SHMEM_WAIT(3)

👑 SHMEM_WAIT(3) — okay bestie, listen

Hi hi hiii 💖 you just opened the Open MPI manual page for the whole shmem_wait family, and honestly? It's giving "I will sit here and wait patiently for you to text back" but make it a processor. 🌸

The full girlies in this crew are:

• shmem_int_wait, shmem_int_wait_until
• shmem_int4_wait, shmem_int4_wait_until
• shmem_int8_wait, shmem_int8_wait_until
• shmem_long_wait, shmem_long_wait_until
• shmem_longlong_wait, shmem_longlong_wait_until
• shmem_short_wait, shmem_short_wait_until
• shmem_wait, shmem_wait_until

and their whole job — their entire purpose, their moment, their slay — is described in the NAME section:

Waits for a variable on the local processing element (PE) to change. ✨ periodt.

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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 issued by a different processor. That's the whole vibe check. 🦋

These routines can be used for point-to-point directed synchronization — iykyk. And here's the tea:

• A call to shmem_wait does not return until some other processor writes a value into ivar on the waiting processor, and that value is not equal to cmp_value.
• A call to shmem_wait_until does not return until some other processor changes ivar to satisfy the condition implied by cmp and cmp_value.

This mechanism is useful when a processor needs to tell another processor that it has completed some action. 🫶 like a little "done!!" text but for PEs.

the arguments, bestie 🍓

• target — the remotely accessible integer data object to be updated on the remote PE. If you're on C/C++, the type of target should match what the SYNOPSIS section implies. If you're 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're on C/C++, the type of value should match what the SYNOPSIS section implies. If you're on 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're on Fortran, it must be a default integer value.
• ivar — a remotely accessible integer variable that is being updated by another PE. If you're on C/C++, the type of ivar should match what the SYNOPSIS section implies. If you're on Fortran, ivar must be a specific-sized integer type according to the function being called, like so:
  • 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're on Fortran, it must be default kind. If you're on C/C++, the type of cmp should match what the SYNOPSIS section implies. Here are the supported cmp values, no cap:
  • 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 — must be of type integer. If you're on C/C++, the type of cmp_value should match that implied in the SYNOPSIS section. If you're on 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. And that compare condition is defined by the ivar argument compared with the cmp_value using the comparison operator, cmp. 🌈

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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)

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👑 Copyright

2003-2026, The Open MPI Community

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okay that's the whole page, you ate it, you read the whole thing, main character energy fr fr 💜 quick sign-off though — this little number is from Sep 10, 2026, with source and version credit going to the Open MPI girlies and the copyright at the bottom reading 2003-2026, The Open MPI Community. now go forth and wait until your condition is true, bestie 🦋💖✨