D7net
Home
Console
Upload
information
Create File
Create Folder
About
Tools
:
/
proc
/
self
/
root
/
opt
/
alt
/
ruby34
/
share
/
ri
/
system
/
Kernel
/
Filename :
select-i.ri
back
Copy
U:RDoc::AnyMethod[iI" select:ETI"Kernel#select;TF: publico:RDoc::Markup::Document: @parts[6o: RDoc::Markup::Paragraph; [ I"JInvokes system call {select(2)}[https://linux.die.net/man/2/select], ;TI"/which monitors multiple file descriptors, ;TI"7waiting until one or more of the file descriptors ;TI"3becomes ready for some class of I/O operation.;To: RDoc::Markup::BlankLine o; ; [I"&Not implemented on all platforms.;T@o; ; [I"DEach of the arguments +read_ios+, +write_ios+, and +error_ios+ ;TI" is an array of IO objects.;T@o; ; [I"NArgument +timeout+ is a numeric value (such as integer or float) timeout ;TI"interval in seconds.;T@o; ; [I"DThe method monitors the \IO objects given in all three arrays, ;TI"#waiting for some to be ready; ;TI"2returns a 3-element array whose elements are:;T@o:RDoc::Markup::List: @type: BULLET: @items[o:RDoc::Markup::ListItem: @label0; [o; ; [I"FAn array of the objects in +read_ios+ that are ready for reading.;To;;0; [o; ; [I"GAn array of the objects in +write_ios+ that are ready for writing.;To;;0; [o; ; [I"DAn array of the objects in +error_ios+ have pending exceptions.;T@o; ; [I"NIf no object becomes ready within the given +timeout+, +nil+ is returned.;T@o; ; [ I"I\IO.select peeks the buffer of \IO objects for testing readability. ;TI"EIf the \IO buffer is not empty, \IO.select immediately notifies ;TI"Jreadability. This "peek" only happens for \IO objects. It does not ;TI"@happen for IO-like objects such as OpenSSL::SSL::SSLSocket.;T@o; ; [ I"FThe best way to use \IO.select is invoking it after non-blocking ;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"Hwith \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"KEspecially, the combination of non-blocking 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"EThis 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"Gsome 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"KThe combination of non-blocking 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"8see {select(2)}[https://linux.die.net/man/2/select];T@o; ; [I"DInvoking \IO.select before IO#readpartial works well as usual. ;TI"6However 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">So, <tt>IO#write(two or more bytes)</tt> can block after ;TI"Kwritability 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;0o; ; [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" Output:;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"XIO.select(read_ios, write_ios = [], error_ios = [], timeout = nil) -> array or nil ;T0[ I"$(p1, p2 = v2, p3 = v3, p4 = v4);T@�FI" Kernel;TcRDoc::NormalModule00