GPIO_V2_LINE_EVENT_READ - Read edge detection events for lines from a request.


int read(int req_fd, void *buf, size_t count)



The file descriptor of the GPIO character device, as returned in the request.fd by GPIO_V2_GET_LINE_IOCTL.


The buffer to contain the events.


The number of bytes available in buf, which must be at least the size of a gpio_v2_line_event.


Read edge detection events for lines from a request.

Edge detection must be enabled for the input line using either GPIO_V2_LINE_FLAG_EDGE_RISING or GPIO_V2_LINE_FLAG_EDGE_FALLING, or both. Edge events are then generated whenever edge interrupts are detected on the input line.

The kernel captures and timestamps edge events as close as possible to their occurrence and stores them in a buffer from where they can be read by userspace at its convenience using read().

Events read from the buffer are always in the same order that they were detected by the kernel, including when multiple lines are being monitored by the one request.

The size of the kernel event buffer is fixed at the time of line request creation, and can be influenced by the request.event_buffer_size. The default size is 16 times the number of lines requested.

The buffer may overflow if bursts of events occur quicker than they are read by userspace. If an overflow occurs then the oldest buffered event is discarded. Overflow can be detected from userspace by monitoring the event sequence numbers.

To minimize the number of calls required to copy events from the kernel to userspace, read() supports copying multiple events. The number of events copied is the lower of the number available in the kernel buffer and the number that will fit in the userspace buffer (buf).

Changing the edge detection flags using GPIO_V2_LINE_SET_CONFIG_IOCTL does not remove or modify the events already contained in the kernel event buffer.

The read() will block if no event is available and the req_fd has not been set O_NONBLOCK.

The presence of an event can be tested for by checking that the req_fd is readable using poll() or an equivalent.

Return Value

On success the number of bytes read, which will be a multiple of the size of a gpio_v2_line_event event.

On error -1 and the errno variable is set appropriately. Common error codes are described in GPIO Error Codes.