SHMEM_INT_WAIT(3)

💖 SHMEM_INT_WAIT(3) — Open MPI, but make it make sense 💖

okay bestie, so this one is a whole family of routines from Open MPI, all hanging out under one manpage roof. you ready? because there are like a million of them and they all do the same cute little thing: wait for a variable on your local processing element (PE) to change. 💅

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✨ NAME ✨

shmem_int_wait(3), shmem_int_wait(3)_until, shmem_int4_wait(3), shmem_int4_wait(3)_until, shmem_int8_wait(3), shmem_int8_wait(3)_until, shmem_long_wait(3), shmem_long_wait(3)_until, shmem_longlong_wait(3), shmem_longlong_wait(3)_until, shmem_short_wait(3), shmem_short_wait(3)_until, shmem_wait(3), shmem_wait(3)_until — Waits for a variable on the local processing element (PE) to change 🦋

(yes, that's the whole squad. they're all here. no gatekeeping.)

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🌸 SYNOPSIS 🌸

C or C++:

#include <mpp/shmem.h>

void shmem_int_wait(volatile int *var, int value)

void shmem_int_wait_until(volatile int *var, int cond, int value)

void shmem_long_wait(volatile long *var, long value)

void shmem_long_wait_until(volatile long *var, int cond, long value)

void shmem_longlong_wait(volatile long long *var, long long value)

void shmem_longlong_wait_until(volatile long long *var, int cond,
  long long value)

void shmem_short_wait(volatile short *var, short value)

void shmem_short_wait_until(volatile short *var, int cond,
  short value)

void shmem_wait(volatile long *ivar, long cmp_value)

void shmem_wait_until(volatile long *ivar, int cmp, long value)

Fortran:

INCLUDE "mpp/shmem.fh"

CALL SHMEM_INT4_WAIT(ivar, cmp_value)

CALL SHMEM_INT4_WAIT_UNTIL(ivar, cmp, cmp_value)

CALL SHMEM_INT8_WAIT(ivar, cmp_value)

CALL SHMEM_INT8_WAIT_UNTIL(ivar, cmp, cmp_value)

CALL SHMEM_WAIT(ivar, cmp_value)

CALL SHMEM_WAIT_UNTIL(ivar, cmp, cmp_value)

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🎀 DESCRIPTION 🎀

so shmem_wait and shmem_wait_until are literally just sitting there waiting for ivar to get changed by a remote write or an atomic swap from a different processor. it's giving point-to-point directed synchronization, honestly. 👑

here's the vibe: a call to shmem_wait does not return until some other processor writes a value into ivar (on the waiting processor, obviously) that is not equal to cmp_value. and a call to shmem_wait_until does not return until some other processor changes ivar so it satisfies the condition implied by cmp and cmp_value. iconic behavior. this whole mechanism is useful when a processor needs to tell another processor that it has completed some action. friendship, but make it distributed. 🫶

the arguments, bestie:

• target — The remotely accessible integer data object to be updated on the remote PE. If you are using C/C++, the type of target should match that implied in the SYNOPSIS section. If you are using the Fortran compiler, it must be of type integer with an element size of 4 bytes for SHMEM_INT4_ADD and 8 bytes for SHMEM_INT8_ADD.
• value — The value to be atomically added to target. If you are using C/C++, the type of value should match that implied in the SYNOPSIS section. If you are using Fortran, it must be of type integer with an element size of target.
• pe — An integer that indicates the PE number upon which target is to be updated. If you are using Fortran, it must be a default integer value.
• ivar — A remotely accessible integer variable that is being updated by another PE. If you are using C/C++, the type of ivar should match that implied in the SYNOPSIS section. If you are using Fortran, ivar must be a specific sized integer type according to the function being called, as follows:
  • shmem_wait, shmem_wait_until:** default INTEGER
  • shmem_int4_wait, shmem_int4_wait_until:** INTEGER*4
  • shmem_int8_wait, shmem_int8_wait_until:** INTEGER*8
• cmp — The compare operator that compares ivar with cmp_value. cmp must be of type integer. If you are using Fortran, it must be of default kind. If you are using C/C++, the type of cmp should match that implied in the SYNOPSIS section. The following cmp values are supported:
  • SHMEM_CMP_EQ — Equal
  • SHMEM_CMP_NE — Not equal
  • SHMEM_CMP_GT — Greater than
  • SHMEM_CMP_LE — Less than or equal to
  • SHMEM_CMP_LT — Less than
  • SHMEM_CMP_GE — Greater than or equal to
• cmp_value — cmp_value must be of type integer. If you are using C/C++, the type of cmp_value should match thatimplied in the SYNOPSIS section. If you are using Fortran, cmp_value must be an integer of the same size and kind as ivar. The shmem_wait routines return when ivar is no longer equal to cmp_value. The shmem_wait_until routines return when the compare condition is true. The compare condition is defined by the ivar argument compared with the cmp_value using the comparison operator, cmp.

💀 real talk: these functions block. they do not come back until the condition is met. don't be out here calling them expecting a lil peek and a return, because that is not the assignment.

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🔥 EXAMPLES 🔥

Example 1: The following call returns when variable ivar is not equal to 100:

INTEGER*8 IVAR

CALL SHMEM_INT8_WAIT(IVAR, INT8(100))

Example 2: The following call to SHMEM_INT8_WAIT_UNTIL is equivalent to the call to SHMEM_INT8_WAIT in example 1:

INTEGER*8 IVAR

CALL SHMEM_INT8_WAIT_UNTIL(IVAR, SHMEM_CMP_NE, INT8(100))

Example 3: The following C/C++ call waits until the sign bit in ivar is set by a transfer from a remote PE:

int ivar;

shmem_int_wait_until(&ivar, SHMEM_CMP_LT, 0);

Example 4: The following Fortran example is in the context of a subroutine:

SUBROUTINE EXAMPLE()
  INTEGER FLAG_VAR
  COMMON/FLAG/FLAG_VAR
  . . .
  FLAG_VAR = FLAG_VALUE ! initialize the event variable
  . . .
  IF (FLAG_VAR .EQ. FLAG_VALUE) THEN
    CALL SHMEM_WAIT(FLAG_VAR, FLAG_VALUE)
  ENDIF
  FLAG_VAR = FLAG_VALUE ! reset the event variable for next time
  . . .
END

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🌈 SEE ALSO 🌈

• intro_shmem(3)
• shmem_put(3)

(yes go read those next, iykyk 💜)

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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and that's the whole page, bestie — go forth and synchronize responsibly. 💖✨🎀

signed, sealed, delivered by the Open MPI crew — source/version vibes: Open MPI, dated May 14, 2026 — mwah, catch you on the next manpage 🦋💅