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select-i.ri
U:RDoc::AnyMethod[iI" select:ETI"Kernel#select;TF: publico:RDoc::Markup::Document: @parts[-o: RDoc::Markup::Paragraph; [ I""Calls select(2) system call. ;TI"HIt monitors given arrays of IO objects, waits until one or more of ;TI"GIO objects are ready for reading, are ready for writing, and have ;TI"Ipending exceptions respectively, and returns an array that contains ;TI"Carrays of those IO objects. It will return +nil+ if optional ;TI"@<i>timeout</i> value is given and no IO object is ready in ;TI" <i>timeout</i> seconds.;To: RDoc::Markup::BlankLine o; ; [ I"GIO.select peeks the buffer of IO objects for testing readability. ;TI"CIf the IO buffer is not empty, IO.select immediately notifies ;TI"Ireadability. This "peek" only happens for IO objects. It does not ;TI"@happen for IO-like objects such as OpenSSL::SSL::SSLSocket.;T@o; ; [ I"DThe best way to use IO.select is invoking it after nonblocking ;TI"Hmethods such as #read_nonblock, #write_nonblock, etc. The methods ;TI"Araise an exception which is extended by IO::WaitReadable or ;TI"FIO::WaitWritable. The modules notify how the caller should wait ;TI"Gwith IO.select. If IO::WaitReadable is raised, the caller should ;TI"Iwait for reading. If IO::WaitWritable is raised, the caller should ;TI"wait for writing.;T@o; ; [I"<So, blocking read (#readpartial) can be emulated using ;TI"-#read_nonblock and IO.select as follows:;T@o:RDoc::Markup::Verbatim; [I" begin ;TI". result = io_like.read_nonblock(maxlen) ;TI" rescue IO::WaitReadable ;TI" IO.select([io_like]) ;TI" retry ;TI" rescue IO::WaitWritable ;TI"! IO.select(nil, [io_like]) ;TI" retry ;TI" end ;T: @format0o; ; [ I"IEspecially, the combination of nonblocking methods and IO.select is ;TI"Hpreferred for IO like objects such as OpenSSL::SSL::SSLSocket. It ;TI"Hhas #to_io method to return underlying IO object. IO.select calls ;TI"2#to_io to obtain the file descriptor to wait.;T@o; ; [I"DThis means that readability notified by IO.select doesn't mean ;TI"5readability from OpenSSL::SSL::SSLSocket object.;T@o; ; [I"GThe most likely situation is that OpenSSL::SSL::SSLSocket buffers ;TI"Esome data. IO.select doesn't see the buffer. So IO.select can ;TI"Bblock when OpenSSL::SSL::SSLSocket#readpartial doesn't block.;T@o; ; [I"8However, several more complicated situations exist.;T@o; ; [ I"5SSL is a protocol which is sequence of records. ;TI",The record consists of multiple bytes. ;TI"BSo, the remote side of SSL sends a partial record, IO.select ;TI"Gnotifies readability but OpenSSL::SSL::SSLSocket cannot decrypt a ;TI"=byte and OpenSSL::SSL::SSLSocket#readpartial will block.;T@o; ; [ I"FAlso, the remote side can request SSL renegotiation which forces ;TI".the local SSL engine to write some data. ;TI"FThis means OpenSSL::SSL::SSLSocket#readpartial may invoke #write ;TI"#system call and it can block. ;TI"GIn such a situation, OpenSSL::SSL::SSLSocket#read_nonblock raises ;TI"+IO::WaitWritable instead of blocking. ;TI"CSo, the caller should wait for ready for writability as above ;TI" example.;T@o; ; [I"IThe combination of nonblocking methods and IO.select is also useful ;TI"Ifor streams such as tty, pipe socket socket when multiple processes ;TI"read from a stream.;T@o; ; [ I";Finally, Linux kernel developers don't guarantee that ;TI"Jreadability of select(2) means readability of following read(2) even ;TI"for a single process. ;TI".See select(2) manual on GNU/Linux system.;T@o; ; [I"CInvoking IO.select before IO#readpartial works well as usual. ;TI"5However it is not the best way to use IO.select.;T@o; ; [ I"8The writability notified by select(2) doesn't show ;TI""how many bytes are writable. ;TI"AIO#write method blocks until given whole string is written. ;TI"BSo, <code>IO#write(two or more bytes)</code> can block after ;TI"Jwritability is notified by IO.select. IO#write_nonblock is required ;TI"to avoid the blocking.;T@o; ; [I"GBlocking write (#write) can be emulated using #write_nonblock and ;TI"GIO.select as follows: IO::WaitReadable should also be rescued for ;TI"2SSL renegotiation in OpenSSL::SSL::SSLSocket.;T@o; ; [I" while 0 < string.bytesize ;TI" begin ;TI"2 written = io_like.write_nonblock(string) ;TI" rescue IO::WaitReadable ;TI" IO.select([io_like]) ;TI" retry ;TI" rescue IO::WaitWritable ;TI"# IO.select(nil, [io_like]) ;TI" retry ;TI" end ;TI". string = string.byteslice(written..-1) ;TI" end ;T; 0S:RDoc::Markup::Heading: leveli: textI"Parameters;To:RDoc::Markup::List: @type: NOTE: @items[ o:RDoc::Markup::ListItem: @label[I"read_array;T; [o; ; [I":an array of IO objects that wait until ready for read;To;;[I"write_array;T; [o; ; [I";an array of IO objects that wait until ready for write;To;;[I"error_array;T; [o; ; [I"4an array of IO objects that wait for exceptions;To;;[I" timeout;T; [o; ; [I" a numeric value in second;T@S;;i;I" Example;T@o; ; [I"rp, wp = IO.pipe ;TI"mesg = "ping " ;TI"100.times { ;TI"H # IO.select follows IO#read. Not the best way to use IO.select. ;TI"' rs, ws, = IO.select([rp], [wp]) ;TI" if r = rs[0] ;TI" ret = r.read(5) ;TI" print ret ;TI" case ret ;TI" when /ping/ ;TI" mesg = "pong\n" ;TI" when /pong/ ;TI" mesg = "ping " ;TI" end ;TI" end ;TI" if w = ws[0] ;TI" w.write(mesg) ;TI" end ;TI"} ;T; 0o; ; [I"<em>produces:</em>;T@o; ; [ I"ping pong ;TI"ping pong ;TI"ping pong ;TI"(snipped) ;TI" ping;T; 0: @fileI" io.c;T:0@omit_headings_from_table_of_contents_below0I"WIO.select(read_array [, write_array [, error_array [, timeout]]]) -> array or nil ;T0[ I"$(p1, p2 = v2, p3 = v3, p4 = v4);T@¼FI" Kernel;TcRDoc::NormalModule00