## File: clib.doc

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swi-prolog 8.0.2+dfsg-3+deb10u1
 123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504505506507508509510511512513514515516517518519520521522523524525526527528529530531532533534535536537538539540541542543544545546547548549550551552553554555556557558559560561562563564565566567568569570571572573574575576577578579580581582583584585586587588589590591592593594595596597598599600601602603604605606607608609610611612613614615616617618619620621622623624625626627628629630631632633634635636637638639640641642643644645646647648649650651652653654655656657658659660661662663664665666667668669670671672673674675676677678679680681682683684685686687688689690691692693694695696697698699700701702703704705706707708709710711712713714715716717718719720721722723724725726727728729730731732733734735736737738739740741742743744745746747748749750751752753754755756757758759760761762763764765766767768769 \documentclass[11pt]{article} \usepackage{times} \usepackage{pl} \usepackage{plpage} \usepackage{html} \sloppy \makeindex \onefile \htmloutput{.} % Output directory \htmlmainfile{clib} % Main document file \bodycolor{white} % Page colour \renewcommand{\runningtitle}{CLIB -- System interfaces} \begin{document} \title{SWI-Prolog C-library} \author{Jan Wielemaker \\ VU University of Amsterdam \\ The Netherlands \\ E-mail: \email{J.Wielemaker@vu.nl}} \maketitle \begin{abstract} This document describes commonly used foreign language extensions to \href{http://www.swi-prolog.org}{SWI-Prolog} distributed as a package known under the name {\em clib}. The package defines a number of Prolog libraries with accompagnying foreign libraries. On Windows systems, the \pllib{unix} library can only be used if the whole SWI-Prolog suite is compiled using \href{http://www.cygwin.com}{Cygwin}. The other libraries have been ported to native Windows. \end{abstract} \vfill \pagebreak \tableofcontents \vfill \vfill \newpage \section{Introduction} \label{sec:clib-intro} Many useful facilities offered by one or more of the operating systems supported by SWI-Prolog are not supported by the SWI-Prolog kernel distribution. Including these would enlarge the {\em footprint} and complicate portability matters while supporting only a limited part of the user-community. This document describes \pllib{unix} to deal with the Unix process API, \pllib{socket} to deal with inet-domain TCP and UDP sockets, \pllib{cgi} to deal with getting CGI form-data if SWI-Prolog is used as a CGI scripting language, \pllib{crypt} to provide password encryption and verification, \pllib{sha} providing cryptographic hash functions and \pllib{memfile} providing in-memorty pseudo files. \input{process.tex} \input{filesex.tex} \InputIfFileExists{uid.tex}{}{} \InputIfFileExists{syslog.tex}{}{} \input{socket.tex} \subsection{UDP protocol support} \label{sec:udp-sockets} The current library provides limited support for UDP packets. The UDP protocol is a \emph{connection-less} and \emph{unreliable} datagram based protocol. That means that messages sent may or may not arrive at the client side and may arrive in a different order as they are sent. UDP messages are often used for streaming media or for service discovery using the broadcasting mechanism. \begin{description} \predicate{udp_socket}{1}{-Socket} Similar to tcp_socket/1, but create a socket using the \const{SOCK_DGRAM} protocol, ready for UDP connections. \predicate{udp_receive}{4}{+Socket, -Data, -From, +Options} Wait for and return the next datagram. The data is returned as a Prolog string object (see string_to_list/2). \arg{From} is a term of the format \mbox{ip(\arg{A},\arg{B},\arg{C},\arg{D}):\arg{Port}} indicating the sender of the message. \arg{Socket} can be waited for using wait_for_input/3. Defined \arg{Options}: \begin{description} \termitem{as}{+Type} Defines the returned term-type. \arg{Type} is one of \const{atom}, \const{codes} or \const{string} (default). \termitem{max_message_size}{+Size} Specify the maximum number of bytes to read from a UDP datagram. Size must be within the range 0-65535. If unspecified, a maximum of 4096 bytes will be read. \end{description} The typical sequence to receive UDP data is: \begin{code} receive(Port) :- udp_socket(S), tcp_bind(S, Port), repeat, udp_receive(Socket, Data, From, [as(atom)]), format('Got ~q from ~q~n', [Data, From]), fail. \end{code} \predicate{udp_send}{4}{+Socket, +Data, +To, +Options} Send a UDP message. Data is a string, atom or code-list providing the data. \arg{To} is an address of the form \arg{Host}:\arg{Port} where Host is either the hostname or a term ip/4. \arg{Options} is currently unused. A simple example to send UDP data is: \begin{code} send(Host, Port, Message) :- udp_socket(S), udp_send(S, Message, Host:Port, []), tcp_close_socket(S). \end{code} A broadcast is achieved by using \term{tcp_setopt}{Socket, broadcast} prior to sending the datagram and using the local network broadcast address as a ip/4 term. \end{description} The normal mechanism to discover a service on the local network is for the client to send a broadcast message to an agreed port. The server receives this message and replies to the client with a message indicating further details to establish the communication. \section{The stream_pool library} \label{sec:stream-pools} The \pllib{streampool} library dispatches input from multiple streams based on wait_for_input/3. It is part of the clib package as it is used most of the time together with the \pllib{socket} library. On non-Unix systems it often can only be used with socket streams. With SWI-Prolog 5.1.x, multi-threading often provides a good alternative to using this library. In this schema one thread watches the listening socket waiting for connections and either creates a thread per connection or processes the accepted connections with a pool of \jargon{worker threads}. The library \pllib{http/thread_httpd} provides an example realising a mult-threaded HTTP server. \begin{description} \predicate{add_stream_to_pool}{2}{+Stream, :Goal} Add \arg{Stream}, which must be an input stream and ---on non-unix systems--- connected to a socket to the pool. If input is available on \arg{Stream}, \arg{Goal} is called. \predicate{delete_stream_from_pool}{1}{+Stream} Delete the given stream from the pool. Succeeds, even if \arg{Stream} is no member of the pool. If \arg{Stream} is unbound the entire pool is emtied but unlike close_stream_pool/0 the streams are not closed. \predicate{close_stream_pool}{0}{} Empty the pool, closing all streams that are part of it. \predicate{dispatch_stream_pool}{1}{+TimeOut} Wait for maximum of \arg{TimeOut} for input on any of the streams in the pool. If there is input, call the \arg{Goal} associated with add_stream_to_pool/2. If \arg{Goal} fails or raises an exception a message is printed. \arg{TimeOut} is described with wait_for_input/3. If \arg{Goal} is called, there is \emph{some} input on the associated stream. \arg{Goal} must be careful not to block as this will block the entire pool.% \footnote{This is hard to achieve at the moment as none of the Prolog read-commands provide for a timeout.} \predicate{stream_pool_main_loop}{0}{} Calls dispatch_stream_pool/1 in a loop until the pool is empty. \end{description} Below is a very simple example that reads the first line of input and echos it back. \begin{code} :- use_module(library(streampool)). server(Port) :- tcp_socket(Socket), tcp_bind(Socket, Port), tcp_listen(Socket, 5), tcp_open_socket(Socket, In, _Out), add_stream_to_pool(In, accept(Socket)), stream_pool_main_loop. accept(Socket) :- tcp_accept(Socket, Slave, Peer), tcp_open_socket(Slave, In, Out), add_stream_to_pool(In, client(In, Out, Peer)). client(In, Out, _Peer) :- read_line_to_codes(In, Command), close(In), format(Out, 'Please to meet you: ~s~n', [Command]), close(Out), delete_stream_from_pool(In). \end{code} \input{uri.tex} \section{CGI Support library} \label{sec:cgi} This is currently a very simple library, providing support for obtaining the form-data for a CGI script: \begin{description} \predicate{cgi_get_form}{1}{-Form} Decodes standard input and the environment variables to obtain a list of arguments passed to the CGI script. This predicate both deals with the CGI {\bf GET} method as well as the {\bf POST} method. If the data cannot be obtained, an \const{existence_error} exception is raised. \end{description} Below is a very simple CGI script that prints the passed parameters. To test it, compile this program using the command below, copy it to your cgi-bin directory (or make it otherwise known as a CGI-script) and make the query \verb$http://myhost.mydomain/cgi-bin/cgidemo?hello=world$ \begin{code} % pl -o cgidemo --goal=main --toplevel=halt -c cgidemo.pl \end{code} \begin{code} :- use_module(library(cgi)). main :- set_stream(current_output, encoding(utf8)), cgi_get_form(Arguments), format('Content-type: text/html; charset=UTF-8~n~n', []), format('~n', []), format('~n', []), format('Simple SWI-Prolog CGI script~n', []), format('~n~n', []), format('~n', []), format('
~n', [Name, Value]), print_args(T). \end{code} \subsection{Some considerations} \label{sec:cgi-considerations} Printing an HTML document using format/2 is not a neat way of producing HTML because it is vulnerable to required escape sequences. A high-level alternative is provided by \pllib{http/html_write} from the HTTP library. The startup-time of Prolog is relatively long, in particular if the program is large. In many cases it is much better to use the SWI-Prolog HTTP server library and make the main web-server relay requests to the SWI-Prolog webserver. See the SWI-Prolog \href{http://www.swi-prolog.org/pldoc/package/http.html}{HTTP package} for details. The CGI standard is unclear about handling Unicode data. The above two declarations ensure the CGI script will send all data in UTF-8 and thus provide full support of Unicode. It is assumed that browsers generally send form-data using the same encoding as the page in which the form appears, UTF-8 or ISO Latin-1. The current version of cgi_get_form/1 assumes the CGI data is in UTF-8. \section{Password encryption library} \label{sec:crypt} The \pllib{crypt} library defines crypt/2 for encrypting and testing passwords. The clib package also provides crytographic hashes as described in \secref{sha} \begin{description} \predicate{crypt}{2}{+Plain, ?Encrypted} This predicate can be used in three modes. To test whether a password matches an encrypted version thereof, simply run with both arguments fully instantiated. To generate a default encrypted version of \arg{Plain}, run with unbound \arg{Encrypted} and this argument is unified to a list of character codes holding an encrypted version. The library supports two encryption formats: traditional Unix DES-hashes\footnote{On non-Unix systems, crypt() is provided by the NetBSD library. The license header is added at the end of this document.} and FreeBSD compatible MD5 hashes (all platforms). MD5 hashes start with the magic sequence \verb|$1$|, followed by an up to 8 character \jargon{salt}. DES hashes start with a 2 character \jargon{salt}. Note that a DES hash considers only the first 8 characters. The MD5 considers the whole string. Salt and algorithm can be forced by instantiating the start of \arg{Encrypted} with it. This is typically used to force MD5 hashes: \begin{code} ?- phrase("$1$", E, _), crypt("My password", E), format('~s~n', [E]). $1$qdaDeDZn\$ZUxSQEESEHIDCHPNc3fxZ1 \end{code} \arg{Encrypted} is always a list of ASCII character codes. \arg{Plain} only supports ISO-Latin-1 passwords in the current implementation. \arg{Plain} is either an atom, SWI-Prolog string, list of characters or list of character-codes. It is not advised to use atoms, as this implies the password will be available from the Prolog heap as a defined atom. \textbf{NOTE}: crypt/2 provides an interface to the Unix password hashing API. Above we already introduced support for classical DES and MD5 hashes, both hashes that are considered \emph{insecure} by today's standards.\footnote{\emph{Insecure} means that the password can realistically be derived from the password hash using a brute-force attack. This implies that leaking the password database is an immediate security risk.} The crypt() API of modern Unix systems typically support more secure hashes. Using crypt/2 is suitable if compatibility with OS passwords is required. If strong hashes and platform independence are important to you, use crypto_password_hash/2 provided by library \pllib{crypto} from the \href{http://www.swi-prolog.org/pldoc/package/ssl.html}{ssl~package}. \end{description} \InputIfFileExists{uuid.tex}{}{} \section{SHA* Secure Hash Algorithms} \label{sec:sha} The library \pllib{sha} provides \jargon{Secure Hash Algorihms} approved by FIPS (\jargon{Federal Information Processing Standard}). Quoting \href{http://en.wikipedia.org/wiki/SHA-1}{Wikipedia}: \textit{The SHA (Secure Hash Algorithm) hash functions refer to five FIPS-approved algorithms for computing a condensed digital representation (known as a message digest) that is, to a high degree of probability, unique for a given input data sequence (the message). These algorithms are called secure' because (in the words of the standard), for a given algorithm, it is computationally infeasible 1) to find a message that corresponds to a given message digest, or 2) to find two different messages that produce the same message digest. Any change to a message will, with a very high probability, result in a different message digest.''} The current library supports all 5 approved algorithms, both computing the hash-key from data and the \jargon{hash Message Authentication Code} (HMAC). A general secure hash interface is provided by \pllib{crypto}, part of the \href{http://www.swi-prolog.org/pldoc/package/ssl.html}{ssl~package}. Input is text, represented as an atom, packed string object or code-list. Note that these functions operate on byte-sequences and therefore are not meaningful on Unicode text. The result is returned as a list of byte-values. This is the most general format that is comfortable supported by standard Prolog and can easily be transformed in other formats. Commonly used text formats are ASCII created by encoding each byte as two hexadecimal digits and ASCII created using \jargon{base64} encoding. Representation as a large integer can be desirable for computational processing. \begin{description} \predicate{sha_hash}{3}{+Data, -Hash, +Options} Hash is the SHA hash of Data. \arg{Data} is either an atom, packed string or list of character codes. \arg{Hash} is unified with a list of bytes (integers in the range 0..255) representing the hash. See hash_atom/2 to convert this into the more commonly seen hexadecimal representation. The conversion is controlled by Options: \begin{description} \termitem{algorithm}{+Algorithm} One of \const{sha1} (default), \const{sha224}, \const{sha256}, \const{sha384} or \const{sha512} \termitem{encoding}{+Encoding} This option defines the mapping from Prolog (Unicode) text to bytes on which the SHA algorithm is performed. It has two values. The defualt is \const{utf8}, which implies that Unicode text is encoded as UTF-8 bytes. This option can deal with any atom. The alternative is \const{octet}, which implies that the text is considered as a sequence of bytes. This is suitable for e.g., atoms that represent binary data. An error is raised if the text contains code-points outside the range 0..255. \end{description} \predicate{hmac_sha}{4}{+Key, +Data, -HMAC, +Options} Quoting \href{http://en.wikipedia.org/wiki/HMAC}{Wikipedia}: \textit{A keyed-hash message authentication code, or HMAC, is a type of message authentication code (MAC) calculated using a cryptographic hash function in combination with a secret key. As with any MAC, it may be used to simultaneously verify both the data integrity and the authenticity of a message. Any iterative cryptographic hash function, such as MD5 or SHA-1, may be used in the calculation of an HMAC; the resulting MAC algorithm is termed HMAC-MD5 or HMAC-SHA-1 accordingly. The cryptographic strength of the HMAC depends upon the cryptographic strength of the underlying hash function, on the size and quality of the key and the size of the hash output length in bits.''} \arg{Key} and \arg{Data} are either an atom, packed string or list of character codes. \arg{HMAC} is unified with a list of integers representing the authentication code. \arg{Options} is the same as for sha_hash/3, but currently only \const{sha1} and \const{sha256} are supported. \predicate{hash_atom}{2}{+Hash, -HexAtom} True when \arg{HexAtom} is the commonly used hexadecimal encoding of the hash code. E.g., \begin{code} ?- sha_hash('SWI-Prolog', Hash, []), hash_atom(Hash, Hex). Hash = [61, 128, 252, 38, 121, 69, 229, 85, 199|...], Hex = '3d80fc267945e555c730403bd0ab0716e2a68c68'. \end{code} \end{description} \subsection{License terms} \label{sec:sha-license} The underlying SHA-2 library is an unmodified copy created by Dr Brian Gladman, Worcester, UK. It is distributed under the license conditions below. The free distribution and use of this software in both source and binary form is allowed (with or without changes) provided that: \begin{enumerate} \item distributions of this source code include the above copyright notice, this list of conditions and the following disclaimer; \item distributions in binary form include the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other associated materials; \item the copyright holder's name is not used to endorse products built using this software without specific written permission. \end{enumerate} ALTERNATIVELY, provided that this notice is retained in full, this product may be distributed under the terms of the GNU General Public License (GPL), in which case the provisions of the GPL apply INSTEAD OF those given above. \input{md5.tex} \input{hashstream.tex} \section{Memory files} \label{sec:memory-files} The \pllib{memfile} provides an alternative to temporary files, intended for temporary buffering of data. Memory files in general are faster than temporary files and do not suffer from security risks or naming conflicts associated with temporary-file management. There is no limit to the number of memory streams, nor the size of them. However, a single memory file cannot have multiple streams at the same time, i.e., a memory file cannot be opened multiple times, not even for reading. Memory files are thread-safe and subject to (atom) garbage collection. These predicates are first of all intended for building higher-level primitives such as open_codes_stream/3. See also format/3, atom_to_term/3, term_to_atom/2, term_string/2, etc. \begin{description} \predicate{new_memory_file}{1}{-Handle} Create a new memory file and return a unique opaque handle to it. \predicate{free_memory_file}{1}{+Handle} Discard the memory file and its contents. If the file is open it is first closed. \predicate{open_memory_file}{3}{+Handle, +Mode, -Stream} Open the memory-file. \arg{Mode} is one of \const{read}, \const{write}, \const{append}, \const{update} or \const{insert}. The resulting \arg{Stream} must be closed using close/1. When opened for \const{update} or \const{insert}, the current location is initialized at the start of the data and can be modified using seek/2 or set_stream_position/2. In \const{update} mode, existing content is replaced, while the size is enlarged after hitting the end of the data. In \const{insert} mode, the new data is inserted at the current point. \predicate{open_memory_file}{4}{+Handle, +Mode, -Stream, +Options} Open a memory-file as open_memory_file/3. Options: \begin{description} \termitem{encoding}{+Encoding} Set the encoding for a memory file and the created stream. Encoding names are the same as used with open/4. By default, memoryfiles represent UTF-8 streams, making them capable of storing arbitrary Unicode text. In practice the only alternative is \const{octet}, turning the memoryfile into binary mode. Please study SWI-Prolog Unicode and encoding issues before using this option. \termitem{free_on_close}{+Bool} If \const{true} (default \const{false} and the memory file is opened for reading, discard the file (see free_memory_file/1) if the input is closed. This is used to realise open_chars_stream/2 in library(charsio). \end{description} \predicate{size_memory_file}{2}{+Handle, -Size} Return the content-length of the memory-file in characters in the current encoding of the memory file. The file should be closed and contain data. \predicate{size_memory_file}{3}{+Handle, -Size, +Encoding} Return the content-length of the memory-file in characters in the given \arg{Encoding}. The file should be closed and contain data. \predicate{atom_to_memory_file}{2}{+Atom, -Handle} Turn an atom into a read-only memory-file containing the (shared) characters of the atom. Opening this memory-file in mode \const{write} yields a permission error. \predicate{insert_memory_file}{3}{+Handle, +Offset, +Data} Insert \arg{Data} into the memory file at location \arg{Offset}. The offset is specified in characters. \arg{Data} can be an atom, string, code or character list. Other terms are first serialized using writeq/1. This predicate raises a domain_error exception if \arg{Offset} is out of range and a permission_error if the memory file is read-only or opened. \predicate{delete_memory_file}{3}{+Handle, +Offset, +Length} Delete a \arg{Length} characters from the memory file, starting at \arg{Offset}. This predicate raises a domain_error exception if \arg{Offset} or \arg{Offset+Length} is out of range and a permission_error if the memory file is read-only or opened. \predicate{memory_file_to_atom}{2}{+Handle, -Atom} Return the content of the memory-file in \arg{Atom}. \predicate{memory_file_to_atom}{3}{+Handle, -Atom, +Encoding} Return the content of the memory-file in \arg{Atom}, pretending the data is in the given \arg{Encoding}. This can be used to convert from one encoding into another, typically from/to bytes. For example, if we must convert a set of bytes that contain text in UTF-8, open the memory file as octet stream, fill it, and get the result using \arg{Encoding} is \const{utf8}. \predicate{memory_file_to_codes}{2}{+Handle, -Codes} Return the content of the memory-file as a list of character-codes in \arg{Codes}. \predicate{memory_file_to_codes}{3}{+Handle, -Codes, +Encoding} Return the content of the memory-file as a list of character-codes in \arg{Codes}, pretending the data is in the given \arg{Encoding}. \predicate{memory_file_to_string}{2}{+Handle, -String} Return the content of the memory-file as a string in \arg{-String}. \predicate{memory_file_to_string}{3}{+Handle, -String, +Encoding} Return the content of the memory-file as a string in \arg{String}, pretending the data is in the given \arg{Encoding}. \predicate{memory_file_substring}{5}{+Handle, ?Before, ?Length, ?After, -SubString} \arg{SubString} is a substring of the memory file. There are \arg{Before} characters in the memory file before \arg{SubString}, \arg{SubString} contains \arg{Length} character and is followed by \arg{After} characters in the memory file. The signature is the same as sub_string/5 and sub_atom/5, but currently at least two of the 3 position arguments must be specified. Future versions might implement the full functionality of sub_string/5. \predicate{memory_file_line_position}{4}{+MF, ?Line, ?LinePos, ?Offset} True if the character offset \arg{Offset} corresponds with the \arg{LinePos} character on line \arg{Line}. Lines are counted from one (1). Note that \arg{LinePos} is \emph{not} the \jargon{column} as each character counts for one, including backspace and tab. \end{description} \section{Time and alarm library} \label{sec:time-and-alarm} The \pllib{time} provides timing and alarm functions. Alarms are thread-specific, i.e., creating an alarm causes the alarm goal to be called in the thread that created it. The predicate current_alarm/4 only reports alarms that are related to the calling thread. If a thread terminates, all remaining alarms are silently removed. Most applications use call_with_time_limit/2. \begin{description} \predicate{alarm}{4}{+Time, :Callable, -Id, +Options} Schedule \arg{Callable} to be called \arg{Time} seconds from now. \arg{Time} is a number (integer or float). \arg{Callable} is called on the next pass through a call- or redo-port of the Prolog engine, or a call to the PL_handle_signals() routine from SWI-Prolog. \arg{Id} is unified with a reference to the timer. The resolution of the alarm depends on the underlying implementation, which is based on pthread_cond_timedwait() (on Windows on the pthread emulation thereof). Long-running foreign predicates that do not call PL_handle_signals() may further delay the alarm. The relation to blocking system calls (sleep, reading from slow devices, etc.) is undefined and varies between implementations. \arg{Options} is a list of \term{\arg{Name}}{Value} terms. Defined options are: \begin{description} \termitem{remove}{Bool} If \const{true} (default \const{false}), the timer is removed automatically after fireing. Otherwise it must be destroyed explicitly using remove_alarm/1. \termitem{install}{Bool} If \const{false} (default \const{true}), the timer is allocated but not scheduled for execution. It must be started later using install_alarm/1. \end{description} \predicate{alarm}{3}{+Time, :Callable, -Id} Same as \term{alarm}{Time, Callable, Id, []}. \predicate{alarm_at}{4}{+Time, :Callable, -Id, +Options} as alarm/3, but \arg{Time} is the specification of an absolute point in time. Absolute times are specified in seconds after the Jan 1, 1970 epoch. See also date_time_stamp/2. \predicate{install_alarm}{1}{+Id} Activate an alarm allocated using alarm/4 with the option \term{install}{false} or stopped using uninstall_alarm/1. \predicate{install_alarm}{2}{+Id, +Time} As install_alarm/1, but specifies a new (relative) timeout value. \predicate{uninstall_alarm}{1}{+Id} Deactivate a running alarm, but do not invalidate the alarm identifier. Later, the alarm can be reactivated using either install_alarm/1 or install_alarm/2. Reinstalled using install_alarm/1, it will fire at the originally scheduled time. Reinstalled using install_alarm/2 causes the alarm to fire at the specified time from now. \predicate{remove_alarm}{1}{+Id} Remove an alarm. If it is not yet fired, it will not be fired any more. \predicate{current_alarm}{4}{?At, ?:Callable, ?Id, ?Status} Enumerate the not-yet-removed alarms. \arg{Status} is one of \const{done} if the alarm has been called, \const{next} if it is the next to be fired and \arg{scheduled} otherwise. \predicate{call_with_time_limit}{2}{+Time, :Goal} True if \arg{Goal} completes within \arg{Time} seconds. \arg{Goal} is executed as in once/1. If \arg{Goal} doesn't complete within \arg{Time} seconds (wall time), exit using the exception \const{time_limit_exceeded}. See catch/3. Please note that this predicate uses alarm/4 and therefore its effect on long-running foreign code and system calls is undefined. Blocking I/O can be handled using the timeout option of read_term/3. \end{description} \InputIfFileExists{unix.tex}{}{} \section{Limiting process resources} \label{sec:limit-process-resources} The \pllib{rlimit} library provides an interface to the POSIX getrlimit()/setrlimit() API that control the maximum resource-usage of a process or group of processes. This call is especially useful for servers such as CGI scripts and inetd-controlled servers to avoid an uncontrolled script claiming too much resources. \begin{description} \predicate{rlimit}{3}{+Resource, -Old, +New} Query and/or set the limit for \arg{Resource}. Time-values are in seconds and size-values are counted in bytes. The following values are supported by this library. Please note that not all resources may be available and accessible on all platforms. This predicate can throw a variety of exceptions. In portable code this should be guarded with catch/3. The defined resources are: \begin{quote} \begin{tabular}{ll} \const{as} & Max address space \\ \const{cpu} & CPU time in seconds \\ \const{fsize} & Maximum filesize \\ \const{data} & max data size \\ \const{stack} & max stack size \\ \const{core} & max core file size \\ \const{rss} & max resident set size \\ \const{nproc} & max number of processes \\ \const{nofile} & max number of open files \\ \const{memlock} & max locked-in-memory address \\ \end{tabular} \end{quote} When the process hits a limit POSIX systems normally send the process a signal that terminates it. These signals may be caught using SWI-Prolog's on_signal/3 primitive. The code below illustrates this behaviour. Please note that asynchronous signal handling is dangerous, especially when using threads. 100\% fail-safe operation cannot be guaranteed, but this procedure will inform the user properly most of the time'. \begin{code} rlimit_demo :- rlimit(cpu, _, 2), on_signal(xcpu, _, cpu_exceeded), ( repeat, fail ). cpu_exceeded(_Sig) :- format(user_error, 'CPU time exceeded~n', []), halt(1). \end{code} \end{description} \input{udpbroadcast.tex} \input{prologstream.tex} \section*{NetBSD Crypt license} \begin{code} * Copyright (c) 1989, 1993 * The Regents of the University of California. All rights reserved. * * This code is derived from software contributed to Berkeley by * Tom Truscott. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * 3. Neither the name of the University nor the names of its contributors * may be used to endorse or promote products derived from this software * without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS AS IS'' AND * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. \end{code} \printindex \end{document}