XL Fortran Enterprise Edition for AIX : User's Guide - IBM

Order of Arguments in Argument List
The argument list is constructed in the following order. Items in the same bullet
appear in the same order as in the procedure declaration, whether or not argument
keywords are used in the call.
v All addresses or values (or both) of actual arguments 4
v “Present” indicators for optional arguments that are passed by value
v Length arguments for strings 4
Linkage Convention for Function Calls
A routine has two symbols associated with it: a function descriptor (name) and an
entry point (.name). When a call is made to a routine, the program branches to the
entry point directly. Excluding the loading of parameters (if any) in the proper
registers, compilers expand calls to functions to the following two-instruction
sequence:
BL .foo # Branch to foo
ORI R0,R0,0x0000
# Special NOP
The linker does one of two things when it encounters a BL instruction:
1. If foo is imported (not in the same object module), the linker changes the BL to
.foo to a BL to .glink (global linkage routine) of foo and inserts the .glink
into the object module. Also, if a NOP instruction (ORI R0,R0,0x0000)
immediately follows the BL instruction, the linker replaces the NOP instruction
with the LOAD instruction L R2, 20(R1).
2. If foo is bound in the same object module as its caller and a LOAD instruction
L R2,20(R1) for 32-bit and L R2,40(R1) for 64-bit, or ORI R0,R0,0 immediately
follows the BL instruction, the linker replaces the LOAD instruction with a
NOP (ORI R0,R0,0).
Note: For any export, the linker inserts the procedure’s descriptor into the object
module.
Pointers to Functions
A function pointer is a data type whose values range over procedure names.
Variables of this type appear in several programming languages, such as C and
Fortran. In Fortran, a dummy argument that appears in an EXTERNAL statement
is a function pointer. Fortran provides support for the use of function pointers in
contexts such as the target of a call statement or an actual argument of such a
statement.
A function pointer is a fullword quantity that is the address of a function
descriptor. The function descriptor is a 3-word object. The first word contains the
address of the entry point of the procedure. The second has the address of the
TOC of the object module in which the procedure is bound. The third is the
environment pointer for some non-Fortran languages. There is only one function
descriptor per entry point. It is bound into the same object module as the function
it identifies if the function is external. The descriptor has an external name, which
is the same as the function name but with a different storage class that uniquely
identifies it. This descriptor name is used in all import or export operations.
4. There may be other items in this list during Fortran-Fortran calls. However, they will not be visible to non-Fortran procedures
that follow the calling rules in this section.
Interlanguage Calls 365