Calling C Math Functions from Fortran Using ISO_C_BINDING

Desired outcome

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Invoke a C library routine (e.g., the sin function from libm) from a Fortran program and obtain the same result as Fortran’s intrinsic SIN.

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Prerequisites

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• A Fortran 2003‑or‑later compiler that provides the intrinsic module ISO_C_BINDING (e.g., gfortran ≥ 4.6, ifort, nagfor).
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• A C compiler and the standard math library (libm on Linux/macOS, msvcrt on Windows).
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• Access to a terminal or command prompt with permission to compile and link executables.
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Procedure

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1. Create a Fortran source file, e.g., call_csin.f90, that uses ISO_C_BINDING to declare the C function.

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   program call_csin\n  use, intrinsic :: iso_c_binding\n  implicit none\n  real(c_double) :: x, y_c, y_f\n  interface\n     real(c_double) function c_sin(x) bind(c, name='sin')\n        import :: c_double\n        real(c_double), value :: x\n     end function c_sin\n  end interface\n\n  x = 0.5_c_double\n  y_c = c_sin(x)          ! call the C sin\n  y_f = sin(x)            ! Fortran intrinsic for comparison\n  print *, 'C sin(', x, ') = ', y_c\n  print *, 'Fortran sin(', x, ') = ', y_f\n  print *, 'Difference = ', abs(y_c - y_f)\nend program call_csin\n

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2. Compile the Fortran source to an object file. Run the command in a terminal where you have write permission for the current directory.

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   gfortran -c -Wall -fcheck=all call_csin.f90 -o call_csin.o

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   Replace gfortran with your Fortran compiler if needed. The flags -Wall and -fcheck=all help catch mismatches early.

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3. Compile a trivial C wrapper (optional) or directly link the Fortran object with the math library. The C function sin resides in libm, so we link with -lm.

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   gfortran call_csin.o -o call_csin -lm

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   On Windows with MSVC, you would link against legacy_stdio_def.lib and vcruntime.lib; the principle is the same.

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4. Run the resulting executable.

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   ./call_csin

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Expected checks

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• Compilation completes without warnings about type mismatches or missing BIND(C) attributes.
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• Linking succeeds; the linker does not report undefined references to sin.
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• Execution prints two nearly identical numbers and a difference close to machine epsilon (e.g., 1.000000000000000E-016 or smaller).
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• No segmentation fault or runtime error occurs.
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Recovery options

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• Linking fails with “undefined reference to `sin’”: ensure you added -lm (or the platform‑specific math library) after the object file. Verify spelling and case; on some systems the symbol may be prefixed with an underscore.
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• Wrong numeric results: check that the Fortran kind matches the C type. Use integer(c_int), real(c_double), etc., as shown. Avoid default integer or real unless you have confirmed they are interchangeable with the C types on your platform.
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• Suspected name‑mangling issues: inspect the generated symbol with nm call_csin.o or objdump -t call_csin.o. The Fortran compiler may append an underscore; adjust the name='' argument in the bind(c, name='…') clause accordingly.
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• Debugging: recompile with -g and use gdb to step into the C function; verify that the argument value received by C matches the Fortran input.
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