MPI_ISEND(3)

💌 MPI_ISEND(3) — the Open MPI glow-up 💌

okay so bestie, welcome to the MPI_ISEND(3) page from the Open MPI manual ✨ this is the one where you yeet a message into the void without waiting for it to land, and honestly? it's giving main character energy 🎀

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🌸 NAME 🌸

MPI_Isend — starts a standard-mode, nonblocking send. that's it. that's the vibe 💅

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✨ SYNTAX ✨

💻 C Syntax

#include <mpi.h>

int MPI_Isend(const void *buf, int count, MPI_Datatype datatype, int dest,
     int tag, MPI_Comm comm, MPI_Request *request)

🧚 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_ISEND(BUF, COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR)
     <type>  BUF(*)
     INTEGER COUNT, DATATYPE, DEST, TAG, COMM, REQUEST, IERROR

🦋 Fortran 2008 Syntax

USE mpi_f08
MPI_Isend(buf, count, datatype, dest, tag, comm, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: buf
     INTEGER, INTENT(IN) :: count, dest, tag
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🍓 INPUT PARAMETERS 🍓

• buf — initial address of send buffer (choice). like, where your actual data lives bestie 🫶
• count — number of elements in send buffer (integer). counting is self-care ✨
• datatype — datatype of each send buffer element (handle) 🎀
• dest — rank of destination (integer). who's receiving this masterpiece 👑
• tag — message tag (integer). your little label so they know what it is 🏷️
• comm — communicator (handle). the group chat, basically 💬

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💜 OUTPUT PARAMETERS 💜

• request — communication request (handle). hold onto this, you'll need it later 👀
• ierror — Fortran only: error status (integer).

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🔥 DESCRIPTION 🔥

okay so MPI_Isend starts a standard-mode, nonblocking send — nonblocking being the whole point, the way that it returns immediately instead of making you stand there waiting 😭

nonblocking calls allocate a communication request object and associate it with the request handle (the request argument, iykyk). you can use that request later to query the status of the communication or to wait for its completion. it's literally your lil receipt 🧾

here's the part you cannot skip: a nonblocking send call indicates that the system may start copying data out of the send buffer. so the sender should not modify any part of the send buffer after a nonblocking send operation is called, until the send completes. no cap. don't touch the buffer. leave it alone. it's busy 💀

you can determine that a send request is completed by calling MPI_Wait, MPI_Waitany, MPI_Test, or MPI_Testany with the request returned by this function. those four are your check-in crew 🌈

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💀 ERRORS 💀

almost all MPI routines return an error value — C routines as the return result of the function, and Fortran routines in the last argument. classic 🙈

before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. if no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. when MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. the initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec, or the info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. if no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

⚠️ heads up bestie: MPI does not guarantee that an MPI program can continue past an error. so like... don't count on it.

Open MPI includes three predefined error handlers that can be used:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. dramatic? yes. effective? also yes 💅
• MPI_ERRORS_ABORT — an error handler that can be invoked on a communicator, window, file, or session. when called on a communicator, it acts as if MPI_Abort was called on that communicator. if called on a window or file, it acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. if called on a session, it aborts only the local process.
• MPI_ERRORS_RETURN — returns an error code to the application. the chill one 🫶

MPI applications can also implement their own error handlers by calling:

• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler

note that MPI does not guarantee that an MPI program can continue past an error (yes, saying it twice, it's that serious 😤).

see the MPI man page for a full list of MPI error codes. see the Error Handling section of the MPI-3.1 standard for more information 📚

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🌟 SEE ALSO 🌟

• MPI_Send
• MPI_Wait
• MPI_Waitany
• MPI_Test
• MPI_Testany

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🎀 COPYRIGHT 🎀

2003-2026, The Open MPI Community — respect the elders 💖

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okay that's the whole page, go forth and nonblocking-send responsibly, mwah 💋✨

— MPI_ISEND(3) · Open MPI · Sep 11, 2026 — 🦋💜🌸