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<h4 class="subsection">7.1.4 Interoperable Subroutines and Functions</h4>
<p>Subroutines and functions have to have the <code>BIND(C)</code> attribute to
be compatible with C. The dummy argument declaration is relatively
straightforward. However, one needs to be careful because C uses
call-by-value by default while Fortran behaves usually similar to
call-by-reference. Furthermore, strings and pointers are handled
differently. Note that only explicit size and assumed-size arrays are
supported but not assumed-shape or allocatable arrays.
<p>To pass a variable by value, use the <code>VALUE</code> attribute.
Thus the following C prototype
<pre class="smallexample"> <code>int func(int i, int *j)</code>
</pre>
<p>matches the Fortran declaration
<pre class="smallexample"> integer(c_int) function func(i,j)
use iso_c_binding, only: c_int
integer(c_int), VALUE :: i
integer(c_int) :: j
</pre>
<p>Note that pointer arguments also frequently need the <code>VALUE</code> attribute,
see <a href="Working-with-Pointers.html#Working-with-Pointers">Working with Pointers</a>.
<p>Strings are handled quite differently in C and Fortran. In C a string
is a <code>NUL</code>-terminated array of characters while in Fortran each string
has a length associated with it and is thus not terminated (by e.g.
<code>NUL</code>). For example, if one wants to use the following C function,
<pre class="smallexample"> #include <stdio.h>
void print_C(char *string) /* equivalent: char string[] */
{
printf("%s\n", string);
}
</pre>
<p>to print “Hello World” from Fortran, one can call it using
<pre class="smallexample"> use iso_c_binding, only: C_CHAR, C_NULL_CHAR
interface
subroutine print_c(string) bind(C, name="print_C")
use iso_c_binding, only: c_char
character(kind=c_char) :: string(*)
end subroutine print_c
end interface
call print_c(C_CHAR_"Hello World"//C_NULL_CHAR)
</pre>
<p>As the example shows, one needs to ensure that the
string is <code>NUL</code> terminated. Additionally, the dummy argument
<var>string</var> of <code>print_C</code> is a length-one assumed-size
array; using <code>character(len=*)</code> is not allowed. The example
above uses <code>c_char_"Hello World"</code> to ensure the string
literal has the right type; typically the default character
kind and <code>c_char</code> are the same and thus <code>"Hello World"</code>
is equivalent. However, the standard does not guarantee this.
<p>The use of strings is now further illustrated using the C library
function <code>strncpy</code>, whose prototype is
<pre class="smallexample"> char *strncpy(char *restrict s1, const char *restrict s2, size_t n);
</pre>
<p>The function <code>strncpy</code> copies at most <var>n</var> characters from
string <var>s2</var> to <var>s1</var> and returns <var>s1</var>. In the following
example, we ignore the return value:
<pre class="smallexample"> use iso_c_binding
implicit none
character(len=30) :: str,str2
interface
! Ignore the return value of strncpy -> subroutine
! "restrict" is always assumed if we do not pass a pointer
subroutine strncpy(dest, src, n) bind(C)
import
character(kind=c_char), intent(out) :: dest(*)
character(kind=c_char), intent(in) :: src(*)
integer(c_size_t), value, intent(in) :: n
end subroutine strncpy
end interface
str = repeat('X',30) ! Initialize whole string with 'X'
call strncpy(str, c_char_"Hello World"//C_NULL_CHAR, &
len(c_char_"Hello World",kind=c_size_t))
print '(a)', str ! prints: "Hello WorldXXXXXXXXXXXXXXXXXXX"
end
</pre>
<p>The intrinsic procedures are described in <a href="Intrinsic-Procedures.html#Intrinsic-Procedures">Intrinsic Procedures</a>.
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