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MPI_Init_thread(3) man page (version 1.10.1)

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MPI_Init_thread - Initializes the MPI execution environment


C Syntax

#include <mpi.h>
int MPI_Init_thread(int *argc, char ***argv,
    int required, int *provided)

Fortran Syntax

INCLUDE ’mpif.h’

C++ Syntax

#include <mpi.h>
int MPI::Init_thread(int& argc, char**& argv, int required)
int MPI::Init_thread(int required)

Input Parameters

C/C++ only: Pointer to the number of arguments.
C/C++ only: Argument vector.
Desired level of thread support (integer).

Output Parameters

Available level of thread support (integer).
Fortran only: Error status (integer).


This routine, or MPI_Init, must be called before any other MPI routine (apart from MPI_Initialized) is called. MPI can be initialized at most once; subsequent calls to MPI_Init or MPI_Init_thread are erroneous.

MPI_Init_thread, as compared to MPI_Init, has a provision to request a certain level of thread support in required:

Only one thread will execute.
If the process is multithreaded, only the thread that called MPI_Init_thread will make MPI calls.
If the process is multithreaded, only one thread will make MPI library calls at one time.
If the process is multithreaded, multiple threads may call MPI at once with no restrictions.

The level of thread support available to the program is set in provided, except in C++, where it is the return value of the function. In Open MPI, the value is dependent on how the library was configured and built. Note that there is no guarantee that provided will be greater than or equal to required.

Also note that calling MPI_Init_thread with a required value of MPI_THREAD_SINGLE is equivalent to calling MPI_Init.

All MPI programs must contain a call to MPI_Init or MPI_Init_thread. Open MPI accepts the C/C++ argc and argv arguments to main, but neither modifies, interprets, nor distributes them:

        /* declare variables */
        MPI_Init_thread(&argc, &argv, req, &prov);
        /* parse arguments */
        /* main program */


The Fortran version does not have provisions for argc and argv and takes only IERROR.

It is the caller’s responsibility to check the value of provided, as it may be less than what was requested in required.

The MPI Standard does not say what a program can do before an MPI_Init_thread or after an MPI_Finalize. In the Open MPI implementation, it should do as little as possible. In particular, avoid anything that changes the external state of the program, such as opening files, reading standard input, or writing to standard output.


MPI_THREAD_MULTIPLE support is included if Open MPI was configured with the --enable-mpi-thread-multiple configure switch. You can check the output of ompi_info(1) to see if Open MPI has MPI_THREAD_MULTIPLE support:

shell$ ompi_info | grep -i thread
          Thread support: posix (mpi: yes, progress: no)

The "mpi: yes" portion of the above output indicates that Open MPI was compiled with MPI_THREAD_MULTIPLE support.

Note that MPI_THREAD_MULTIPLE support is only lightly tested. It likely does not work for thread-intensive applications. Also note that only the MPI point-to-point communication functions for the BTL’s listed below are considered thread safe. Other support functions (e.g., MPI attributes) have not been certified as safe when simultaneously used by multiple threads.


Note that Open MPI’s thread support is in a fairly early stage; the above devices are likely to work, but the latency is likely to be fairly high. Specifically, efforts so far have concentrated on correctness, not performance (yet).


Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

See Also


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