watch_time – fpm-watch

Sleep utilities and low-CPU waiting.

fpm-watch is primarily poll-based. This module provides two waiting strategies:

Spin wait (sleep_spin): uses system_clock in a tight loop. This is portable and offers good timing resolution, but it can consume CPU.
OS sleep (sleep_os): delegates to a small C shim (fpm_watch_sleep_seconds) which performs an efficient, platform-native sleep:
- Windows: Sleep(milliseconds)
- POSIX: nanosleep(timespec) (with EINTR retry)

The public entry point sleep_seconds dispatches between these strategies depending on the global low-CPU mode flag set by set_low_cpu.

Design notes

The OS sleep is implemented in C to avoid link-time issues where referencing platform-specific symbols (e.g. Sleep or nanosleep) from Fortran can break linking on the “other” platform even if the routine is never called.
sleep_seconds is intentionally lightweight and safe to call frequently.

Usage

use watch_time, only: set_low_cpu, sleep_seconds

call set_low_cpu(.true.)   ! prefer OS sleep (near-zero idle CPU)
call sleep_seconds(0.25)   ! sleep for ~250 ms

Threading/Signals

Spin sleep does not yield; it runs until time has elapsed.
OS sleep yields to the OS scheduler; on POSIX the C shim typically retries on interrupt (EINTR) to honor the requested duration.

Uses

- fpm_environment
- iso_c_binding
- iso_fortran_env
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Used by

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Graph Key

Nodes of different colours represent the following:

Solid arrows point from a submodule to the (sub)module which it is descended from. Dashed arrows point from a module or program unit to modules which it uses.
Where possible, edges connecting nodes are given different colours to make them easier to distinguish in large graphs.

Variables

Type	Visibility	Attributes		Name		Initial
logical,	private,	save	::	low_cpu_mode	=	.false.	Global mode switch that selects OS sleep vs spin sleep. `.false.` (default): `sleep_seconds` uses `sleep_spin` (busy wait). `.true.` : `sleep_seconds` uses `sleep_os` (OS-backed sleep).

Interfaces

interface

C shim providing a portable OS sleep implementation.

The actual platform-specific sleep is implemented in C to prevent link-time failures that can occur if Fortran directly references Sleep (Windows) or nanosleep (POSIX) symbols in the same object.

Arguments

sec: requested sleep duration in seconds.

private subroutine fpm_watch_sleep_seconds(sec) bind(C, name="fpm_watch_sleep_seconds")

Arguments

Type	Intent	Optional	Attributes		Name
real(kind=c_double),			value	::	sec

Subroutines

public subroutine set_low_cpu(flag)

Enable or disable low-CPU mode.

Arguments

Type	Intent	Optional	Attributes		Name
logical,	intent(in)			::	flag

public subroutine sleep_seconds(s)

Sleep for approximately s seconds.

Arguments

Type	Intent	Optional	Attributes		Name
real,	intent(in)			::	s

private subroutine sleep_os(s)

OS-backed sleep for approximately s seconds.

Arguments

Type	Intent	Optional	Attributes		Name
real,	intent(in)			::	s

private subroutine sleep_spin(s)

Busy-wait for approximately s seconds.

Arguments

Type	Intent	Optional	Attributes		Name
real,	intent(in)			::	s

watch_time Module

Contents

Variables

Interfaces

Subroutines

Design notes

Usage

Threading/Signals

Uses

Used by

Variables

Interfaces

interface

Arguments

private subroutine fpm_watch_sleep_seconds(sec) bind(C, name="fpm_watch_sleep_seconds")

Arguments

Subroutines

public subroutine set_low_cpu(flag)

Arguments

public subroutine sleep_seconds(s)

Arguments

private subroutine sleep_os(s)

Arguments

private subroutine sleep_spin(s)

Arguments