Optional Arguments

Access to Optional Arguments which are Not Present

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Optional Arguments and IF (PRESENT(...))

If an optional argument is accessed during a sub-program call in which it is not present, an error is likely to occur. Access to optional arguments may be guarded by IF (PRESENT( <argument> )) constructs.

What Happens if an Access is Made when the Argument is Not Present?

The possibilities are (At least):

The optional argument is referenced through a null pointer. The program will probably crash.
A valid pointer remains in place from a previous call, and the object which it points to is static, or is still at the same address on the heap or in a stack frame. The last value will be used. No error will be seen though the result will not be as intended.
A valid pointer remains but the data to which it points is no longer present. Something will probably be fetched, and if the argument has INTENT(OUT) or INTENT(INOUT) something will be overwritten. No immediate error may be seen but an error will occur.

Testing for IF (PRESENT(...))

The fpt command to test whether optional arguments are guarded is:

CHECK OPTIONAL ARGUMENTS

fpt finds each executable statement which contains an optional argument and traces program flow backwards towards the entry to the routine in which it occurs. If there is a path to the start of the routine, the occurrence is marked as a potential unguarded reference.

Why Only a Potential Unguarded Reference?

For at least two reasons:

There may be groups of optional arguments which are always either all present or all absent. A test for any one of them may be sufficient.
The presence of one or more arguments may be registered in a variable which may be tested. fpt does not yet track this possibility.

The end of the report, including a summary of the number of occurrences, is shown below for WRFV3.6.1.

------------------------------------------------------------------------------ !!! WARNING !!! Number 3637 Severity 1 (Worst 2) Count 12175 Line 1655 File: ...cation/test/UCAR_clean_build_WRFV3.6.1/WRFV3/phys/module_pbl_driver.F ,z = z,t_phy = t_phy,qc_curr = qc_curr,qi_curr = qi_curr,z_at_w = z_at_w & ^ Unguarded use of an OPTIONAL argument which is always PRESENT ------------------------------------------------------------------------------ ------------------------------------------------------------------------------ !!! WARNING !!! Number 3637 Severity 1 (Worst 2) Count 12176 Line 1664 File: ...st/UCAR_clean_build_WRFV3.6.1/WRFV3/phys/module_microphysics_driver.F #if ( WRF_CHEM == 1 ) qv_b4mp(its:ite,kts:kte,jts:jte) = qv_curr(its:ite,kts:kte,jts:jte) qc_b4mp(its:ite,kts:kte,jts:jte) = qc_curr(its:ite,kts:kte,jts:jte) qi_b4mp(its:ite,kts:kte,jts:jte) = qi_curr(its:ite,kts:kte,jts:jte) qs_b4mp(its:ite,kts:kte,jts:jte) = qs_curr(its:ite,kts:kte,jts:jte) #endif ,t_phy = t_phy,pi_phy = pi_phy,z_at_w = z_at_w,qfx = qfx & ^ Unguarded use of an OPTIONAL argument which is always PRESENT ------------------------------------------------------------------------------ Check for unguarded access to OPTIONAL arguments ================================================ Optional arguments: 3194 Not always present and with unguarded references (Errors): 35 Always present but with unguarded references (Warnings): 1325 Arguments with possible unguarded references (Warnings): 1 Total number of unguarded references: 2529

A run-time check for unguarded references

The fpt command:

INSERT CHECK OF OPTIONAL ARGUMENTS

inserts a check for presence before every access to an optional argument where no guard has already been identified. Within the check, a call is inserted to a subroutine which counts the number of visits to this location. The count is held in an array indexed by a unique integer identifier for each call.

A section of the instrumented code from WRFV3.6.1 is shown below:

Target found at line: 438 in file: .../UCAR_clean_build_WRFV3.6.1/WRFV3/phys/module_lightning_driver.F CALL wrf_debug(100,' lightning_driver: using option-default IC:CG method') ICCG_METHOD_DEFAULT:SELECT CASE(lightning_option) ! CASE(ltng_crm_pr92w,ltng_crm_pr92z,ltng_cpm_pr92z)ICCG_METHOD_DEFAULT IF (.NOT. PRESENT(cg_flashrate)) THEN CALL fpt_check_present(386) ENDIF IF (.NOT. PRESENT(ic_flashrate)) THEN CALL fpt_check_present(909) ENDIF CALL iccg_boccippio( & xlat,xlon, & iccg_prescribed_num,iccg_prescribed_den, & ! Order dependent args for domain, mem, and tile dims ids,ide,jds,jde,kds,kde, & ims,ime,jms,jme,kms,kme, &