Systems With Weak symbols

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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_Send = PMPI_Send
int PMPI_Send(/* appropriate args */)
    /* Useful content */        

The effect of this #pragma is to define the external symbol MPI_Send 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. This type of situation is illustrated in Fig. gif, in which a profiling library has been written that profiles calls to MPI_Send() but not calls to MPI_Bcast(). On systems with weak links the link step for an application would be something like

% cc ... -lprof -lmpi

References to MPI_Send() are resolved in the profiling library, where the routine then calls PMPI_Send() which is resolved in the MPI library. In this case the weak link to PMPI_Send() is ignored. However, since MPI_Bcast() is not included in the profiling library, references to it are resolved via a weak link to PMPI_Bcast() in the MPI library.

Figure: Resolution of MPI calls on systems with weak links.

Jack Dongarra
Fri Sep 1 06:16:55 EDT 1995