hal_stream(3hal)

NAME
hal_stream - non-blocking realtime streams

SYNOPSIS

DESCRIPTION
A HAL stream provides a limited ability for two components to communicate data which does not fit within the model of HAL pins. A reader and a writer must agree on a key (32-bit integer identifier) and a data structure specified by typestring. They must also agree which component (the first one loaded) will hal_stream_create the stream, and which component (the second one loaded) will hal_stream_attach to the already-created stream. The userspace part can be halstreamer or halsampler. In the case of halstreamer the key is 0x48535430 plus the channel number. In the case of halsampler the key is 0x48534130 plus the channel number.

  hal_stream_create
Create the given stream, initializing the stream which is passed by reference. It is an undiagnosed error if a stream has already been created with the same key.

  hal_stream_destroy
Destroy the given stream. It is an undiagnosed error if the stream is still attached by another component. It is an undiagnosed error if the stream was attached with hal_stream_attach rather than created with hal_stream_create. It is an undiagnosed error if the call to hal_stream_destroy is omitted.

  hal_stream_attach
Attach the given stream, which was already created by hal_stream_create. If the typestring is specified, this call fails if it does not match the typestring the stream was created with. If the typestring argument is NULL, then any typestring is accepted.

  hal_stream_detach
Detach the given stream. It is an undiagnosed error if the stream was created with hal_stream_create rather than attached with hal_stream_attach. It is an undiagnosed error if the call to hal_stream_detach is omitted.

  hal_stream_element_count
Returns the number of pins.

  hal_stream_element_type
Returns the type of the given pin number.

  hal_stream_readable
Returns true if the stream has at least one sample to read

  hal_stream_read
If the stream has one sample to read, stores it in buf.

  hal_stream_writable
Returns true if the stream has room for at least one sample to be written.

  hal_stream_depth
Returns the number of samples waiting to be read.

  hal_stream_maxdepth
Returns the depth argument that the stream was created with.

  hal_stream_num_overruns
Returns a number which is incremented each time hal_stream_write is called without space available.

  hal_stream_num_underruns
Returns a number which is incremented each time hal_stream_read is called without a sample available.

  hal_stream_wait_readable
Waits until the stream is readable or the stop flag is set.

  hal_stream_wait_writable
Waits until the stream is writable or the stop flag is set.

  hal_stream_read
Reads a record from stream. If successful, it is stored in the given buffer. Optionally, the sample number can be retrieved. If no sample is available, num_underruns is incremented. It is an undetected error if more than one component or real-time function calls hal_stream_read concurrently.

  hal_stream_write
Writes a record to the stream. If successful, it copied from the given buffer. If no room is available, num_overruns is incremented. In either case, the internal sampleno value is incremented.
  It is an undetected error if more than one component or real-time function calls hal_stream_write concurrently.

ARGUMENTS

stream
    A pointer to a stream object. In the case of hal_stream_create and hal_stream_attach this is an uninitialized stream; in other cases, it must be a stream created or attached by an earlier call and not yet detached or destroyed.

hal_id
    An HAL component identifier returned by an earlier call to hal_init.

key The key for the shared memory segment.

depth
    The number of samples that can be unread before any samples are lost (overrun)

typestring
    A typestring is a case-insensitive string which consists of one or more of the following type characters:

    B   for bool / hal_bit_t

    S   for int32_t / hal_s32_t

    U   for uint32_t / hal_u32_t

    F   for real_t / hal_float_t
A typestring is limited to 16 characters.

buf A buffer big enough to hold all the data in one sample.

sampleno
    If non-NULL, the last sample number is stored here. Gaps in this sequence indicate that an overrun occurred between the previous read and this one. May be NULL, in which case the sample number is not retrieved.

stop
    A pointer to a value which is monitored while waiting. If it is nonzero, the wait operation returns early. This allows a wait call to be safely terminated in the case of a signal.

SAMPLE CODE
In the source tree under src/hal/components, sampler.c and streamer.c are realtime components that read and write hal streams.

REALTIME CONSIDERATIONS
hal_stream_read, hal_stream_readable, hal_stream_write, hal_stream_writable, hal_stream_element_count, hal_tream_pin_type, hal_stream_depth, hal_stream_maxdepth, hal_stream_num_underruns, hal_stream_number_overruns may be called from realtime code. hal_stream_wait_writable, hal_stream_wait_writable may be called from ULAPI code. Other functions may be called in any context, including realtime contexts.

RETURN VALUE
hal_stream_create , hal_stream_attach , hal_stream_read , hal_stream_write , hal_stream_detach and hal_stream_destroy return an RTAPI status code. Other functions' return values are explained above.

BUGS
The memory overhead of a stream can be large. Each element in a record uses 8 bytes, and the implicit sample number also uses 8 bytes. As a result, a stream which is used to transport 8-bit values uses 94% of its memory as overhead. However, for modest stream sizes this overhead is not important. (this memory is part of its own shared memory region and does not count against the HAL shared memory region used for pins, parameters and signals)

SEE ALSO
sampler(9), streamer(9), halsampler(1), halstreamer(1)