| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1999-2014 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being "Funding Free Software", the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| "GNU Free Documentation License". |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development. --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>The GNU Fortran Compiler: Interoperable Subroutines and Functions</title> |
| |
| <meta name="description" content="The GNU Fortran Compiler: Interoperable Subroutines and Functions"> |
| <meta name="keywords" content="The GNU Fortran Compiler: Interoperable Subroutines and Functions"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Interoperability-with-C.html#Interoperability-with-C" rel="up" title="Interoperability with C"> |
| <link href="Working-with-Pointers.html#Working-with-Pointers" rel="next" title="Working with Pointers"> |
| <link href="Interoperable-Global-Variables.html#Interoperable-Global-Variables" rel="prev" title="Interoperable Global Variables"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Interoperable-Subroutines-and-Functions"></a> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| <hr> |
| <a name="Interoperable-Subroutines-and-Functions-1"></a> |
| <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 in Fortran 2003 and 2008 only explicit size |
| and assumed-size arrays are supported but not assumed-shape or |
| deferred-shape (i.e. allocatable or pointer) arrays. However, those |
| are allowed since the Technical Specification 29113, see |
| <a href="Further-Interoperability-of-Fortran-with-C.html#Further-Interoperability-of-Fortran-with-C">Further Interoperability of Fortran with C</a> |
| </p> |
| <p>To pass a variable by value, use the <code>VALUE</code> attribute. |
| Thus, the following C prototype |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"><code>int func(int i, int *j)</code> |
| </pre></div> |
| |
| <p>matches the Fortran declaration |
| </p> |
| <div class="smallexample"> |
| <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></div> |
| |
| <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> |
| <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, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> #include <stdio.h> |
| void print_C(char *string) /* equivalent: char string[] */ |
| { |
| printf("%s\n", string); |
| } |
| </pre></div> |
| |
| <p>to print “Hello World” from Fortran, one can call it using |
| </p> |
| <div class="smallexample"> |
| <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></div> |
| |
| <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> |
| <p>The use of strings is now further illustrated using the C library |
| function <code>strncpy</code>, whose prototype is |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> char *strncpy(char *restrict s1, const char *restrict s2, size_t n); |
| </pre></div> |
| |
| <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: |
| </p> |
| <div class="smallexample"> |
| <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></div> |
| |
| <p>The intrinsic procedures are described in <a href="Intrinsic-Procedures.html#Intrinsic-Procedures">Intrinsic Procedures</a>. |
| </p> |
| <hr> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| |
| |
| |
| </body> |
| </html> |
