2.3. Procedure Specification

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MPI procedures are specified using a language-independent notation. The arguments of procedure calls are marked as IN, OUT or INOUT. The meanings of these are:

There is one special case --- if an argument is a handle to an opaque object (these terms are defined in Section Opaque Objects ), and the object is updated by the procedure call, then the argument is marked OUT. It is marked this way even though the handle itself is not modified --- we use the OUT attribute to denote that what the handle references is updated. Thus, in C++, IN arguments are either references or pointers to const objects.


The definition of MPI tries to avoid, to the largest possible extent, the use of INOUT arguments, because such use is error-prone, especially for scalar arguments. ( End of rationale.)
MPI's use of IN, OUT and INOUT is intended to indicate to the user how an argument is to be used, but does not provide a rigorous classification that can be translated directly into all language bindings (e.g., INTENT in Fortran 90 bindings or const in C bindings). For instance, the ``constant'' MPI_BOTTOM can usually be passed to OUT buffer arguments. Similarly, MPI_STATUS_IGNORE can be passed as the OUT status argument.

A common occurrence for MPI functions is an argument that is used as IN by some processes and OUT by other processes. Such an argument is, syntactically, an INOUT argument and is marked as such, although, semantically, it is not used in one call both for input and for output on a single process.

Another frequent situation arises when an argument value is needed only by a subset of the processes. When an argument is not significant at a process then an arbitrary value can be passed as an argument.

Unless specified otherwise, an argument of type OUT or type INOUT cannot be aliased with any other argument passed to an MPI procedure. An example of argument aliasing in C appears below. If we define a C procedure like this,

void copyIntBuffer( int *pin, int *pout, int len ) 
{   int i; 
    for (i=0; i<len; ++i) *pout++ = *pin++; 
then a call to it in the following code fragment has aliased arguments.
int a[10]; 
copyIntBuffer( a, a+3, 7); 
Although the C language allows this, such usage of MPI procedures is forbidden unless otherwise specified. Note that Fortran prohibits aliasing of arguments.

All MPI functions are first specified in the language-independent notation. Immediately below this, the ANSI C version of the function is shown followed by a version of the same function in Fortran and then the C++ binding. Fortran in this document refers to Fortran 90; see Section Language Binding .

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