MPI library implementation

On a Unix system, in which the MPI library is implemented in C, then there are various possible options, of which two of the most obvious are presented here. Which is better depends on whether the linker and compiler support weak symbols.

Systems with weak symbols

If the compiler and linker support weak external symbols (e.g. Solaris 2.x, other system V.4 machines), then only a single library is required through the use of #pragma weak thus

 
 
#pragma weak MPI_Example = PMPI_Example 

int PMPI_Example(/* appropriate args */) 
{ 
    /* Useful content */         
} 

Systems without weak symbols

In the absence of weak symbols then one possible solution would be to use the C macro pre-processor thus

 
 
#ifdef PROFILELIB 
#    ifdef __STDC__ 
#        define FUNCTION(name) P##name 
#    else 
#        define FUNCTION(name) P/**/name 
#    endif 
#else 
#    define FUNCTION(name) name 
#endif 

 
 
int FUNCTION(MPI_Example)(/* appropriate args */) 
{ 
    /* Useful content */         
} 

It is required that the standard MPI library be built in such a way that the inclusion of MPI functions can be achieved one at a time. This is a somewhat unpleasant requirement, since it may mean that each external function has to be compiled from a separate file. However this is necessary so that the author of the profiling library need only define those MPI functions which she wishes to intercept, references to any others being fulfilled by the normal MPI library. Therefore the link step can look something like this