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

The Intel oneAPI compiler needs the -fpscomp logicals flag to use C_BOOL values correctly between C, C++ and Fortran. This flag 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("$<$<AND:$<COMPILE_LANGUAGE:Fortran>,$<NOT:$<BOOL:${WIN32}>>>:-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.