SHMEM_SWAP(3)

💅 SHMEM_SWAP(3) 💅

okay so bestie, welcome to the Open MPI manual page for the swap girlies 💖 — yes, we're talking about shmem_double_swap(3), shmem_float_swap(3), shmem_int_swap(3), shmem_long_swap(3), shmem_swap(3), shmem_int4_swap(3), shmem_int8_swap(3), shmem_real4_swap(3), shmem_real8_swap(3), and shmem_longlong_swap(3). These iconic little functions perform an atomic swap to a remote data object and honestly? they ate. 🔥

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✨ SYNOPSIS ✨

C or C++ 🎀

#include <mpp/shmem.h>

double shmem_double_swap(double *target, double value,
  int pe);

float shmem_float_swap(float *target, float value, int pe);

int shmem_int_swap(int *target, int value, int pe);

long shmem_long_swap(long *target, long value, int pe);

long long shmem_longlong_swap(long long *target,
  long long value, int pe);

long shmem_swap(long *target, long value, int pe);

Fortran 🦋

INCLUDE "mpp/shmem.fh"

INTEGER pe

INTEGER SHMEM_SWAP
ires = SHMEM_SWAP(target, value, pe)

INTEGER(KIND=4) SHMEM_INT4_SWAP
ires = SHMEM_INT4_SWAP(target, value, pe)

INTEGER(KIND=8) SHMEM_INT8_SWAP
ires = SHMEM_INT8_SWAP(target, value, pe)

REAL(KIND=4) SHMEM_REAL4_SWAP
res = SHMEM_REAL4_SWAP(target, value, pe)

REAL(KIND=8) SHMEM_REAL8_SWAP
res = SHMEM_REAL8_SWAP(target, value, pe)

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🌸 DESCRIPTION 🌸

So like, here's the vibe: the atomic swap routines write value to address target on PE pe, and they return the previous contents of target — all in one atomic operation. no half-swaps, no messy in-between states, it's giving main character energy who has her life together fr fr 💅

Now let's meet the arguments, bestie:

• target — the remotely accessible integer data object that's getting updated on the remote PE. if you're in C/C++ land, the type of target should match that implied in the SYNOPSIS section (iykyk, type-matching is self care). if you're doing Fortran, it's gotta be one of these bad boys:
  • SHMEM_SWAP: Integer of default kind
  • SHMEM_INT4_SWAP: 4-byte integer
  • SHMEM_INT8_SWAP: 8-byte integer
  • SHMEM_REAL4_SWAP: 4-byte real
  • SHMEM_REAL8_SWAP: 8-byte real
• value — the value being atomically written to the remote PE. value is the same type as target. matching types, matching vibes, periodt 💖
• pe — an integer that tells you which PE number target lives on and is gonna be updated. if you're in Fortran, it must be a default integer value. no funny business 🌸

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🧚 NOTES 🧚

the term remotely accessible is defined in intro_shmem(3). go read that one if you need the full lore drop, iykyk 📖✨

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👑 RETURN VALUES 👑

what you get back is the contents that had been at the target address on the remote PE prior to the swap. like, the old thing. the before-times. the swap hands it to you on a little platter 🫶

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🦋 SEE ALSO 🦋

intro_shmem(3) — the whole friend group, honestly 💜

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so yeah, that's the whole page! here's the official tea at the bottom: this is from gitclone, the Unreleased developer copy, and the page itself is dated SHMEM_SWAP(3). bye bestie, go swap something atomically, slay 💅✨🎀