathered atomically, which introduces an error in the result. The different values have the following relation to each other.

Values beginning with an uppercase letter is not part of the result. The total value is supposed to be the total amount of memory dynamically allocated by the emulator. Shared libraries, the code of the emulator itself, and the emulator stacks are not supposed to be included. That is, the total value is not supposed to be equal to the total size of all pages mapped to the emulator.

Also, because of fragmentation and prereservation of memory areas, the size of the memory segments containing the dynamically allocated memory blocks can be much larger than the total size of the dynamically allocated memory blocks. As from ERTS 5. Returns the memory size in bytes allocated for memory of type Type.

Returns the smallest of Term1 and Term2. Returns true if the module Module is loaded as current code ; otherwise, false. It does not attempt to load the module. Sends a monitor request of type Type to the entity identified by Item. The monitor request is an asynchronous signal.

That is, it takes time before the signal reaches its destination. A process or port monitor is triggered only once, after that it is removed from both monitoring process and the monitored entity. Monitors are fired when the monitored process or port terminates, does not exist at the moment of creation, or if the connection to it is lost.

If the connection to it is lost, we do not know if it still exists. A process or port monitor by name resolves the RegisteredName to pid or port only once at the moment of monitor instantiation, later changes to the name registration will not affect the existing monitor. When a process or port monitor is triggered, a 'DOWN' message is sent that has the following pattern:. In the monitor message MonitorRef and Type are the same as described earlier, and:. The monitored entity, which triggered the event.

When monitoring a process or a local port, Object will be equal to the pid or port that was being monitored. Either the exit reason of the process, noproc process or port did not exist at the time of monitor creation , or noconnection no connection to the node where the monitored process resides. Creates monitor between the current process and another process identified by Item , which can be a pid local or remote , an atom RegisteredName or a tuple {RegisteredName, Node} for a registered process, located elsewhere.

Before ERTS Now, such a call to monitor will instead succeed and a monitor is created. But the monitor will only supervise the connection. Creates monitor between the current process and a port identified by Item , which can be a port only local , an atom RegisteredName or a tuple {RegisteredName, Node} for a registered port, located on this node.

Note, that attempt to monitor a remote port will result in badarg. Monitors changes in time offset between Erlang monotonic time and Erlang system time.

In this case it serves as an identifier of the runtime system internal clock service at current runtime system instance. The monitor is triggered when the time offset is changed. This either if the time offset value is changed, or if the offset is changed from preliminary to final during finalization of the time offset when the single time warp mode is used.

When a change from preliminary to final time offset is made, the monitor is triggered once regardless of whether the time offset value was changed or not. If the runtime system is in multi time warp mode , the time offset is changed when the runtime system detects that the OS system time has changed. The runtime system does, however, not detect this immediately when it occurs.

A task checking the time offset is scheduled to execute at least once a minute, so under normal operation this is to be detected within a minute, but during heavy load it can take longer time.

The monitor is not automatically removed after it has been triggered. That is, repeated changes of the time offset trigger the monitor repeatedly. When the monitor is triggered a 'CHANGE' message is sent to the monitoring process.

According to the legend, when the last move of the puzzle is completed, the world will end. There are many variations on this legend.

For instance, in some tellings, the temple is a monastery , and the priests are monks. The temple or monastery may be in various locales including Hanoi , and may be associated with any religion. In some versions, other elements are introduced, such as the fact that the tower was created at the beginning of the world, or that the priests or monks may make only one move per day.

The puzzle can be played with any number of disks, although many toy versions have around 7 to 9 of them. A simple solution for the toy puzzle is to alternate moves between the smallest piece and a non-smallest piece. When moving the smallest piece, always move it to the next position in the same direction to the right if the starting number of pieces is even, to the left if the starting number of pieces is odd.

If there is no tower position in the chosen direction, move the piece to the opposite end, but then continue to move in the correct direction.

For example, if you started with three pieces, you would move the smallest piece to the opposite end, then continue in the left direction after that.

When the turn is to move the non-smallest piece, there is only one legal move. Doing this will complete the puzzle in the fewest moves. Considering those constraints after the first move, there is only one legal move at every subsequent turn.

The sequence of these unique moves is an optimal solution to the problem equivalent to the iterative solution described above. The key to solving a problem recursively is to recognize that it can be broken down into a collection of smaller sub-problems, to each of which that same general solving procedure that we are seeking applies, and the total solution is then found in some simple way from those sub-problems' solutions. Each of these created sub-problems being "smaller" guarantees that the base case s will eventually be reached.

Thence, for the Towers of Hanoi:. Assuming all n disks are distributed in valid arrangements among the pegs; assuming there are m top disks on a source peg, and all the rest of the disks are larger than m , so they can be safely ignored; to move m disks from a source peg to a target peg using a spare peg, without violating the rules:.

The full Tower of Hanoi solution then consists of moving n disks from the source peg A to the target peg C, using B as the spare peg. This approach can be given a rigorous mathematical proof with mathematical induction and is often used as an example of recursion when teaching programming.

First, observe that the problem is symmetric for permutations of the names of the pegs symmetric group S 3. If a solution is known moving from peg A to peg C , then, by renaming the pegs, the same solution can be used for every other choice of starting and destination peg. If there is only one disk or even none at all , the problem is trivial. This is called recursion. This algorithm can be schematized as follows. Identify the disks in order of increasing size by the natural numbers from 0 up to but not including h.

The following is a procedure for moving a tower of h disks from a peg A onto a peg C , with B being the remaining third peg:. By means of mathematical induction , it is easily proven that the above procedure requires the minimal number of moves possible and that the produced solution is the only one with this minimal number of moves.

The following Python code demonstrates the recursive solution. The list of moves for a tower being carried from one peg onto another one, as produced by the recursive algorithm, has many regularities. When counting the moves starting from 1, the ordinal of the disk to be moved during move m is the number of times m can be divided by 2. Hence every odd move involves the smallest disk. It can also be observed that the smallest disk traverses the pegs f , t , r , f , t , r , etc.

for odd height of the tower and traverses the pegs f , r , t , f , r , t , etc. for even height of the tower. This provides the following algorithm, which is easier, carried out by hand, than the recursive algorithm.

For the very first move, the smallest disk goes to peg t if h is odd and to peg r if h is even. With this knowledge, a set of disks in the middle of an optimal solution can be recovered with no more state information than the positions of each disk:.

Disk positions may be determined more directly from the binary base-2 representation of the move number the initial state being move 0, with all digits 0, and the final state being with all digits 1 , using the following rules:. The source and destination pegs for the m th move can also be found elegantly from the binary representation of m using bitwise operations. Furthermore, the disk to be moved is determined by the number of times the move count m can be divided by 2 i.

the number of zero bits at the right , counting the first move as 1 and identifying the disks by the numbers 0, 1, 2, etc. in order of increasing size. This permits a very fast non-recursive computer implementation to find the positions of the disks after m moves without reference to any previous move or distribution of disks.

The operation, which counts the number of consecutive zeros at the end of a binary number, gives a simple solution to the problem: the disks are numbered from zero, and at move m , disk number count trailing zeros is moved the minimal possible distance to the right circling back around to the left as needed. The binary numeral system of Gray codes gives an alternative way of solving the puzzle. In the Gray system, numbers are expressed in a binary combination of 0s and 1s, but rather than being a standard positional numeral system , the Gray code operates on the premise that each value differs from its predecessor by only one and exactly one bit changed.

If one counts in Gray code of a bit size equal to the number of disks in a particular Tower of Hanoi, begins at zero and counts up, then the bit changed each move corresponds to the disk to move, where the least-significant bit is the smallest disk, and the most-significant bit is the largest.

This technique identifies which disk to move, but not where to move it to. For the smallest disk, there are always two possibilities. For the other disks there is always one possibility, except when all disks are on the same peg, but in that case either it is the smallest disk that must be moved or the objective has already been achieved. Luckily, there is a rule that does say where to move the smallest disk to.

Let f be the starting peg, t the destination peg, and r the remaining third peg. The position of the bit change in the Gray code solution gives the size of the disk moved at each step: 1, 2, 1, 3, 1, 2, 1, 4, 1, 2, 1, 3, 1, 2, 1, sequence A in the OEIS , [12] a sequence also known as the ruler function , or one more than the power of 2 within the move number.

The game can be represented by an undirected graph , the nodes representing distributions of disks and the edges representing moves. For one disk, the graph is a triangle:. The nodes at the vertices of the outermost triangle represent distributions with all disks on the same peg.

The sides of the outermost triangle represent the shortest ways of moving a tower from one peg to another one. The edge in the middle of the sides of the largest triangle represents a move of the largest disk. The edge in the middle of the sides of each next smaller triangle represents a move of each next smaller disk. The sides of the smallest triangles represent moves of the smallest disk.

In general, for a puzzle with n disks, there are 3 n nodes in the graph; every node has three edges to other nodes, except the three corner nodes, which have two: it is always possible to move the smallest disk to one of the two other pegs, and it is possible to move one disk between those two pegs except in the situation where all disks are stacked on one peg.

The corner nodes represent the three cases where all the disks are stacked on one peg. As more disks are added, the graph representation of the game will resemble a fractal figure, the Sierpiński triangle.

Incidentally, this longest non-repetitive path can be obtained by forbidding all moves from a to c. The Hamiltonian cycle for three disks is:. This gives N h to be 2, 12, , , sequence A in the OEIS. If all moves must be between adjacent pegs i. The solution uses all 3 n valid positions, always taking the unique move that does not undo the previous move. The position with all disks at peg B is reached halfway, i. In Cyclic Hanoi, we are given three pegs A, B, C , which are arranged as a circle with the clockwise and the counterclockwise directions being defined as A — B — C — A and A — C — B — A respectively.

The moving direction of the disk must be clockwise. The solution can be found using two mutually recursive procedures:. Let C n and A n represent moving n disks clockwise and counterclockwise, then we can write down both formulas:. Although the three-peg version has a simple recursive solution long been known, the optimal solution for the Tower of Hanoi problem with four pegs called Reve's puzzle was not verified until , by Bousch.

However, in case of four or more pegs, the Frame—Stewart algorithm is known without proof of optimality since For the formal derivation of the exact number of minimal moves required to solve the problem by applying the Frame—Stewart algorithm and other equivalent methods , see the following paper.

For other variants of the four-peg Tower of Hanoi problem, see Paul Stockmeyer's survey paper. The so-called Towers of Bucharest and Towers of Klagenfurt game configurations yield ternary and pentary Gray codes. A curious generalization of the original goal of the puzzle is to start from a given configuration of the disks where all disks are not necessarily on the same peg and to arrive in a minimal number of moves at another given configuration.

In general, it can be quite difficult to compute a shortest sequence of moves to solve this problem. A solution was proposed by Andreas Hinz and is based on the observation that in a shortest sequence of moves, the largest disk that needs to be moved obviously one may ignore all of the largest disks that will occupy the same peg in both the initial and final configurations will move either exactly once or exactly twice.

The mathematics related to this generalized problem becomes even more interesting when one considers the average number of moves in a shortest sequence of moves between two initial and final disk configurations that are chosen at random.

In Magnetic Tower of Hanoi, each disk has two distinct sides North and South typically colored "red" and "blue".

Disks must not be placed with the similar poles together—magnets in each disk prevent this illegal move. Also, each disk must be flipped as it is moved. This variation of the famous Tower of Hanoi puzzle was offered to grade 3—6 students at 2ème Championnat de France des Jeux Mathématiques et Logiques held in July The rules of the puzzle are essentially the same: disks are transferred between pegs one at a time.

At no time may a bigger disk be placed on top of a smaller one. The difference is that now for every size there are two disks: one black and one white. Also, there are now two towers of disks of alternating colors.

The goal of the puzzle is to make the towers monochrome same color. The biggest disks at the bottom of the towers are assumed to swap positions. A variation of the puzzle has been adapted as a solitaire game with nine playing cards under the name Tower of Hanoy. The Tower of Hanoi is frequently used in psychological research on problem-solving.

There also exists a variant of this task called Tower of London for neuropsychological diagnosis and treatment of executive functions. Zhang and Norman [30] used several isomorphic equivalent representations of the game to study the impact of representational effect in task design.

They demonstrated an impact on user performance by changing the way that the rules of the game are represented, using variations in the physical design of the game components. This knowledge has impacted on the development of the TURF framework [31] for the representation of human—computer interaction. As mentioned above, the Tower of Hanoi is popular for teaching recursive algorithms to beginning programming students.

A pictorial version of this puzzle is programmed into the emacs editor, accessed by typing M-x hanoi. There is also a sample algorithm written in Prolog.

The Tower of Hanoi is also used as a test by neuropsychologists trying to evaluate frontal lobe deficits. In , researchers published the results of an experiment that found that the ant species Linepithema humile were successfully able to solve the 3-disk version of the Tower of Hanoi problem through non-linear dynamics and pheromone signals. In , scientists synthesized multilayered palladium nanosheets with a Tower of Hanoi like structure.

In the science fiction story "Now Inhale", by Eric Frank Russell , a human is held prisoner on a planet where the local custom is to make the prisoner play a game until it is won or lost before his execution. The protagonist knows that a rescue ship might take a year or more to arrive, so he chooses to play Towers of Hanoi with 64 disks.

This story makes reference to the legend about the Buddhist monks playing the game until the end of the world.

In the Doctor Who story The Celestial Toymaker , the eponymous villain forces the Doctor to play a ten-piece 1,move Tower of Hanoi game entitled The Trilogic Game with the pieces forming a pyramid shape when stacked.

In , the concept of the Towers Of Hanoi problem was used in Professor Layton and the Diabolical Box in puzzles 6, 83, and 84, but the disks had been changed to pancakes. The puzzle was based around a dilemma where the chef of a restaurant had to move a pile of pancakes from one plate to the other with the basic principles of the original puzzle i. three plates that the pancakes could be moved onto, not being able to put a larger pancake onto a smaller one, etc.

In the film Rise of the Planet of the Apes , this puzzle, called in the film the "Lucas Tower", is used as a test to study the intelligence of apes. The puzzle is featured regularly in adventure and puzzle games.

An HTML form is a section of a document containing normal content, markup, special elements called controls checkboxes, radio buttons, menus, etc. Users generally "complete" a form by modifying its controls entering text, selecting menu items, etc. Here's a simple form that includes labels, radio buttons, and push buttons reset the form or submit it :. This specification includes more detailed information about forms in the subsections on form display issues.

Users interact with forms through named controls. A control's "control name" is given by its name attribute. The scope of the name attribute for a control within a FORM element is the FORM element. Each control has both an initial value and a current value, both of which are character strings.

Please consult the definition of each control for information about initial values and possible constraints on values imposed by the control. In general, a control's "initial value" may be specified with the control element's value attribute. However, the initial value of a TEXTAREA element is given by its contents, and the initial value of an OBJECT element in a form is determined by the object implementation i.

The control's "current value" is first set to the initial value. Thereafter, the control's current value may be modified through user interaction and scripts. A control's initial value does not change. Thus, when a form is reset, each control's current value is reset to its initial value.

If a control does not have an initial value, the effect of a form reset on that control is undefined. When a form is submitted for processing, some controls have their name paired with their current value and these pairs are submitted with the form. Authors should specify the scripting language of a push button script through a default script declaration with the META element. Authors create buttons with the BUTTON element or the INPUT element.

Please consult the definitions of these elements for details about specifying different button types. Authors should note that the BUTTON element offers richer rendering capabilities than the INPUT element. Several checkboxes in a form may share the same control name. Thus, for example, checkboxes allow users to select several values for the same property. The INPUT element is used to create a checkbox control.

Since user agent behavior differs, authors should ensure that in each set of radio buttons that one is initially "on". The elements used to create controls generally appear inside a FORM element, but may also appear outside of a FORM element declaration when they are used to build user interfaces. This is discussed in the section on intrinsic events. Note that controls outside a form cannot be successful controls. The default value for this attribute is the reserved string "UNKNOWN".

User agents may interpret this value as the character encoding that was used to transmit the document containing this FORM element. The FORM element acts as a container for controls.

It specifies:. A form can contain text and markup paragraphs, lists, etc. in addition to form controls. The following example shows a form that is to be processed by the "adduser" program when submitted. The form will be sent to the program using the HTTP "post" method.

Please consult the section on form submission for information about how user agents must prepare form data for servers and how user agents should handle expected responses. Further discussion on the behavior of servers that receive form data is beyond the scope of this specification. The control type defined by the INPUT element depends on the value of the type attribute:. Application designers should note that this mechanism affords only light security protection.

Although the password is masked by user agents from casual observers, it is transmitted to the server in clear text, and may be read by anyone with low-level access to the network. When a pointing device is used to click on the image, the form is submitted and the click coordinates passed to the server. The x value is measured in pixels from the left of the image, and the y value in pixels from the top of the image.

The submitted data includes name. If the server takes different actions depending on the location clicked, users of non-graphical browsers will be disadvantaged. For this reason, authors should consider alternate approaches:. The following sample HTML fragment defines a simple form that allows the user to enter a first name, last name, email address, and gender. When the submit button is activated, the form will be sent to the program specified by the action attribute.

In the section on the LABEL element, we discuss marking up labels such as "First name". In this next example, the JavaScript function name verify is triggered when the "onclick" event occurs:. Please consult the section on intrinsic events for more information about scripting and events.

The following example shows how the contents of a user-specified file may be submitted with a form. The user is prompted for his or her name and a list of file names whose contents should be submitted with the form.

Buttons created with the BUTTON element function just like buttons created with the INPUT element, but they offer richer rendering possibilities: the BUTTON element may have content.

For example, a BUTTON element that contains an image functions like and may resemble an INPUT element whose type is set to "image", but the BUTTON element type allows content. The following example expands a previous example, but creates submit and reset buttons with BUTTON instead of INPUT.

The buttons contain images by way of the IMG element. Recall that authors must provide alternate text for an IMG element. It is illegal to associate an image map with an IMG that appears as the contents of a BUTTON element.

The SELECT element creates a menu. Each choice offered by the menu is represented by an OPTION element. A SELECT element must contain at least one OPTION element.

The OPTGROUP element allows authors to group choices logically. This is particularly helpful when the user must choose from a long list of options; groups of related choices are easier to grasp and remember than a single long list of options.

In HTML 4, all OPTGROUP elements must be specified directly within a SELECT element i. Zero or more choices may be pre-selected for the user.

User agents should determine which choices are pre-selected as follows:. Since user agent behavior differs, authors should ensure that each menu includes a default pre-selected OPTION. Implementors are advised that future versions of HTML may extend the grouping mechanism to allow for nested groups i. This will allow authors to represent a richer hierarchy of choices. When rendering a menu choice , user agents should use the value of the label attribute of the OPTION element as the choice.

If this attribute is not specified, user agents should use the contents of the OPTION element. The label attribute of the OPTGROUP element specifies the label for a group of choices. In this example, we create a menu that allows the user to select which of seven software components to install. The first and second components are pre-selected but may be deselected by the user. The remaining components are not pre-selected. The size attribute states that the menu should only have 4 rows even though the user may select from among 7 options.

The other options should be made available through a scrolling mechanism. The SELECT is followed by submit and reset buttons. Only selected options will be successful using the control name "component-select". When no options are selected, the control is not successful and neither the name nor any values are submitted to the server when the form is submitted.

Note that where the value attribute is set, it determines the control's initial value , otherwise it's the element's contents. In this example we use the OPTGROUP element to group choices. The following markup:. Visual user agents may allow users to select from option groups through a hierarchical menu or some other mechanism that reflects the structure of choices.

This image shows a SELECT element rendered as cascading menus. The top label of the menu displays the currently selected value PortMaster 3, 3. The user has unfurled two cascading menus, but has not yet selected the new value PortMaster 2, 3. Note that each cascading menu displays the label of an OPTGROUP or OPTION element. The TEXTAREA element creates a multi-line text input control. User agents should use the contents of this element as the initial value of the control and should render this text initially.

This example creates a TEXTAREA control that is 20 rows by 80 columns and contains two lines of text initially. The TEXTAREA is followed by submit and reset buttons. Setting the readonly attribute allows authors to display unmodifiable text in a TEXTAREA. This differs from using standard marked-up text in a document because the value of TEXTAREA is submitted with the form. ISINDEX is deprecated. This element creates a single-line text input control.

Authors should use the INPUT element to create text input controls. The ISINDEX element creates a single-line text input control that allows any number of characters.

In this example, we assign an access key to a link defined by the A element. If one of the participants of a link terminates, it will send an exit signal to the other participant. See the next note for an example of when an atom exists in the source code for an Erlang module but not in the compiled version of the same module. Before ERTS Similarly, users must give a TEXTAREA focus in order to enter text into it. This option sets a transformation matrix, for use by subsequent -draw or -transform options. User agents should ignore leading zeros.

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