The Portability Library contains library functions that are not built-in to the Fortran language. These functions are in the user name space. Whilst the corresponding library is available by default, these functions are linked only when there is no global definition in your program that satisfies the global reference.
While not required, inserting a USE statement to interface to these functions is recommended as the easiest way to pick up the correct interface and avoid hard-to-debug errors due to type mismatches. If pollution of the user namespace is a concern, specify the ONLY clause with the USE statement.
If you want to include interfaces for all the routines in your program, you can either?
include the file iflport.f90 from the INCLUDE directory of your compiler distribution,
add a USE IFLPORT statement to access the INTERFACEs for all portability functions.
A complete list of portability library functions is provided below. For more information on these library routines, please consult the Intel® Fortran Libraries Reference.