Inter-process communications.

RTAI FIFO communication functions. More...

Files
file	rtai_fifos.h
	Interface of the RTAI FIFO module.
file	fifos.c
	Implementation of the RTAI FIFO module.

Functions
int	rtf_suspend_timed (int fd, int ms_delay)
	Suspend a process for some time. More...
int	rtf_open_sized (const char *dev, int perm, int size)
	Create a real-time FIFO. More...
int	rtf_read_all_at_once (int fd, void *buf, int count)
	Read data from FIFO in user space, waiting for all of them. More...
int	rtf_read_timed (int fd, void *buf, int count, int ms_delay)
	Read data from FIFO in user space, with timeout. More...
int	rtf_write_timed (int fd, void *buf, int count, int ms_delay)
	Write data to FIFO in user space, with timeout. More...
int	rtf_set_async_sig (int fd, int signum)
	Activate asynchronous notification of data availability. More...
RTAI_SYSCALL_MODE int	rtf_reset (unsigned int minor)
	Reset a real-time FIFO. More...
RTAI_SYSCALL_MODE int	rtf_resize (unsigned int minor, int size)
	Resize a real-time FIFO. More...
RTAI_SYSCALL_MODE int	rtf_create (unsigned int minor, int size)
	Create a real-time FIFO. More...
RTAI_SYSCALL_MODE int	rtf_destroy (unsigned int minor)
	Close a real-time FIFO. More...
int	rtf_create_handler (unsigned int minor, void *handler)
	Install a FIFO handler function. More...
RTAI_SYSCALL_MODE int	rtf_put (unsigned int minor, void *buf, int count)
	Write data to FIFO. More...
RTAI_SYSCALL_MODE int	rtf_get (unsigned int minor, void *buf, int count)
	Read data from FIFO. More...

Detailed Description

RTAI FIFO communication functions.

RTAI fifos maintain full compatibility with those available in NMT_RTLinux while adding many other useful services that avoid the clumsiness of Unix/Linux calls. So if you need portability you should bent yourself to the use of select for timing out IO operations, while if you have not to satisfy such constraints use the available simpler, and more direct, RTAI fifos specific services.

In the table below the standard Unix/Linux services in user space are enclosed in []. See standard Linux man pages if you want to use them, they need not be explained here.

Called from RT task	Called from Linux process
rtf_create	rtf_open_sized [open]
rtf_destroy	[close]
rtf_reset	rtf_reset
rtf_resize	rtf_resize
rtf_get	[read] rtf_read_timed rtf_read_all_at_once
rtf_put	[write] rtf_write_timed
rtf_create_handler
	rtf_suspend_timed
	rtf_set_async_sig

In Linux, fifos have to be created by :

$ mknod /dev/rtf<x> c 150 <x> 

where <x> is the minor device number, from 0 to 63; thus on the Linux side RTL fifos can be used as standard character devices. As it was said above to use standard IO operations on such devices there is no need to explain anything, go directly to Linux man pages. RTAI fifos specific services available in kernel and user space are instead explained here.

What is important to remember is that in the user space side you address fifos through the file descriptor you get at fifo device opening while in kernel space you directly address them by their minor number. So you will mate the fd you get in user space by using

open(/dev/rtfxx,...) 

to the integer xx you will use in kernel space.

Note

RTAI fifos should be used just with applications that use only real time interrupt handlers, so that no RTAI scheduler is installed, or if you need compatibility with NMT RTL. If you are working with any RTAI scheduler already installed you are strongly invited to think about avoiding them, use LXRT instead.

It is far better and flexible, and if you really like it the fifos way mailboxes are a one to one, more effective, substitute. After all RTAI fifos are implemented on top of them.

Function Documentation

rtf_create()

RTAI_SYSCALL_MODE int rtf_create	(	unsigned int	minor,
		int	size
	)

Create a real-time FIFO.

rtf_create creates a real-time fifo (RT-FIFO) of initial size size and assigns it the identifier fifo. It must be used only in kernel space.

Parameters

minor	is a positive integer that identifies the fifo on further operations. It has to be less than RTF_NO.
size	is the requested size for the fifo.

fifo may refer to an existing RT-FIFO. In this case the size is adjusted if necessary.

The RT-FIFO is a character based mechanism to communicate among real-time tasks and ordinary Linux processes. The rtf_* functions are used by the real-time tasks; Linux processes use standard character device access functions such as read, write, and select.

If this function finds an existing fifo of lower size it resizes it to the larger new size. Note that the same condition apply to the standard Linux device open, except that when it does not find any already existing fifo it creates it with a default size of 1K bytes.

It must be remarked that practically any fifo size can be asked for. In fact if size is within the constraint allowed by kmalloc such a function is used, otherwise vmalloc is called, thus allowing any size that can fit into the available core memory.

Multiple calls of this function are allowed, a counter is kept internally to track their number, and avoid destroying/closing a fifo that is still used.

Return values

0	on success
ENODEV	if fifo is greater than or equal to RTF_NO
ENOMEM	if the necessary size could not be allocated for the RT-FIFO.

References rtf_resize().

rtf_create_handler()

int rtf_create_handler	(	unsigned int	minor,
		void *	handler
	)

Install a FIFO handler function.

rtf_create_handler installs a handler which is executed when data is written to or read from a real-time fifo.

Parameters

minor	is an RT-FIFO that must have previously been created with a call to rtf_create().
handler	is a pointer on a function wich will be called whenever a Linux process accesses that fifo.

rtf_create_handler is often used in conjunction with rtf_get() to process data acquired asynchronously from a Linux process. The installed handler calls rtf_get() when data is present. Because the handler is only executed when there is activity on the fifo, polling is not necessary.

Return values

0	on success.
EINVAL	if fifo is greater than or equal to RTF_NO, or handler is NULL.

Note

rtf_create_handler does not check if FIFO referred by fifo is open or not. The next call of rtf_create will uninstall the handler just "installed".

rtf_destroy()

RTAI_SYSCALL_MODE int rtf_destroy

(

unsigned int

minor

)

Close a real-time FIFO.

rtf_destroy closes, in kernel space, a real-time fifo previously created or reopened with rtf_create() or rtf_open_sized(). An internal mechanism counts how many times a fifo was opened. Opens and closes must be in pair. rtf_destroy should be called as many times as rtf_create was. After the last close the fifo is really destroyed.

No need for any particular function for the same service in user space, simply use the standard Unix close.

Returns

a non-negative value on success. Actually it is the open counter, that means how many times rtf_destroy should be called yet to destroy the fifo.

a a negative value is returned as described below.

Return values

ENODEV	if fifo is greater than or equal to RTF_NO.
EINVAL	if fifo refers to a not opened fifo.

Note

The equivalent of rtf_destroy in user space is the standard UNIX close.

rtf_get()

RTAI_SYSCALL_MODE int rtf_get	(	unsigned int	minor,
		void *	buf,
		int	count
	)

Read data from FIFO.

rtf_get tries to read a block of data from a real-time fifo previously created with a call to rtf_create().

Parameters

minor	is the ID with which the RT-FIFO was created.
buf	points a buffer provided by the caller.
count	is the size of buf in bytes.

This mechanism is available only to real-time tasks; Linux processes use a read from the corresponding fifo device to dequeue data from a fifo. Similarly, Linux processes use write or similar functions to write the data to be read via rtf_put() by a real-time task.

rtf_get is often used in conjunction with rtf_create_handler() to process data received asynchronously from a Linux process. A handler is installed via rtf_create_handler(); this handler calls rtf_get to receive any data present in the RT-FIFO as it becomes available. In this way, polling is not necessary; the handler is called only when data is present in the fifo.

Returns

the size of the received data block on success. Note that this value may be less than count if count bytes of data is not available in the fifo.

Return values

ENODEV	if fifo is greater than or equal to RTF_NO.
EINVAL	if fifo refers to a not opened fifo.

Note

The equivalent of rtf_get in user space is the standard UNIX read, which can be either blocking or nonblocking according to how you opened the related device.

rtf_open_sized()

int rtf_open_sized	(	const char *	dev,
		int	perm,
		int	size
	)

Create a real-time FIFO.

rtf_open_sized is the equivalent of rtf_create() in user space; it creates a real-time fifo (RT-FIFO) of initial size size.

Parameters

size	is the requested size for the fifo.

The RT-FIFO is a character based mechanism to communicate among real-time tasks and ordinary Linux processes. The rtf_* functions are used by the real-time tasks; Linux processes use standard character device access functions such as read, write, and select.

If this function finds an existing fifo of lower size it resizes it to the larger new size. Note that the same condition apply to the standard Linux device open, except that when it does not find any already existing fifo it creates it with a default size of 1K bytes.

It must be remarked that practically any fifo size can be asked for. In fact if size is within the constraint allowed by kmalloc such a function is used, otherwise vmalloc is called, thus allowing any size that can fit into the available core memory.

Multiple calls of this function are allowed, a counter is kept internally to track their number, and avoid destroying/closing a fifo that is still used.

Returns

the usual Unix file descriptor on succes, to be used in standard reads and writes.

Return values

-ENOMEM

if the necessary size could not be allocated for the RT-FIFO.

Note

In user space, the standard UNIX open acts like rtf_open_sized with a default 1K size.

rtf_put()

RTAI_SYSCALL_MODE int rtf_put	(	unsigned int	minor,
		void *	buf,
		int	count
	)

Write data to FIFO.

rtf_put tries to write a block of data to a real-time fifo previously created with rtf_create().

Parameters

minor	is the ID with which the RT-FIFO was created.
buf	points the block of data to be written.
count	is the size of the block in bytes.

This mechanism is available only in kernel space, i.e. either in real-time tasks or handlers; Linux processes use a write to the corresponding /dev/fifo<n> device to enqueue data to a fifo. Similarly, Linux processes use read or similar functions to read the data previously written via rtf_put by a real-time task.

Returns

the number of bytes written on succes. Note that this value may be less than count if count bytes of free space is not available in the fifo.

Return values

ENODEV	if fifo is greater than or equal to RTF_NO.
EINVAL	if fifo refers to a not opened fifo.

Note

The equivalent of rtf_put in user space is the standard UNIX write, which can be either blocking or nonblocking according to how you opened the related device.

rtf_read_all_at_once()

int rtf_read_all_at_once	(	int	fd,
		void *	buf,
		int	count
	)

Read data from FIFO in user space, waiting for all of them.

rtf_read_all_at_once reads a block of data from a real-time fifo identified by the file descriptor fd blocking till all waiting at most count bytes are available, whichever option was used at the related device opening.

Parameters

fd	is the file descriptor returned at fifo open.
buf	points the block of data to be written.
count	is the size in bytes of the buffer.

Returns

the number of bytes read on success.

Return values

-EINVAL

if fd refers to a not opened fifo.

rtf_read_timed()

int rtf_read_timed	(	int	fd,
		void *	buf,
		int	count,
		int	ms_delay
	)

Read data from FIFO in user space, with timeout.

rtf_read_timed reads a block of data from a real-time fifo identified by the file descriptor fd waiting at most delay milliseconds to complete the operation.

Parameters

fd	is the file descriptor returned at fifo open.
buf	points the block of data to be written.
count	is the size of the block in bytes.
ms_delay	is the timeout time in milliseconds.

Returns

the number of bytes read is returned on success or timeout. Note that this value may be less than count if count bytes of free space is not available in the fifo or a timeout occured.

Return values

-EINVAL

if fd refers to a not opened fifo.

Note

The standard, clumsy, Unix way to achieve the same result is to use select.

rtf_reset()

RTAI_SYSCALL_MODE int rtf_reset

(

unsigned int

minor

)

Reset a real-time FIFO.

rtf_reset resets RT-FIFO fd_fifo by setting its buffer pointers to zero, so that any existing data is discarded and the fifo started anew like at its creations. It can be used both in kernel and user space.

Parameters

minor

is a file descriptor returned by standard UNIX open in user space while it is directly the chosen fifo number in kernel space.

Return values

0	on succes.
ENODEV	if fd_fifo is greater than or equal to RTF_NO.
EINVAL	if fd_fifo refers to a not opened fifo.
EFAULT	if the operation was unsuccessful.

rtf_resize()

RTAI_SYSCALL_MODE int rtf_resize	(	unsigned int	minor,
		int	size
	)

Resize a real-time FIFO.

rtf_resize modifies the real-time fifo fifo, previously created with, rtf_create(), to have a new size of size. Any data in the fifo is discarded.

Parameters

minor	is a file descriptor returned by standard UNIX open in user space while it is directly the chosen fifo number in kernel space.
size	is the requested new size.

Return values

size	on success.
-ENODEV	if fifo is greater than or equal to RTF_NO.
-EINVAL	if fifo refers to a not opened fifo.
-ENOMEM	if size bytes could not be allocated for the RT-FIFO. Fifo
-EBUSY	if size is smaller than actual content size is unchanged.

Referenced by rtf_create().

rtf_set_async_sig()

int rtf_set_async_sig	(	int	fd,
		int	signum
	)

Activate asynchronous notification of data availability.

rtf_set_async_sig activate an asynchronous signals to notify data availability by catching a user set signal signum.

Parameters

signum

is a user chosen signal number to be used, default is SIGIO.

Return values

-EINVAL

if fd refers to a not opened fifo.

rtf_suspend_timed()

int rtf_suspend_timed	(	int	fd,
		int	ms_delay
	)

Suspend a process for some time.

rtf_suspend_timed suspends a Linux process according to delay.

Parameters

fd	is the file descriptor returned at fifo open, rtf_suspend_timed needs a fifo support.
ms_delay	is the timeout time in milliseconds.

Note

The standard, clumsy, way to achieve the same result is to use select with null file arguments, for long sleeps, with seconds resolution, sleep is also available.

rtf_write_timed()

int rtf_write_timed	(	int	fd,
		void *	buf,
		int	count,
		int	ms_delay
	)

Write data to FIFO in user space, with timeout.

rtf_write_timed writes a block of data to a real-time fifo identified by the file descriptor fd waiting at most @æ delay milliseconds to complete the operation.

Parameters

fd	is the file descriptor returned at fifo open.
buf	points the block of data to be written.
count	is the size of the block in bytes.
ms_delay	is the timeout time in milliseconds.

Returns

the number of bytes written on succes. Note that this value may be less than count if count bytes of free space is not available in the fifo.

Return values

-EINVAL

if fd refers to a not opened fifo.

Note

The standard, clumsy, Unix way to achieve the same result is to use select.