3.2.19. ioctl DMX_QBUF, DMX_DQBUF Name

DMX_QBUF - DMX_DQBUF - Exchange a buffer with the driver


this API is still experimental Synopsis


int ioctl(int fd, DMX_QBUF, struct dmx_buffer *argp)


int ioctl(int fd, DMX_DQBUF, struct dmx_buffer *argp) Arguments


File descriptor returned by open().


Pointer to struct dmx_buffer. Description

Applications call the DMX_QBUF ioctl to enqueue an empty (capturing) or filled (output) buffer in the driver’s incoming queue. The semantics depend on the selected I/O method.

To enqueue a buffer applications set the index field. Valid index numbers range from zero to the number of buffers allocated with ioctl DMX_REQBUFS (struct dmx_requestbuffers count) minus one. The contents of the struct dmx_buffer returned by a ioctl DMX_QUERYBUF ioctl will do as well.

When DMX_QBUF is called with a pointer to this structure, it locks the memory pages of the buffer in physical memory, so they cannot be swapped out to disk. Buffers remain locked until dequeued, until the device is closed.

Applications call the DMX_DQBUF ioctl to dequeue a filled (capturing) buffer from the driver’s outgoing queue. They just set the index field with the buffer ID to be queued. When DMX_DQBUF is called with a pointer to struct dmx_buffer, the driver fills the remaining fields or returns an error code.

By default DMX_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, DMX_DQBUF returns immediately with an EAGAIN error code when no buffer is available.

The struct dmx_buffer structure is specified in Buffers. Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.


Non-blocking I/O has been selected using O_NONBLOCK and no buffer was in the outgoing queue.


The index is out of bounds, or no buffers have been allocated yet.


DMX_DQBUF failed due to an internal error. Can also indicate temporary problems like signal loss or CRC errors.