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| <a name="Argument-passing-conventions-1"></a> |
| <h4 class="subsection">7.4.2 Argument passing conventions</h4> |
| |
| <p>Subroutines do not return a value (matching C99’s <code>void</code>) while |
| functions either return a value as specified in the platform ABI or |
| the result variable is passed as hidden argument to the function and |
| no result is returned. A hidden result variable is used when the |
| result variable is an array or of type <code>CHARACTER</code>. |
| </p> |
| <p>Arguments are passed according to the platform ABI. In particular, |
| complex arguments might not be compatible to a struct with two real |
| components for the real and imaginary part. The argument passing |
| matches the one of C99’s <code>_Complex</code>. Functions with scalar |
| complex result variables return their value and do not use a |
| by-reference argument. Note that with the <samp>-ff2c</samp> option, |
| the argument passing is modified and no longer completely matches |
| the platform ABI. Some other Fortran compilers use <code>f2c</code> |
| semantic by default; this might cause problems with |
| interoperablility. |
| </p> |
| <p>GNU Fortran passes most arguments by reference, i.e. by passing a |
| pointer to the data. Note that the compiler might use a temporary |
| variable into which the actual argument has been copied, if required |
| semantically (copy-in/copy-out). |
| </p> |
| <p>For arguments with <code>ALLOCATABLE</code> and <code>POINTER</code> |
| attribute (including procedure pointers), a pointer to the pointer |
| is passed such that the pointer address can be modified in the |
| procedure. |
| </p> |
| <p>For dummy arguments with the <code>VALUE</code> attribute: Scalar arguments |
| of the type <code>INTEGER</code>, <code>LOGICAL</code>, <code>REAL</code> and |
| <code>COMPLEX</code> are passed by value according to the platform ABI. |
| (As vendor extension and not recommended, using <code>%VAL()</code> in the |
| call to a procedure has the same effect.) For <code>TYPE(C_PTR)</code> and |
| procedure pointers, the pointer itself is passed such that it can be |
| modified without affecting the caller. |
| </p> |
| <p>For Boolean (<code>LOGICAL</code>) arguments, please note that GCC expects |
| only the integer value 0 and 1. If a GNU Fortran <code>LOGICAL</code> |
| variable contains another integer value, the result is undefined. |
| As some other Fortran compilers use <em>-1</em> for <code>.TRUE.</code>, |
| extra care has to be taken – such as passing the value as |
| <code>INTEGER</code>. (The same value restriction also applies to other |
| front ends of GCC, e.g. to GCC’s C99 compiler for <code>_Bool</code> |
| or GCC’s Ada compiler for <code>Boolean</code>.) |
| </p> |
| <p>For arguments of <code>CHARACTER</code> type, the character length is passed |
| as hidden argument. For deferred-length strings, the value is passed |
| by reference, otherwise by value. The character length has the type |
| <code>INTEGER(kind=4)</code>. Note with C binding, <code>CHARACTER(len=1)</code> |
| result variables are returned according to the platform ABI and no |
| hidden length argument is used for dummy arguments; with <code>VALUE</code>, |
| those variables are passed by value. |
| </p> |
| <p>For <code>OPTIONAL</code> dummy arguments, an absent argument is denoted |
| by a NULL pointer, except for scalar dummy arguments of type |
| <code>INTEGER</code>, <code>LOGICAL</code>, <code>REAL</code> and <code>COMPLEX</code> |
| which have the <code>VALUE</code> attribute. For those, a hidden Boolean |
| argument (<code>logical(kind=C_bool),value</code>) is used to indicate |
| whether the argument is present. |
| </p> |
| <p>Arguments which are assumed-shape, assumed-rank or deferred-rank |
| arrays or, with <samp>-fcoarray=lib</samp>, allocatable scalar coarrays use |
| an array descriptor. All other arrays pass the address of the |
| first element of the array. With <samp>-fcoarray=lib</samp>, the token |
| and the offset belonging to nonallocatable coarrays dummy arguments |
| are passed as hidden argument along the character length hidden |
| arguments. The token is an oparque pointer identifying the coarray |
| and the offset is a passed-by-value integer of kind <code>C_PTRDIFF_T</code>, |
| denoting the byte offset between the base address of the coarray and |
| the passed scalar or first element of the passed array. |
| </p> |
| <p>The arguments are passed in the following order |
| </p><ul> |
| <li> Result variable, when the function result is passed by reference |
| </li><li> Character length of the function result, if it is a of type |
| <code>CHARACTER</code> and no C binding is used |
| </li><li> The arguments in the order in which they appear in the Fortran |
| declaration |
| </li><li> The the present status for optional arguments with value attribute, |
| which are internally passed by value |
| </li><li> The character length and/or coarray token and offset for the first |
| argument which is a <code>CHARACTER</code> or a nonallocatable coarray dummy |
| argument, followed by the hidden arguments of the next dummy argument |
| of such a type |
| </li></ul> |
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| Previous: <a href="Naming-conventions.html#Naming-conventions" accesskey="p" rel="prev">Naming conventions</a>, Up: <a href="Naming-and-argument_002dpassing-conventions.html#Naming-and-argument_002dpassing-conventions" accesskey="u" rel="up">Naming and argument-passing conventions</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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