BidirMMapPipe Class Reference

BidirMMapPipe creates a bidirectional channel between the current process and a child it forks.

Author

Manuel Schiller manue.nosp@m.l.sc.nosp@m.hille.nosp@m.r@ni.nosp@m.khef..nosp@m.nl

Date

2013-07-07

This class creates a bidirectional channel between this process and a child it creates with fork().

The channel is comrised of a small shared pool of buffer memory mmapped into both process spaces, and two pipes to synchronise the exchange of data. The idea behind using the pipes at all is to have some primitive which we can block on without having to worry about atomic operations or polling, leaving these tasks to the OS. In case the anonymous mmap cannot be performed on the OS the code is running on (for whatever reason), the code falls back to mmapping /dev/zero, mmapping a temporary file, or (if those all fail), a dynamically allocated buffer which is then transmitted through the pipe(s), a slightly slower alternative (because the data is copied more often).

The channel supports five major operations: read(), write(), flush(), purge() and close(). Reading and writing may block until the required buffer space is available. Writes may queue up data to be sent to the other end until either enough pages are full, or the user calls flush which forces any unsent buffers to be sent to the other end. flush forces any data that is to be sent to be sent. purge discards any buffered data waiting to be read and/or sent. Closing the channel on the child returns zero, closing it on the parent returns the child's exit status.

The class also provides operator<< and operator>> for C++-style I/O for basic data types (bool, char, short, int, long, long long, float, double and their unsigned counterparts). Data is transmitted binary (i.e. no formatting to strings like std::cout does). There are also overloads to support C-style zero terminated strings and std::string. In terms of performance, the former is to be preferred.

If the caller needs to multiplex input and output to/from several pipes, the class provides the poll() method which allows to block until an event occurs on any of the polled pipes.

After the BidirMMapPipe is closed, no further operations may be performed on that object, save for the destructor which may still be called.

If the BidirMMapPipe has not properly been closed, the destructor will call close. However, the exit code of the child is lost in that case.

Closing the object causes the mmapped memory to be unmapped and the two pipes to be closed. We also install an atexit handler in the process of creating BidirMMapPipes. This ensures that when the current process terminates, a SIGTERM signal is sent to the child processes created for all unclosed pipes to avoid leaving zombie processes in the OS's process table.

BidirMMapPipe creation, closing and destruction are thread safe. If the BidirMMapPipe is used in more than one thread, the other operations have to be protected with a mutex (or something similar), though.

End of file (other end closed its pipe, or died) is indicated with the eof() method, serious I/O errors set a flags (bad(), fail()), and also throw exceptions. For normal read/write operations, they can be suppressed (i.e. error reporting only using flags) with a constructor argument.

Technicalities:

• there is a pool of mmapped pages, half the pages are allocated to the parent process, half to the child
• when one side has accumulated enough data (or a flush forces dirty pages out to the other end), it sends these pages to the other end by writing a byte containing the page number into the pipe
• the other end (which has the pages mmapped, too) reads the page number(s) and puts the corresponding pages on its busy list
• as the other ends reads, it frees busy pages, and eventually tries to put them on the its list; if a page belongs to the other end of the connection, it is sent back
• lists of pages are sent across the pipe, not individual pages, in order to minimise the number of read/write operations needed
• when mmap works properly, only one bytes containing the page number of the page list head is sent back and forth; the contents of that page allow to access the rest of the page list sent, and page headers on the list tell the receiving end if the page is free or has to be added to the busy list
• when mmap does not work, we transfer one byte to indicate the head of the page list sent, and for each page on the list of sent pages, the page header and the page payload is sent (if the page is free, we only transmit the page header, and we never transmit more payload than the page actually contains)
• in the child, all open BidirMMapPipes but the current one are closed. this is done for two reasons: first, to conserve file descriptors and address space. second, if more than one process is meant to use such a BidirMMapPipe, synchronisation issues arise which can lead to bugs that are hard to find and understand. it's much better to come up with a design which does not need pipes to be shared among more than two processes.

Here is a trivial example of a parent and a child talking to each other over a BidirMMapPipe:

#include <string>
#include <iostream>
#include <cstdlib>
 
#include "BidirMMapPipe.h"
 
int simplechild(BidirMMapPipe& pipe)
{
    // child does an echo loop
    while (pipe.good() && !pipe.eof()) {
        // read a string
        std::string str;
        pipe >> str;
        if (!pipe) return -1;
        if (pipe.eof()) break;
        // check if parent wants us to shut down
        if (!str.empty()) {
            std::cout << "[CHILD] :  read: " << str << std::endl;
            str = "... early in the morning?";
        }
        pipe << str << BidirMMapPipe::flush;
        if (str.empty()) break;
        if (!pipe) return -1;
        std::cout << "[CHILD] : wrote: " << str << std::endl;
    }
    // send shutdown request acknowledged
    pipe << "" << BidirMMapPipe::flush;
 
    pipe.close();
    return 0;
}
 
BidirMMapPipe* spawnChild(int (*childexec)(BidirMMapPipe&))
{
    BidirMMapPipe *p = new BidirMMapPipe();
    if (p->isChild()) {
        int retVal = childexec(*p);
        delete p;
        std::exit(retVal);
    }
    return p;
}
 
int main()
{
    std::cout << "[PARENT]: simple challenge-response test, one child:" <<
            std::endl;
    BidirMMapPipe* pipe = spawnChild(simplechild);
    for (int i = 0; i < 5; ++i) {
        std::string str("What shall we do with a drunken sailor...");
        *pipe << str << BidirMMapPipe::flush;
        if (!*pipe) return -1;
        std::cout << "[PARENT]: wrote: " << str << std::endl;
        *pipe >> str;
        if (!*pipe) return -1;
        std::cout << "[PARENT]:  read: " << str << std::endl;
    }
    // ask child to shut down
    pipe << "" << BidirMMapPipe::flush;
    // wait for it to see the shutdown request
    std::string s;
    pipe >> s;
    std::cout << "[PARENT]: exit status of child: " << pipe->close() <<
            std::endl;
    delete pipe;
    return 0;
}

When designing your own protocols to use over the pipe, there are a few things to bear in mind:

• Do as http does: When building a request, send all the options and properties of that request with the request itself in a single go (one flush). Then, the server has everything it needs, and hopefully, it'll shut up for a while and to let the client do something useful in the meantime... The same goes when the server replies to the request: include everything there is to know about the result of the request in the reply.
• The expensive operation should be the request that is made, all other operations should somehow be formulated as options or properties to that request.
• Include a shutdown handshake in whatever protocol you send over the pipe. That way, you can shut things down in a controlled way. Otherwise, and depending on your OS's scheduling quirks, you may catch a SIGPIPE if one end closes its pipe while the other is still trying to read.

Definition at line 380 of file BidirMMapPipe.h.

Classes
class	PollEntry
	for poll() interface More...

Public Types
enum	{   eofbit = 1 , failbit = 2 , rderrbit = 4 , wrerrbit = 8 ,   badbit = rderrbit | wrerrbit , exceptionsbit = 16 }
	flag bits for partial C++ iostream compatibility More...
typedef BidirMMapPipe_impl::BidirMMapPipeException	Exception
	convenience typedef for BidirMMapPipeException
enum	PollFlags {   None = 0 , Readable = 1 , Writable = 2 , ReadError = 4 ,   WriteError = 8 , Error = ReadError | WriteError , ReadEndOfFile = 32 , WriteEndOfFile = 64 ,   EndOfFile = ReadEndOfFile | WriteEndOfFile , ReadInvalid = 64 , WriteInvalid = 128 , Invalid = ReadInvalid | WriteInvalid }
	condition flags for poll More...
typedef std::vector< PollEntry >	PollVector
	convenience typedef for poll() interface
typedef std::size_t	size_type
	type used to represent sizes

Public Member Functions
	BidirMMapPipe (bool useExceptions=true, bool useSocketpair=false)
	constructor (forks!)
	~BidirMMapPipe ()
	destructor
bool	bad () const
	true on I/O error
size_type	bytesReadableNonBlocking ()
	number of bytes that can be read without blocking
size_type	bytesWritableNonBlocking ()
	number of bytes that can be written without blocking
int	close ()
	flush buffers, close pipe
bool	closed () const
	true if closed
bool	eof () const
	true if end-of-file
bool	fail () const
	logical failure (e.g.
void	flush ()
	flush buffers with unwritten data
bool	good () const
	status of stream is good
bool	isChild () const
	return if this end of the pipe is the child end
bool	isParent () const
	return if this end of the pipe is the parent end
	operator bool () const
	return true if not serious error (fail/bad)
bool	operator! () const
	return true if serious error (fail/bad)
BidirMMapPipe &	operator<< (BidirMMapPipe &(*manip)(BidirMMapPipe &))
	I/O manipulator support.
BidirMMapPipe &	operator<< (const char *str)
	write a C-style string
BidirMMapPipe &	operator<< (const std::string &str)
	write a std::string object
template<class T >
BidirMMapPipe &	operator<< (const T *tptr)
	write raw pointer to T to other side
BidirMMapPipe &	operator>> (BidirMMapPipe &(*manip)(BidirMMapPipe &))
	I/O manipulator support.
BidirMMapPipe &	operator>> (char *(&str))
	read a C-style string
BidirMMapPipe &	operator>> (std::string &str)
	read a std::string object
template<class T >
BidirMMapPipe &	operator>> (T *&tptr)
	read raw pointer to T from other side
pid_t	pidOtherEnd () const
	return PID of the process on the other end of the pipe
void	purge ()
	purge buffered data waiting to be read and/or written
int	rdstate () const
	return flags (end of file, BidirMMapPipe closed, ...)
size_type	read (void *addr, size_type sz)
	read from pipe
	STREAMOP (bool)
	C++ style stream operators for bool.
	STREAMOP (char)
	C++ style stream operators for char.
	STREAMOP (double)
	C++ style stream operators for double.
	STREAMOP (float)
	C++ style stream operators for float.
	STREAMOP (int)
	C++ style stream operators for int.
	STREAMOP (long long)
	C++ style stream operators for long long.
	STREAMOP (long)
	C++ style stream operators for long.
	STREAMOP (short)
	C++ style stream operators for short.
	STREAMOP (unsigned char)
	C++ style stream operators for unsigned char.
	STREAMOP (unsigned int)
	C++ style stream operators for unsigned int.
	STREAMOP (unsigned long long)
	C++ style stream operators for unsigned long long.
	STREAMOP (unsigned long)
	C++ style stream operators for unsigned long.
	STREAMOP (unsigned short)
	C++ style stream operators for unsigned short.
bool	usesPipepair () const
	if BidirMMapPipe uses a pipe pair for communications
bool	usesSocketpair () const
	if BidirMMapPipe uses a socketpair for communications
size_type	write (const void *addr, size_type sz)
	wirte to pipe

Static Public Member Functions
static int	debugflag ()
	return the current setting of the debug flag
static BidirMMapPipe &	flush (BidirMMapPipe &pipe)
	for usage a la "pipe << flush;"
static int	poll (PollVector &pipes, int timeout)
	poll a set of pipes for events (ready to read from, ready to write to, error)
static BidirMMapPipe &	purge (BidirMMapPipe &pipe)
	for usage a la "pipe << purge;"
static void	setDebugflag (int flag)
	set the debug flags

Private Types
enum	{ TotPages = 16 , PagesPerEnd = TotPages / 2 , FlushThresh = (3 * PagesPerEnd) / 4 }
	tuning constants More...
typedef BidirMMapPipe_impl::Page	Page
	convenience typedef for Page

Private Member Functions
	BidirMMapPipe (const BidirMMapPipe &)
	copy-construction forbidden
Page *	busypage ()
	get a busy page to read data from (may block)
Page *	dirtypage ()
	get a dirty page to write data to (may block)
int	doClose (bool force, bool holdlock=false)
	close the pipe (no flush if forced)
void	doFlush (bool forcePartialPages=true)
	perform the flush
void	feedPageLists (Page *plist)
	"feed" the busy and free lists with a list of pages
void	markPageDirty (Page *p)
	put on dirty pages list
BidirMMapPipe &	operator= (const BidirMMapPipe &)
	assignment forbidden
unsigned	recvpages ()
	receive a pages from the other end (may block), queue them
unsigned	recvpages_nonblock ()
	receive pages from other end (non-blocking)
void	sendpages (Page *plist)
	send page(s) to the other end (may block)

Static Private Member Functions
static unsigned	lenPageList (const Page *list)
	return length of a page list
static BidirMMapPipe_impl::PagePool &	pagepool ()
	return page pool
static void	teardownall (void)
	cleanup routine - at exit, we want our children to get a SIGTERM...
static size_type	xferraw (int fd, void *addr, const size_type len, ssize_t(*xferfn)(int, const void *, std::size_t))
	transfer bytes through the pipe (reading, writing, may block)
static size_type	xferraw (int fd, void *addr, size_type len, ssize_t(*xferfn)(int, void *, std::size_t))
	transfer bytes through the pipe (reading, writing, may block)

Private Attributes
Page *	m_busylist
	linked list: busy pages (data to be read)
pid_t	m_childPid
	pid of the child (zero if we're child)
Page *	m_dirtylist
	linked list: dirty pages (data to be sent)
int	m_flags
	flags (e.g. end of file)
Page *	m_freelist
	linked list: free pages
int	m_inpipe
	pipe end from which data may be read
int	m_outpipe
	pipe end to which data may be written
BidirMMapPipe_impl::Pages	m_pages
	mmapped pages
pid_t	m_parentPid
	pid of the parent

Static Private Attributes
static int	s_debugflag = 0
	debug flag
static std::list< BidirMMapPipe * >	s_openpipes
	list of open BidirMMapPipes
static pthread_mutex_t	s_openpipesmutex = PTHREAD_MUTEX_INITIALIZER
	protects s_openpipes
static BidirMMapPipe_impl::PagePool *	s_pagepool = 0
	pool of mmapped pages
static unsigned	s_pagepoolrefcnt = 0
	page pool reference counter

Friends
class	BidirMMapPipe_impl::Page
	page is our friend

#include </home/sftnight/build/workspace/root-makedoc-v626/rootspi/rdoc/src/v6-26-00-patches/roofit/roofitcore/src/BidirMMapPipe.h>

Member Typedef Documentation

Exception

typedef BidirMMapPipe_impl::BidirMMapPipeException BidirMMapPipe::Exception

convenience typedef for BidirMMapPipeException

Definition at line 386 of file BidirMMapPipe.h.

Page

typedef BidirMMapPipe_impl::Page BidirMMapPipe::Page

private

convenience typedef for Page

Definition at line 870 of file BidirMMapPipe.h.

PollVector

typedef std::vector<PollEntry> BidirMMapPipe::PollVector

convenience typedef for poll() interface

Definition at line 537 of file BidirMMapPipe.h.

size_type

typedef std::size_t BidirMMapPipe::size_type

type used to represent sizes

Definition at line 384 of file BidirMMapPipe.h.

Member Enumeration Documentation

anonymous enum

anonymous enum

flag bits for partial C++ iostream compatibility

Enumerator
eofbit	end of file reached
failbit	logical failure (e.g. pipe closed)
rderrbit	read error
wrerrbit	write error
badbit	general I/O error
exceptionsbit	error reporting with exceptions

Definition at line 388 of file BidirMMapPipe.h.

anonymous enum

anonymous enum

private

tuning constants

Enumerator
TotPages	pages shared (child + parent)
PagesPerEnd	pages per pipe end
FlushThresh	flush threshold

Definition at line 873 of file BidirMMapPipe.h.

PollFlags

enum BidirMMapPipe::PollFlags

condition flags for poll

Enumerator
None	nothing special on this pipe
Readable	pipe has data for reading
Writable	pipe can be written to
ReadError	pipe error read end
WriteError	pipe error Write end
Error	pipe error
ReadEndOfFile	read pipe in end-of-file state
WriteEndOfFile	write pipe in end-of-file state
EndOfFile	end of file
ReadInvalid	read end of pipe invalid
WriteInvalid	write end of pipe invalid
Invalid	invalid pipe

Definition at line 508 of file BidirMMapPipe.h.

Constructor & Destructor Documentation

BidirMMapPipe() [1/2]

BidirMMapPipe::BidirMMapPipe	(	bool	useExceptions = true,
		bool	useSocketpair = false
	)

constructor (forks!)

Creates a bidirectional communications channel between this process and a child the constructor forks. On return from the constructor, isParent() and isChild() can be used to tell the parent end from the child end of the pipe. In the child, all other open BidirMMapPipes are closed.

Parameters

useExceptions	read()/write() error reporting also done using exceptions
useSocketpair	use a socketpair instead of a pair or pipes

Normally, exceptions are thrown for all serious I/O errors (apart from end of file). Setting useExceptions to false will force the read() and write() methods to only report serious I/O errors using flags.

When useSocketpair is true, use a pair of Unix domain sockets created using socketpair instead a pair of pipes. The advantage is that only one pair of file descriptors is needed instead of two pairs which are needed for the pipe pair. Performance should very similar on most platforms, especially if mmap works, since only very little data is sent through the pipe(s)/socketpair.

Definition at line 750 of file BidirMMapPipe.cxx.

~BidirMMapPipe()

BidirMMapPipe::~BidirMMapPipe

(

)

destructor

closes this end of pipe

Definition at line 996 of file BidirMMapPipe.cxx.

BidirMMapPipe() [2/2]

BidirMMapPipe::BidirMMapPipe ( const BidirMMapPipe &  )

private

copy-construction forbidden

Definition at line 739 of file BidirMMapPipe.cxx.

Member Function Documentation

bad()

bool BidirMMapPipe::bad ( ) const

inline

true on I/O error

Returns

true on I/O error

Definition at line 725 of file BidirMMapPipe.h.

busypage()

BidirMMapPipe::Page * BidirMMapPipe::busypage ( )

private

get a busy page to read data from (may block)

Definition at line 1241 of file BidirMMapPipe.cxx.

bytesReadableNonBlocking()

BidirMMapPipe::size_type BidirMMapPipe::bytesReadableNonBlocking

(

)

number of bytes that can be read without blocking

Returns

number of bytes that can be read without blocking

Definition at line 1306 of file BidirMMapPipe.cxx.

bytesWritableNonBlocking()

BidirMMapPipe::size_type BidirMMapPipe::bytesWritableNonBlocking

(

)

number of bytes that can be written without blocking

Returns

number of bytes that can be written without blocking

Definition at line 1317 of file BidirMMapPipe.cxx.

close()

int BidirMMapPipe::close

(

)

flush buffers, close pipe

Flush buffers, discard unread data, closes the pipe. If the pipe is in the parent process, it waits for the child.

Returns

exit code of child process in parent, zero in child

Definition at line 910 of file BidirMMapPipe.cxx.

closed()

bool BidirMMapPipe::closed ( ) const

inline

true if closed

Returns

true if stream is closed

Definition at line 737 of file BidirMMapPipe.h.

debugflag()

static int BidirMMapPipe::debugflag ( )

inlinestatic

return the current setting of the debug flag

Returns

an integer with the debug Setting

Definition at line 433 of file BidirMMapPipe.h.

dirtypage()

BidirMMapPipe::Page * BidirMMapPipe::dirtypage ( )

private

get a dirty page to write data to (may block)

Definition at line 1252 of file BidirMMapPipe.cxx.

doClose()

int BidirMMapPipe::doClose ( bool  force, bool  holdlock = false  )

private

close the pipe (no flush if forced)

Definition at line 916 of file BidirMMapPipe.cxx.

doFlush()

void BidirMMapPipe::doFlush ( bool  forcePartialPages = true )

private

perform the flush

Definition at line 1270 of file BidirMMapPipe.cxx.

eof()

bool BidirMMapPipe::eof ( ) const

inline

true if end-of-file

Returns

true if end-of-file

Definition at line 713 of file BidirMMapPipe.h.

fail()

bool BidirMMapPipe::fail ( ) const

inline

logical failure (e.g.

I/O on closed BidirMMapPipe)

Returns

true in case of grave logical error (I/O on closed pipe,...)

Definition at line 719 of file BidirMMapPipe.h.

feedPageLists()

void BidirMMapPipe::feedPageLists ( Page *  plist )

private

"feed" the busy and free lists with a list of pages

Parameters

plist

linked list of pages

goes through plist, puts free pages from plist onto the freelist (or sends them to the remote end if they belong there), and puts non-empty pages on plist onto the busy list

Definition at line 1131 of file BidirMMapPipe.cxx.

flush() [1/2]

void BidirMMapPipe::flush

(

)

flush buffers with unwritten data

This forces unwritten data to be written to the other end. The call will block until this has been done (or the attempt failed with an error).

Definition at line 1267 of file BidirMMapPipe.cxx.

flush() [2/2]

static BidirMMapPipe & BidirMMapPipe::flush ( BidirMMapPipe &  pipe )

inlinestatic

for usage a la "pipe << flush;"

Definition at line 857 of file BidirMMapPipe.h.

good()

bool BidirMMapPipe::good ( ) const

inline

status of stream is good

Returns

true if pipe is good (no errors, eof, ...)

Definition at line 731 of file BidirMMapPipe.h.

isChild()

bool BidirMMapPipe::isChild ( ) const

inline

return if this end of the pipe is the child end

Returns

true if child end of pipe

Definition at line 689 of file BidirMMapPipe.h.

isParent()

bool BidirMMapPipe::isParent ( ) const

inline

return if this end of the pipe is the parent end

Returns

true if parent end of pipe

Definition at line 683 of file BidirMMapPipe.h.

lenPageList()

unsigned BidirMMapPipe::lenPageList ( const Page *  list )

staticprivate

return length of a page list

Definition at line 1124 of file BidirMMapPipe.cxx.

markPageDirty()

void BidirMMapPipe::markPageDirty ( Page *  p )

private

put on dirty pages list

Definition at line 1227 of file BidirMMapPipe.cxx.

operator bool()

BidirMMapPipe::operator bool ( ) const

inline

return true if not serious error (fail/bad)

Returns

true if stream is does not have serious error (fail/bad)

(if EOF, this is still true)

Definition at line 745 of file BidirMMapPipe.h.

operator!()

bool BidirMMapPipe::operator! ( ) const

inline

return true if serious error (fail/bad)

Returns

true if stream has a serious error (fail/bad)

Definition at line 751 of file BidirMMapPipe.h.

operator<<() [1/4]

BidirMMapPipe & BidirMMapPipe::operator<< ( BidirMMapPipe &(*)(BidirMMapPipe &)  manip )

inline

I/O manipulator support.

Parameters

manip

manipulator

Returns

pipe with manipulator applied

example:

pipe << BidirMMapPipe::flush;

Definition at line 840 of file BidirMMapPipe.h.

operator<<() [2/4]

BidirMMapPipe & BidirMMapPipe::operator<<

(

const char *

str

)

write a C-style string

Parameters

str	C-style string

Returns

pipe written to

Definition at line 1570 of file BidirMMapPipe.cxx.

operator<<() [3/4]

BidirMMapPipe & BidirMMapPipe::operator<<

(

const std::string &

str

)

write a std::string object

Parameters

str	string to write

Returns

pipe written to

Definition at line 1591 of file BidirMMapPipe.cxx.

operator<<() [4/4]

template<class T >

BidirMMapPipe & BidirMMapPipe::operator<< ( const T *  tptr )

inline

write raw pointer to T to other side

NOTE: This will not write the pointee! Only the value of the pointer is transferred.

Parameters

tptr	pointer to be written

Returns

pipe written to

Definition at line 816 of file BidirMMapPipe.h.

operator=()

BidirMMapPipe & BidirMMapPipe::operator= ( const BidirMMapPipe &  )

inlineprivate

assignment forbidden

Definition at line 865 of file BidirMMapPipe.h.

operator>>() [1/4]

BidirMMapPipe & BidirMMapPipe::operator>> ( BidirMMapPipe &(*)(BidirMMapPipe &)  manip )

inline

I/O manipulator support.

Parameters

manip

manipulator

Returns

pipe with manipulator applied

example:

pipe >> BidirMMapPipe::purge;

Definition at line 853 of file BidirMMapPipe.h.

operator>>() [2/4]

BidirMMapPipe & BidirMMapPipe::operator>>

(

char *&

str

)

read a C-style string

Parameters

str	pointer to string (space allocated with malloc!)

Returns

pipe read from

since this is for C-style strings, we use malloc/realloc/free for strings. passing in a NULL pointer is valid here, and the routine will use realloc to allocate a chunk of memory of the right size.

Definition at line 1578 of file BidirMMapPipe.cxx.

operator>>() [3/4]

BidirMMapPipe & BidirMMapPipe::operator>>

(

std::string &

str

)

read a std::string object

Parameters

str	string to be read

Returns

pipe read from

Definition at line 1599 of file BidirMMapPipe.cxx.

operator>>() [4/4]

template<class T >

BidirMMapPipe & BidirMMapPipe::operator>> ( T *&  tptr )

inline

read raw pointer to T from other side

NOTE: This will not read the pointee! Only the value of the pointer is transferred.

Parameters

tptr	pointer to be read

Returns

pipe read from

Definition at line 827 of file BidirMMapPipe.h.

pagepool()

BidirMMapPipe_impl::PagePool & BidirMMapPipe::pagepool ( )

staticprivate

return page pool

Definition at line 719 of file BidirMMapPipe.cxx.

pidOtherEnd()

pid_t BidirMMapPipe::pidOtherEnd ( ) const

inline

return PID of the process on the other end of the pipe

Returns

PID of the process running on the remote end

Definition at line 504 of file BidirMMapPipe.h.

poll()

int BidirMMapPipe::poll ( BidirMMapPipe::PollVector &  pipes, int  timeout  )

static

poll a set of pipes for events (ready to read from, ready to write to, error)

Parameters

pipes	set of pipes to check
timeout	timeout in milliseconds

Returns

positive number: number of pipes which have status changes, 0: timeout, or no pipes with status changed, -1 on error

Timeout can be zero (check for specified events, and return), finite (wait at most timeout milliseconds before returning), or -1 (infinite). The poll method returns when the timeout has elapsed, or if an event occurs on one of the pipes being polled, whichever happens earlier.

Pipes is a vector of one or more PollEntries, which each list a pipe and events to poll for. If events is left empty (zero), all conditions are polled for, otherwise only the indicated ones. On return, the revents fields contain the events that occurred for each pipe; error Error, EndOfFile or Invalid events are always set, regardless of wether they were in the set of requested events.

poll may block slightly longer than specified by timeout due to OS timer granularity and OS scheduling. Due to its implementation, the poll call can also return early if the remote end of the page sends a free page while polling (which is put on that pipe's freelist), while that pipe is polled for e.g Reading. The status of the pipe is indicated correctly in revents, and the caller can simply poll again. (The reason this is done this way is because it helps to replenish the pool of free pages and queue busy pages without blocking.)

Here's a piece of example code waiting on two pipes; if they become readable they are read:

#include <unistd.h>
#include <cstdlib>
#include <string>
#include <sstream>
#include <iostream>
 
#include "BidirMMapPipe.h"
 
// what to execute in the child
int randomchild(BidirMMapPipe& pipe)
{
    ::srand48(::getpid());
    for (int i = 0; i < 5; ++i) {
        // sleep a random time between 0 and .9 seconds
       ::usleep(int(1e6 * ::drand48()));
       std::ostringstream buf;
       buf << "child pid " << ::getpid() << " sends message " << i;
       std::cout << "[CHILD] : " << buf.str() << std::endl;
       pipe << buf.str() << BidirMMapPipe::flush;
       if (!pipe) return -1;
       if (pipe.eof()) break;
    }
    // tell parent we're done
    pipe << "" << BidirMMapPipe::flush;
    // wait for parent to acknowledge
    std::string s;
    pipe >> s;
    pipe.close();
    return 0;
}
 
// function to spawn a child
BidirMMapPipe* spawnChild(int (*childexec)(BidirMMapPipe&))
{
    BidirMMapPipe *p = new BidirMMapPipe();
    if (p->isChild()) {
        int retVal = childexec(*p);
        delete p;
        std::exit(retVal);
    }
    return p;
}
 
int main()
{
    typedef BidirMMapPipe::PollEntry PollEntry;
    // poll data structure
    BidirMMapPipe::PollVector pipes;
    pipes.reserve(3);
    // spawn children
    for (int i = 0; i < 3; ++i) {
        pipes.push_back(PollEntry(spawnChild(randomchild),
             BidirMMapPipe::Readable));
    }
    // while at least some children alive
    while (!pipes.empty()) {
        // poll, wait until status change (infinite timeout)
        int npipes = BidirMMapPipe::poll(pipes, -1);
        // scan for pipes with changed status
        for (std::vector<PollEntry>::iterator it = pipes.begin();
                npipes && pipes.end() != it; ) {
            if (!it->revents) {
                // unchanged, next one
                ++it;
                continue;
            }
            --npipes; // maybe we can stop early...
            // read from pipes which are readable
            if (it->revents & BidirMMapPipe::Readable) {
                std::string s;
                *(it->pipe) >> s;
                if (!s.empty()) {
                std::cout << "[PARENT]: Read from pipe " <<
                    it->pipe << ": " << s << std::endl;
                ++it;
                continue;
                } else {
                    // child is shutting down...
                    *(it->pipe) << "" << BidirMMapPipe::flush;
                    goto childcloses;
                }
            }
            // retire pipes with error or end-of-file condition
            if (it->revents & (BidirMMapPipe::Error |
                    BidirMMapPipe::EndOfFile |
                    BidirMMapPipe::Invalid)) {
                std::cout << "[PARENT]: Error on pipe " <<
                    it->pipe << " revents " << it->revents <<
                    std::endl;
childcloses:
                std::cout << "[PARENT]:\tchild exit status: " <<
                    it->pipe->close() << std::endl;
                if (retVal) return retVal;
                delete it->pipe;
                it = pipes.erase(it);
                continue;
            }
        }
    }
    return 0;
}

Definition at line 1434 of file BidirMMapPipe.cxx.

purge() [1/2]

void BidirMMapPipe::purge

(

)

purge buffered data waiting to be read and/or written

Discards all internal buffers.

Definition at line 1289 of file BidirMMapPipe.cxx.

purge() [2/2]

static BidirMMapPipe & BidirMMapPipe::purge ( BidirMMapPipe &  pipe )

inlinestatic

for usage a la "pipe << purge;"

Definition at line 859 of file BidirMMapPipe.h.

rdstate()

int BidirMMapPipe::rdstate ( ) const

inline

return flags (end of file, BidirMMapPipe closed, ...)

Returns

flags (end of file, BidirMMapPipe closed, ...)

Definition at line 707 of file BidirMMapPipe.h.

read()

BidirMMapPipe::size_type BidirMMapPipe::read	(	void *	addr,
		size_type	sz
	)

read from pipe

Parameters

addr	where to put read data
sz	size of data to read (in bytes)

Returns

size of data read, or 0 in case of end-of-file

read may block until data from other end is available. It will return 0 if the other end closed the pipe.

Definition at line 1363 of file BidirMMapPipe.cxx.

recvpages()

unsigned BidirMMapPipe::recvpages ( )

private

receive a pages from the other end (may block), queue them

Returns

number of pages received

this is an application-level scatter read, which gets the list of pages to read from the pipe. if mmap works, it needs only one read call (to get the head of the list of pages transferred). if we need to copy pages through the pipe, we need to add one read for each empty page, and two reads for each non-empty page.

Definition at line 1066 of file BidirMMapPipe.cxx.

recvpages_nonblock()

unsigned BidirMMapPipe::recvpages_nonblock ( )

private

receive pages from other end (non-blocking)

Returns

number of pages received

like recvpages(), but does not block if nothing is available for reading

Definition at line 1099 of file BidirMMapPipe.cxx.

sendpages()

void BidirMMapPipe::sendpages ( Page *  plist )

private

send page(s) to the other end (may block)

Parameters

plist

linked list of pages to send

the implementation gathers the different write(s) whereever possible; if mmap works, this results in a single write to transfer the list of pages sent, if we need to copy things through the pipe, we have one write to transfer which pages are sent, and then one write per page.

Definition at line 1044 of file BidirMMapPipe.cxx.

setDebugflag()

static void BidirMMapPipe::setDebugflag ( int  flag )

inlinestatic

set the debug flags

Parameters

flag	debug flags (if zero, no messages are printed)

Definition at line 439 of file BidirMMapPipe.h.

STREAMOP() [1/13]

BidirMMapPipe::STREAMOP

(

bool

)

C++ style stream operators for bool.

STREAMOP() [2/13]

BidirMMapPipe::STREAMOP

(

char

)

C++ style stream operators for char.

STREAMOP() [3/13]

BidirMMapPipe::STREAMOP

(

double

)

C++ style stream operators for double.

STREAMOP() [4/13]

BidirMMapPipe::STREAMOP

(

float

)

C++ style stream operators for float.

STREAMOP() [5/13]

BidirMMapPipe::STREAMOP

(

int

)

C++ style stream operators for int.

STREAMOP() [6/13]

BidirMMapPipe::STREAMOP

(

long long

)

C++ style stream operators for long long.

STREAMOP() [7/13]

BidirMMapPipe::STREAMOP

(

long

)

C++ style stream operators for long.

STREAMOP() [8/13]

BidirMMapPipe::STREAMOP

(

short

)

C++ style stream operators for short.

STREAMOP() [9/13]

BidirMMapPipe::STREAMOP

(

unsigned char

)

C++ style stream operators for unsigned char.

STREAMOP() [10/13]

BidirMMapPipe::STREAMOP

(

unsigned int

)

C++ style stream operators for unsigned int.

STREAMOP() [11/13]

BidirMMapPipe::STREAMOP

(

unsigned long long

)

C++ style stream operators for unsigned long long.

STREAMOP() [12/13]

BidirMMapPipe::STREAMOP

(

unsigned long

)

C++ style stream operators for unsigned long.

STREAMOP() [13/13]

BidirMMapPipe::STREAMOP

(

unsigned short

)

C++ style stream operators for unsigned short.

teardownall()

void BidirMMapPipe::teardownall ( void  )

staticprivate

cleanup routine - at exit, we want our children to get a SIGTERM...

Definition at line 726 of file BidirMMapPipe.cxx.

usesPipepair()

bool BidirMMapPipe::usesPipepair ( ) const

inline

if BidirMMapPipe uses a pipe pair for communications

Returns

true if BidirMMapPipe uses a pipe pair for communications

Definition at line 701 of file BidirMMapPipe.h.

usesSocketpair()

bool BidirMMapPipe::usesSocketpair ( ) const

inline

if BidirMMapPipe uses a socketpair for communications

Returns

true if BidirMMapPipe uses a socketpair for communications

Definition at line 695 of file BidirMMapPipe.h.

write()

BidirMMapPipe::size_type BidirMMapPipe::write	(	const void *	addr,
		size_type	sz
	)

wirte to pipe

Parameters

addr	where to get data to write from
sz	size of data to write (in bytes)

Returns

size of data written, or 0 in case of end-of-file

write may block until data can be written to other end (depends a bit on available buffer space). It will return 0 if the other end closed the pipe. The data is queued to be written on the next convenient occasion, or it can be forced out with flush().

Definition at line 1399 of file BidirMMapPipe.cxx.

xferraw() [1/2]

static size_type BidirMMapPipe::xferraw ( int  fd, void *  addr, const size_type  len, ssize_t(*)(int, const void *, std::size_t)  xferfn  )

inlinestaticprivate

transfer bytes through the pipe (reading, writing, may block)

Definition at line 934 of file BidirMMapPipe.h.

xferraw() [2/2]

BidirMMapPipe::size_type BidirMMapPipe::xferraw ( int  fd, void *  addr, size_type  len, ssize_t(*)(int, void *, std::size_t)  xferfn  )

staticprivate

transfer bytes through the pipe (reading, writing, may block)

Definition at line 999 of file BidirMMapPipe.cxx.

Friends And Related Symbol Documentation

BidirMMapPipe_impl::Page

friend class BidirMMapPipe_impl::Page

friend

page is our friend

Definition at line 868 of file BidirMMapPipe.h.

Member Data Documentation

m_busylist

Page* BidirMMapPipe::m_busylist

private

linked list: busy pages (data to be read)

Definition at line 902 of file BidirMMapPipe.h.

m_childPid

pid_t BidirMMapPipe::m_childPid

private

pid of the child (zero if we're child)

Definition at line 908 of file BidirMMapPipe.h.

m_dirtylist

Page* BidirMMapPipe::m_dirtylist

private

linked list: dirty pages (data to be sent)

Definition at line 904 of file BidirMMapPipe.h.

m_flags

int BidirMMapPipe::m_flags

private

flags (e.g. end of file)

Definition at line 907 of file BidirMMapPipe.h.

m_freelist

Page* BidirMMapPipe::m_freelist

private

linked list: free pages

Definition at line 903 of file BidirMMapPipe.h.

m_inpipe

int BidirMMapPipe::m_inpipe

private

pipe end from which data may be read

Definition at line 905 of file BidirMMapPipe.h.

m_outpipe

int BidirMMapPipe::m_outpipe

private

pipe end to which data may be written

Definition at line 906 of file BidirMMapPipe.h.

m_pages

BidirMMapPipe_impl::Pages BidirMMapPipe::m_pages

private

mmapped pages

Definition at line 901 of file BidirMMapPipe.h.

m_parentPid

pid_t BidirMMapPipe::m_parentPid

private

pid of the parent

Definition at line 909 of file BidirMMapPipe.h.

s_debugflag

int BidirMMapPipe::s_debugflag = 0

staticprivate

debug flag

Definition at line 895 of file BidirMMapPipe.h.

s_openpipes

std::list< BidirMMapPipe * > BidirMMapPipe::s_openpipes

staticprivate

list of open BidirMMapPipes

Definition at line 889 of file BidirMMapPipe.h.

s_openpipesmutex

pthread_mutex_t BidirMMapPipe::s_openpipesmutex = PTHREAD_MUTEX_INITIALIZER

staticprivate

protects s_openpipes

Definition at line 887 of file BidirMMapPipe.h.

s_pagepool

BidirMMapPipe_impl::PagePool * BidirMMapPipe::s_pagepool = 0

staticprivate

pool of mmapped pages

Definition at line 891 of file BidirMMapPipe.h.

s_pagepoolrefcnt

unsigned BidirMMapPipe::s_pagepoolrefcnt = 0

staticprivate

page pool reference counter

Definition at line 893 of file BidirMMapPipe.h.