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<a name="Interoperable-Subroutines-and-Functions"></a>
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<div class="header">
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<p>
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Next: <a href="Working-with-Pointers.html#Working-with-Pointers" accesskey="n" rel="next">Working with Pointers</a>, Previous: <a href="Interoperable-Global-Variables.html#Interoperable-Global-Variables" accesskey="p" rel="prev">Interoperable Global Variables</a>, Up: <a href="Interoperability-with-C.html#Interoperability-with-C" accesskey="u" rel="up">Interoperability with C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Interoperable-Subroutines-and-Functions-1"></a>
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<h4 class="subsection">7.1.4 Interoperable Subroutines and Functions</h4>
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<p>Subroutines and functions have to have the <code>BIND(C)</code> attribute to
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be compatible with C. The dummy argument declaration is relatively
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straightforward. However, one needs to be careful because C uses
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call-by-value by default while Fortran behaves usually similar to
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call-by-reference. Furthermore, strings and pointers are handled
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differently. Note that in Fortran 2003 and 2008 only explicit size
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and assumed-size arrays are supported but not assumed-shape or
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deferred-shape (i.e. allocatable or pointer) arrays. However, those
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are allowed since the Technical Specification 29113, see
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<a href="Further-Interoperability-of-Fortran-with-C.html#Further-Interoperability-of-Fortran-with-C">Further Interoperability of Fortran with C</a>
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</p>
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<p>To pass a variable by value, use the <code>VALUE</code> attribute.
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Thus, the following C prototype
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</p>
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<div class="smallexample">
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<pre class="smallexample"><code>int func(int i, int *j)</code>
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</pre></div>
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<p>matches the Fortran declaration
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</p>
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<div class="smallexample">
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<pre class="smallexample"> integer(c_int) function func(i,j)
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use iso_c_binding, only: c_int
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integer(c_int), VALUE :: i
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integer(c_int) :: j
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</pre></div>
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<p>Note that pointer arguments also frequently need the <code>VALUE</code> attribute,
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see <a href="Working-with-Pointers.html#Working-with-Pointers">Working with Pointers</a>.
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</p>
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<p>Strings are handled quite differently in C and Fortran. In C a string
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is a <code>NUL</code>-terminated array of characters while in Fortran each string
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has a length associated with it and is thus not terminated (by e.g.
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<code>NUL</code>). For example, if one wants to use the following C function,
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</p>
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<div class="smallexample">
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<pre class="smallexample"> #include <stdio.h>
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void print_C(char *string) /* equivalent: char string[] */
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{
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printf("%s\n", string);
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}
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</pre></div>
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<p>to print “Hello World” from Fortran, one can call it using
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</p>
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<div class="smallexample">
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<pre class="smallexample"> use iso_c_binding, only: C_CHAR, C_NULL_CHAR
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interface
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subroutine print_c(string) bind(C, name="print_C")
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use iso_c_binding, only: c_char
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character(kind=c_char) :: string(*)
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end subroutine print_c
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end interface
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call print_c(C_CHAR_"Hello World"//C_NULL_CHAR)
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</pre></div>
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<p>As the example shows, one needs to ensure that the
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string is <code>NUL</code> terminated. Additionally, the dummy argument
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<var>string</var> of <code>print_C</code> is a length-one assumed-size
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array; using <code>character(len=*)</code> is not allowed. The example
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above uses <code>c_char_"Hello World"</code> to ensure the string
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literal has the right type; typically the default character
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kind and <code>c_char</code> are the same and thus <code>"Hello World"</code>
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is equivalent. However, the standard does not guarantee this.
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</p>
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<p>The use of strings is now further illustrated using the C library
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function <code>strncpy</code>, whose prototype is
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</p>
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<div class="smallexample">
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<pre class="smallexample"> char *strncpy(char *restrict s1, const char *restrict s2, size_t n);
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</pre></div>
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<p>The function <code>strncpy</code> copies at most <var>n</var> characters from
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string <var>s2</var> to <var>s1</var> and returns <var>s1</var>. In the following
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example, we ignore the return value:
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</p>
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<div class="smallexample">
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<pre class="smallexample"> use iso_c_binding
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implicit none
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character(len=30) :: str,str2
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interface
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! Ignore the return value of strncpy -> subroutine
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! "restrict" is always assumed if we do not pass a pointer
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subroutine strncpy(dest, src, n) bind(C)
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import
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character(kind=c_char), intent(out) :: dest(*)
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character(kind=c_char), intent(in) :: src(*)
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integer(c_size_t), value, intent(in) :: n
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end subroutine strncpy
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end interface
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str = repeat('X',30) ! Initialize whole string with 'X'
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call strncpy(str, c_char_"Hello World"//C_NULL_CHAR, &
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len(c_char_"Hello World",kind=c_size_t))
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print '(a)', str ! prints: "Hello WorldXXXXXXXXXXXXXXXXXXX"
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end
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</pre></div>
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<p>The intrinsic procedures are described in <a href="Intrinsic-Procedures.html#Intrinsic-Procedures">Intrinsic Procedures</a>.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Working-with-Pointers.html#Working-with-Pointers" accesskey="n" rel="next">Working with Pointers</a>, Previous: <a href="Interoperable-Global-Variables.html#Interoperable-Global-Variables" accesskey="p" rel="prev">Interoperable Global Variables</a>, Up: <a href="Interoperability-with-C.html#Interoperability-with-C" accesskey="u" rel="up">Interoperability with C</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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