OMPI_AFFINITY_STR(3)

💅 OMPI_AFFINITY_STR(3) — Open MPI, bestie edition 🎀

okay so this is the manual page for OMPI_Affinity_str from Open MPI, and it's genuinely one of the cutest little introspection functions you'll ever meet. It's giving "tell me where I live on this machine" energy. No cap. ✨

🫶 NAME

OMPI_Affinity_str — Obtain prettyprint strings of processor affinity information for this process 💖

🌸 SYNTAX

✨ C Syntax

#include <mpi.h>
#include <mpi-ext.h>

int OMPI_Affinity_str(ompi_affinity_fmt_type_t fmt_type,
                      char ompi_bound[OMPI_AFFINITY_STRING_MAX],
                      char current_binding[OMPI_AFFINITY_STRING_MAX],
                      char exists[OMPI_AFFINITY_STRING_MAX])

😭 Fortran Syntax

There is no Fortran binding for this function. literally none. she's C-only, sorry babe.

💀 Fortran 2008 Syntax

There is no Fortran 2008 binding for this function. bestie I checked, it's not there.

🙈 C++ Syntax

There is no C++ binding for this function. nope, not a thing.

🦋 INPUT PARAMETERS

• fmt_type — an enum that tells the function how to format the ompi_bound and current_binding strings it hands back. OMPI_AFFINITY_RSRC_STRING_FMT makes it return the string as human-readable resource names, like "socket 0, core 0" — super readable, iykyk. 🌸
• OMPI_AFFINITY_LAYOUT_FMT — this one returns ASCII art showing where this MPI process is bound relative to the machine's resource layout. For example "[. B][. .]" means the process that called the routine is bound to socket 0, core 1, on a system with 2 sockets each containing 2 cores. the way that little B just sits there 👑
• See below for more output examples. ⭐

💜 OUTPUT PARAMETERS

• ompi_bound — a prettyprint string describing what processor(s) Open MPI bound this process to, or a string saying Open MPI did not bind this process at all (rude of them, but okay). 🫶
• current_binding — a prettyprint string describing what processor(s) this process is currently bound to, or a string saying the process is bound to all available processors (which is why we call it "unbound"). 💅
• exists — a prettyprint string describing the available sockets and sockets on this host. (yes, it says sockets and sockets, that's what the page says, don't come for me 😭)

🌈 DESCRIPTION

Open MPI may bind a process to specific sockets and/or cores at process launch time. This non-standard Open MPI function call returns prettyprint information about three things, and the three things are iconic:

Where Open MPI bound this process. The string returned in ompi_bound will either indicate that Open MPI did not bind this process to anything, or it will contain a prettyprint description of the processor(s) to which Open MPI bound this process. 💖

Where this process is currently bound. Regardless of whether Open MPI bound this process or not, another entity may have bound it. The string returned in current_binding will indicate what the current binding is of this process, regardless of what Open MPI may have done earlier. The string returned will either indicate that the process is unbound (meaning that it is bound to all available processors) or it will contain a prettyprint description of the sockets and cores to which the process is currently bound. main character energy tbh 🦋

What processors exist. As a convenience to the user, the exists string will contain a prettyprint description of the sockets and cores that this process can see (which is usually all processors in the system). a little gift, just for you 🎀

---

🌟 EXAMPLES

✨ Example 1: Print out processes binding using resource string format.

int rank;
char ompi_bound[OMPI_AFFINITY_STRING_MAX];
char current_binding[OMPI_AFFINITY_STRING_MAX];
char exists[OMPI_AFFINITY_STRING_MAX];

MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);

OMPI_Affinity_str(OMPI_AFFINITY_RSRC_STRING_FMT,
                  ompi_bound, current_binding, exists);
printf("rank %d: \n"
       "       ompi_bound: %s\n"
       "  current_binding: %s\n"
       "           exists: %s\n",
       rank, ompi_bound, current_binding, exists);
...

Output of mpirun -n 2 -bind-to-core a.out:

rank 0:
       ompi_bound: socket 0[core 0]
  current_binding: socket 0[core 0]
           exists: socket 0 has 4 cores
rank 1:
       ompi_bound: socket 0[core 1]
  current_binding: socket 0[core 1]
           exists: socket 0 has 4 cores

Output of mpirun -n 2 -bind-to-socket a.out:

rank 0:
       ompi_bound: socket 0[core 0-3]
  current_binding: Not bound (or bound to all available processors)
           exists: socket 0 has 4 cores
rank 1:
       ompi_bound: socket 0[core 0-3]
  current_binding: Not bound (or bound to all available processors)
           exists: socket 0 has 4 cores

🦋 Example 2: Print out processes binding using layout string format.

int rank;
char ompi_bound[OMPI_AFFINITY_STRING_MAX];
char current_binding[OMPI_AFFINITY_STRING_MAX];
char exists[OMPI_AFFINITY_STRING_MAX];

MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);

OMPI_Affinity_str(OMPI_AFFINITY_LAYOUT_FMT,
                  ompi_bound, current_binding, exists);
printf("rank %d: \n"
       "       ompi_bound: %s\n"
       "  current_binding: %s\n"
       "           exists: %s\n",
       rank, ompi_bound, current_binding, exists);
...

Output of mpirun -n 2 -bind-to-core a.out:

rank 0:
       ompi_bound: [B . . .]
  current_binding: [B . . .]
           exists: [. . . .]
rank 1:
       ompi_bound: [. B . .]
  current_binding: [. B . .]
           exists: [. . . .]

Output of mpirun -n 2 -bind-to-socket a.out:

rank 0:
       ompi_bound: [B B B B]
  current_binding: [B B B B]
           exists: [. . . .]
rank 1:
       ompi_bound: [B B B B]
  current_binding: [B B B B]
           exists: [. . . .]

---

👑 SEE ALSO

• mpirun(1) — go read that one next bestie, it's the whole vibe 🔥

🍓 COPYRIGHT

2003-2026, The Open MPI Community

---

and that's the whole page!! 💖✨🎀🦋🌸💅🫶😭🔥👑🌈⭐💀🙈🍓🧚💜

this one was brought to you by Open MPI, dated Sep 11, 2026 — go be bound to something cute, ily, mwah 🎀