Fortran Unsigned Wrapper (funsignedwrapper)

Motivation

In contrast to C, Fortran does not provide unsigned data types. This can cause problem when interfacing C functions that require unsigned arguments. For example the components of RGB-values or parts of IPv4 adresses are often expressed as unsigned 8-bit integers.

In principle it is possible to pass signed integers with the same bit pattern as the unsigned value one wants to pass (For example, both an unsigned 8-bit integer with the value 128 and a signed integer with the value -128 have the same bit pattern (binary: 1000 0000) if the Two's complement is used as is it the case for most systems. This may, however, lead to code that is difficult to read and maintain.

Functions

There are two types of functions in the module funsignedwrapper that try to address the problem in an easier readable way.

Create an unsigned value

intX_to_uintY stores the numerical value of a larger (i.e., stored using Y bits) integer in a smaller (signed) integer variable (stored using X bits) using a bit pattern that corresponds to a (C-type) unsigned variable of the same size.

Here Y > X.

For X the values 8, 16and 32 are possible. For Y tha values 16, 32 and 64 are possible

The algorithm assumes that the signed Fortran integers are described using the Two's complement. Furthermore it assumes the same endianness for all interger values, i.e. it is not possible to convert big-endian in little-endian variables and vice versa.

Read an unsigned value

uintX_to_intY performs the inverse operation, i.e. the bit pattern of a "smaller" variable is interpreted as the bit pattern of an unsigned integer.

The possible range for X and Y is the same as for the functions that create the bit patterns for the unsigned integers.

Building the Module

By downloading the code and calling make the code of the module is generated including the object file (funsignedwrapper.o for Unix systems) and the module file (funsignedwrapper.mod).

Furthermore a set of test programs are written and executed during the make process that output whether the comversions lead to the expected results.

make clean removes the generated files.

Requirements

The module requires a Fortran compiler that supports the constants int8, int16, int32 and int64 in the module iso_fortran_env. This is the case for all compilers comforming to the Fortran 2008 standard, but may also be supported by Compilers that otherwise do not fully support Fortran 2008.

The test programs further require a C99 comforming C compiler where the typedefs uint8_t, uint16_t, uint_32_t and uint64_t are defined in stdint.h.

The Module has been tested with gfortran and gcc 9.3 on an AMD64 system.

Problems and TODOS

• The performance currently is very poor (approximately two orders of magnitude slower than the suggestions to use Fortran's built-in KIND functions () in https://fortran-lang.discourse.group/t/module-for-dealing-with-unsigned-integers-in-standard-fortran/1242/3. So the conversion large signed ==> small unsigned should only use the functions of this module if readability is much more important than performance. The functions for the conversion small unsigned ==> large unsigned may be more useful.

• Compiling the code will fail if one of the integer sizes 8, 16, 32 or 64 bit is not available.

  If a value is passed to a function that does not " fit" into the smaller unsigned variable, a modulo operation is performed and no error flag is set.

• Cleanup of the Makefile.

• More testing...