Scientific Computing

Gfortran 15.1 has a bug with the -Wexternal-argument-mismatch flag. This flag is implicitly included in the -Wall option. In Gfortran 15.1–fixed in Gfortran 15.2–the -Wexternal-argument-mismatch warning causes compilation to fail on correct code. To work around this bug, suggest in the build system such as CMake to detect GCC 15.1 and disable the -Wexternal-argument-mismatch warning. It’s also relevant to consider if -Wall is a flag one wants to automatically set for the default end-user build, as such a bug could happen for future versions of GCC as well.

In CMake, detect GCC 15.1 and disable the problem flag:

if(CMAKE_Fortran_COMPILER_ID STREQUAL "GNU" AND
   CMAKE_Fortran_COMPILER_VERSION VERSION_GREATER_EQUAL "15.0" AND
   CMAKE_Fortran_COMPILER_VERSION VERSION_LESS "15.2"
   )
  add_compile_options($<$<COMPILE_LANGUAGE:Fortran>:-Wno-external-argument-mismatch>)
endif()

Fortran bind(C) allows Fortran to call C functions and vice versa. However, Fortran arrays are one-based by default, while C arrays are zero-based. This can lead to confusion when passing arrays between Fortran and C. Adding to the confusion is multiple GCC Gfortran versions have known bugs that were not fixed as of their final release.

Specifically, the versions of Gfortran known to be working correctly are Gfortran ≤ 8.5 and Gfortran ≥ 12. Known broken GFortran versions are 9.x, 10.x, 11.x.

We show this by example:

program test_bounds
use iso_fortran_env, only : error_unit
implicit none

real, allocatable :: a(:)
integer :: L1,L2, U1,U2
allocate(a(1:2))

L1 = lbound(a,1)
U1 = ubound(a,1)

call c_bounder(a)

L2 = lbound(a,1)
U2 = ubound(a,1)

if (L1 /= L2 .or. U1 /= U2) then
  write(error_unit, '(a,2i2,a,2i2)') 'FAIL: bounds changed before/after lower:', L1,L2, " upper: ", U1,U2
  error stop
endif

print '(a)', "bounds check OK"

contains

subroutine c_bounder(a) bind(c)
real,  intent(inout) :: a(:)
end subroutine c_bounder

end program

Some code may have integer declarations like u_int, u_long, etc. These types are not part of the C or C++ standard and may not be portable across different platforms. Instead, it’s recommended to use the standard integer types, including those defined in <stdint.h> (C) or <cstdint> (C++). For example, replace “u_int” with unsigned int or a fixed-width type appropriate for the use case. “u_int” is from header <sys/types.h> and is not part of the C standard, and it may not be available on systems like Windows.

GDB debugger is not yet readily available for macOS with Apple Silicon CPU. GDB is installable on Apple Silicon Macs via Homebrew, but it is not functional on Apple Silicon Macs for local executables.

It has long been possible to use a virtual machine with Linux for ARM64 to run GDB within the VM. When trying to use GDB on Apple Silicon for local executables, it doesn’t work:

gdb ./myexe

r

Don’t know how to run. Try “help target”.

To use GDB on an Apple Silicon Mac for local executables, it seems a Linux VM is the best option.

The Mathworks experienced a major outage starting on May 18, 2025 due to a ransomware attack. The Mathworks’ license servers have been effectively required by default for Matlab for ~ five years. Virtually all Mathworks services were down initially for about a week, and then services are gradually being restored. The outage affects users worldwide regarding ability to start Matlab or install toolboxes. Trying to access Mathworks services results in a 503 return code or “no healthy upstream” or similar. Previously, external institutions cited a cyberattack.

Internet discussions have been peppered with recommendations to switch to GNU Octave, which is a free and open-source alternative to Matlab. However, the proprietary toolboxes that add so much value and functionality to Matlab are often not available for GNU Octave. Examine the GNU Octave Help forum to assess issues facing Octave users vs. Matlab Answers forum. It behooves instructors to consider avoiding proprietary toolboxes except where essential to the learning goals. Note that Python packages (and libraries in just about every programming language) have also experienced cyberattacks. Matlab adds serious value to many research and development projects, but the reliance on a single vendor for such a critical tool is a risk that should be considered.

Transitioning to open-source alternatives like GNU Octave or Python take considerable effort. Using the Matlab Engine for Python can help incrementally transition Matlab projects to Python.

Apple has adaptive impedance for the TRRS connector on MacBook. Windows Logo certified computers likewise have standards for voltage levels and impedance for audio input and output. Instead of the built-in computer audio interface, we generally recommend using an external USB sound card to reduce risk of damage to the computer in case of voltage surges. Consider two 3.5mm TRS cables (one each for input and output) to the radio connector (typically the microphone connector that also has fixed level audio output) and a USB sound card. The USB sound card can be used on different computers once it’s known to work with the radio audio interface. At a minimum, a grounded coaxial surge protector should be used in the antenna feedline along with a fused, surge-protected power supply. Consider using an older laptop that still runs a currently supported operating system as a dedicated radio computer. This avoids risking an expensive laptop when an older laptop is often more than adequate for digital ham radio modes.

When a desired website is down or a new website is about to become available, one can use the “curl” program to check the status of the website HTTP response code. Using a periodic check with watch program one can monitor the status for an extended period of time. Keep in mind that the website might have anti-DOS measures that block repeated requests from the same IP address. Set “watch” interval and maximum number of requests to avoid being blocked.

Use a command like the following to check every 60 seconds for up to 60 times (1 hour):

watch -n 60 -q 60 curl -s -w %{response_code} -o /dev/null URL

Curl command reference:

curl -s -w %{response_code} -o /dev/null URL

-o /dev/null

discard the webpage content to avoid screen clutter. On Windows, use -o NUL instead.

-s

silent mode

-w %{response_code}

write the HTTP response code to console

URL

the desired URL to check

C++ attribute specifiers add metadata to declarations and definitions. The metadata can be used by the compiler and by the developer to help understand the code intent.

Commonly used C++17 attributes such as [[maybe_unused]] and [[fallthrough]] suppress compiler warnings and signal developer intent for clarity.

C++20 attributes such as [[likely]] and [[unlikely]] may be used by the compiler to optimize compilation, and also provide human code readers insight into the intent of the algorithm.

GCC ≥ 5 feature test macro __has_cpp_attribute() checks if an attribute is supported by the compiler–even if the command line specified standard is older.

maybe_unused attribute

To enable code to fallback to no attribute with older compilers, use [[maybe_unused]] attribute in declaration AND in the definition to be compatible with GCC:

int windows_fun(int x, [[maybe_unused]] int y) {

#ifdef _WIN32
  return y;
#else
  return x;
#endif
}

On non-Windows systems, the compiler would have issued a warning about y being an unused argument. The [[maybe_unused]] attribute suppresses that warning.

fallthrough attribute

The [[fallthrough]] attribute is used to indicate that a fall-through in a switch statement is intentional. This attribute can be used to suppress warnings about missing break statements in a switch block.

In the header file:

#ifndef __has_cpp_attribute
#define __has_cpp_attribute(x) 0
#endif

#if __has_cpp_attribute(likely)
#define LIKELY [[likely]]
#else
#define LIKELY
#endif

switch (x) {
  case 1:
    x++;
    [[fallthrough]];
  case 2:  LIKELY
    x--;
}

When using cmake_path(GET in PARENT_PATH out) to manipulate paths in CMake, be cautious about trailing slashes. The usual intent is to get the parent directory of a given path. If the trailing slash is not stripped, the same directory is returned, which may be unexpected.

Strip the trailing slash first like:

set(p "/path/to/directory/")

# Strip trailing slash
string(REGEX REPLACE "[\\/]+$" "" p "${p}")

cmake_path(GET p FILENAME n)

h5labview is one way to read and write HDF5 files in Labview.