GPIO_V2_LINEINFO_CHANGED_READ - Read line info changed events for watched lines from the chip.


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



The file descriptor of the GPIO character device returned by open().


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_info_changed event.


Read line info changed events for watched lines from the chip.


Monitoring line info changes is not generally required, and would typically only be performed by a system monitoring component.

These events relate to changes in a line’s request state or configuration, not its value. Use GPIO_V2_LINE_EVENT_READ to receive events when a line changes value.

A line must be watched using GPIO_V2_GET_LINEINFO_WATCH_IOCTL to generate info changed events. Subsequently, a request, release, or reconfiguration of the line will generate an info changed event.

The kernel timestamps events when they occur and stores them in a buffer from where they can be read by userspace at its convenience using read().

The size of the kernel event buffer is fixed at 32 events per chip_fd.

The buffer may overflow if bursts of events occur quicker than they are read by userspace. If an overflow occurs then the most recent event is discarded. Overflow cannot be detected from userspace.

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 chip_fd.

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).

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

The presence of an event can be tested for by checking that the chip_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_info_changed event.

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