SHMEM_LONG_CSWAP(3)

📦

version 5.0.10 · 4 versions

💖 shmem_long_cswap(3) — the Open MPI girlies' guide to the conditional swap ✨

okay so bestie, gather round, because this page is actually a whole family of functions, not just one. The star of the show is shmem_long_cswap, but she brought her sisters:

shmem_int_cswap <#shmem-int-cswap>(3)
shmem_int4_cswap(3)
shmem_int8_cswap(3)
shmem_long_cswap(3)
shmem_longlong_cswap <#shmem-longlong-cswap>(3)

and together they do one (1) thing: Performs an atomic conditional swap to a remote data object. that's it, that's the girlboss job. 👑

✨ SYNOPSIS ✨

C or C++:

#include <mpp/shmem.h>

int shmem_int_cswap(int *target, int cond, int value, int pe)

long shmem_long_cswap(long *target, long cond, long value,
  int pe)

long long shmem_longlong_cswap(longlong *target,
  longlong cond, longlong value, int pe)

Fortran:

INCLUDE "mpp/shmem.fh"

INTEGER pe

INTEGER(KIND=4) SHMEM_INT4_CSWAP
ires = SHMEM_INT4_CSWAP(target, cond, value, pe)

INTEGER(KIND=8) SHMEM_INT8_CSWAP
ires = SHMEM_INT8_CSWAP(target, cond, value, pe)

🌸 DESCRIPTION 🌸

The conditional swap routines conditionally update a target data object on an arbitrary processing element (PE) and return the prior contents of the data object in one atomic operation. like, no rusty little race-condition crumbs left behind. periodt. 💅

The arguments are as follows:

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.

and then, because this page loves a double feature, here's the rundown again with cond joining the party:

target — The remotely accessible integer data object to be updated on the remote PE. If you are using C/C++, the data type of target should match that implied in the SYNOPSIS section. If you are using Fortran, it must be of the following type:
- SHMEM_INT4_CSWAP: 4-byte integer
- SHMEM_INT8_CSWAP: 8-byte integer
cond — cond is compared to the remote target value. If cond and the remote target are equal, then value is swapped into the remote target. Otherwise, the remote target is unchanged. In either case, the old value of the remote target is returned as the function return value. cond must be of the same data type as target.
value — The value to be atomically written to the remote PE. value must be the same data type as 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.

📝 NOTES 📝

The term remotely accessible is defined in intro_shmem(3). lowkey the origin story, you should read it. 🫶

💫 RETURN VALUES 💫

The contents that had been in the target data object on the remote PE prior to the conditional swap. she gives you the before pic, not the after. 📸

🔥 EXAMPLES 🔥

The following call ensures that the first PE to execute the conditional swap will successfully write its PE number to race_winner on PE 0.

main()
{
  static int race_winner = -1;
  int oldval;

  shmem_init();
  oldval = shmem_int_cswap(&race_winner, -1, shmem_my_pe(), 0);
  if (oldval == -1)
    printf("pe %d was first\n",shmem_my_pe());
}