328. MPI Library Implementation Example

If the MPI library is implemented in C on a Unix system, then there are various options, including the two presented here, for supporting the name-shift requirement. The choice between these two options depends partly on whether the linker and compiler support weak symbols.

328.1. 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 as the following example shows:

Example

#pragma weak MPI_Example = PMPI_Example 
 
int PMPI_Example(/* appropriate args */) 
{ 
    /* Useful content */         
} 

The effect of this #pragma is to define the external symbol MPI_Example as a weak definition. This means that the linker will not complain if there is another definition of the symbol (for instance in the profiling library); however if no other definition exists, then the linker will use the weak definition.

328.2. Systems Without Weak Symbols

In the absence of weak symbols then one possible solution would be to use the C macro preprocessor as the following example shows:

Example

#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 that she wishes to intercept, references to any others being fulfilled by the normal MPI library. Therefore the link step can look something like this

% cc ... -lmyprof -lpmpi -lmpi