Fortran compilers typically use 4 bytes for logical while C compilers usually use 1 byte for bool.

oneAPI flag -fpscomp logicals ensures integer values 0,1 corresponding to .false., .true. are used. Otherwise by default unexpected values may cause programs to fail at runtime with impossible boolean values like 255.

In CMake this flag is applied like:

if(CMAKE_Fortran_COMPILER_ID MATCHES "^Intel")
  add_compile_options("$<$<COMPILE_LANGUAGE:Fortran>:-fpscomp;logicals>")
endif()

For C interoperability, Fortran can use:

use, intrinsic :: iso_c_binding

logical(kind=C_BOOL) :: L
logical :: Q

c_sizeof(L) == 1
c_sizeof(Q) == 4
! typically

while C uses:

#include <stdbool.h>
#include <stdio.h>

int main(void) {
bool L;
printf("%zu\n", sizeof(L));
}

and likewise C++ bool is typically 1 byte:

#include <iostream>

int main(){
  bool L;
  std::cout << sizeof(L) << "\n";
}

Always use iso_c_binding when using C or C++ with Fortran modules to produce cross-platform compatible projects.

See “bool” examples for interfacing between C, C++ and Fortran.

Do not use

The Intel oneAPI compiler -standard-semantics flag to use C_BOOL values correctly between C, C++ and Fortran has a BIG DOWNSIDE in that it breaks linking with system libraries–including IntelMPI! All projects and libraries linking to a Fortran library that used oneAPI -standard-semantics must also use -standard-semantics.