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iopause.3
iopause - stateful handling of events based on the poll interface or file descriptors
#include "ioause.h"
int iopause(iopause_fd *x, unsigned int len, struct taia *deadline, struct taia *stamp);
iopause checks for file descriptor readability or writeability as specified by x[0].fd, x[0].events, x[1].fd, x[1].events, ..., x[len-1].fd, x[len-1].events. If x[i].events includes the bit IOPAUSE_READ, iopause checks for readability of the descriptor x[i].fd; if x[i].events includes the bit IOPAUSE_WRITE, iopause checks for writability of the descriptor x[i].fd; other bits in x[i].events have undefined effects.
iopause sets the IOPAUSE_READ bit in x[i].revents if it finds that x[i].fd is readabled and it sets the IOPAUSE_WRITE bit in x[i].revents if it finds that x[i].fd is writable. Beware that readability and writeability may be destroyed at any moment by other processes with access to the same file x[i].fd refers to.
If there is no readability or writeability to report, iopause waits until deadline for something to happen. iopause will return before deadline if a descriptor becomes readable or writeable, or an interrupting signal arrives, or some system-defined amount of time passes. iopause sets revents in any case.
You must put a current timestamp into stamp before calling iopause. In case the the current timestamp is older than the previous one (ie. due to daylight-saving) negative values are omitted.
The current implementation of iopause uses the poll interface if that is available. On some systems, poll needs to dynamically allocate kernel memory. In case not enough memory is available, iopause will return immediately and will report (often incorrectly) that no descriptors are readable or writeable.
If the poll interface is not available, iopause uses the select function. This function cannot see descriptor numbers past a system-defined limit, typically 256 or 1024; iopause will artificially pretend that those descriptors are never readable or writeable.
Future implementations of iopause may work around these problems on some systems, at the expense of chewing up all available CPU time.
Both poll and select use relative timeouts rather than absolute deadlines. Some kernels round the timeout down to a multiple of 10 milliseconds; this can burn quite a bit of CPU time as the deadline approaches. iopause compensates for this by adding 20 milliseconds to the timeout. In case the current timestamp is older than the previous one (ie. due to daylight-saving) the timeout is set to 20 milliseconds.
iopause reads and deploys the return codes of the poll and select call. Only positive return values shall be considered by the calling routines for a succcessful invocation.
Parts of the description are taken from the 'libowfat' man page.
select(2) poll(3) taia(3),