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| <a name="GNU-Fortran-Compiler-Directives-1"></a> |
| <h3 class="section">7.2 GNU Fortran Compiler Directives</h3> |
| |
| <p>The Fortran standard describes how a conforming program shall |
| behave; however, the exact implementation is not standardized. In order |
| to allow the user to choose specific implementation details, compiler |
| directives can be used to set attributes of variables and procedures |
| which are not part of the standard. Whether a given attribute is |
| supported and its exact effects depend on both the operating system and |
| on the processor; see |
| <a href="http://gcc.gnu.org/onlinedocs/gcc/index.html#Top">C Extensions</a> in <cite>Using the GNU Compiler Collection (GCC)</cite> |
| for details. |
| </p> |
| <p>For procedures and procedure pointers, the following attributes can |
| be used to change the calling convention: |
| </p> |
| <ul> |
| <li> <code>CDECL</code> – standard C calling convention |
| </li><li> <code>STDCALL</code> – convention where the called procedure pops the stack |
| </li><li> <code>FASTCALL</code> – part of the arguments are passed via registers |
| instead using the stack |
| </li></ul> |
| |
| <p>Besides changing the calling convention, the attributes also influence |
| the decoration of the symbol name, e.g., by a leading underscore or by |
| a trailing at-sign followed by the number of bytes on the stack. When |
| assigning a procedure to a procedure pointer, both should use the same |
| calling convention. |
| </p> |
| <p>On some systems, procedures and global variables (module variables and |
| <code>COMMON</code> blocks) need special handling to be accessible when they |
| are in a shared library. The following attributes are available: |
| </p> |
| <ul> |
| <li> <code>DLLEXPORT</code> – provide a global pointer to a pointer in the DLL |
| </li><li> <code>DLLIMPORT</code> – reference the function or variable using a global pointer |
| </li></ul> |
| |
| <p>For dummy arguments, the <code>NO_ARG_CHECK</code> attribute can be used; in |
| other compilers, it is also known as <code>IGNORE_TKR</code>. For dummy arguments |
| with this attribute actual arguments of any type and kind (similar to |
| <code>TYPE(*)</code>), scalars and arrays of any rank (no equivalent |
| in Fortran standard) are accepted. As with <code>TYPE(*)</code>, the argument |
| is unlimited polymorphic and no type information is available. |
| Additionally, the argument may only be passed to dummy arguments |
| with the <code>NO_ARG_CHECK</code> attribute and as argument to the |
| <code>PRESENT</code> intrinsic function and to <code>C_LOC</code> of the |
| <code>ISO_C_BINDING</code> module. |
| </p> |
| <p>Variables with <code>NO_ARG_CHECK</code> attribute shall be of assumed-type |
| (<code>TYPE(*)</code>; recommended) or of type <code>INTEGER</code>, <code>LOGICAL</code>, |
| <code>REAL</code> or <code>COMPLEX</code>. They shall not have the <code>ALLOCATE</code>, |
| <code>CODIMENSION</code>, <code>INTENT(OUT)</code>, <code>POINTER</code> or <code>VALUE</code> |
| attribute; furthermore, they shall be either scalar or of assumed-size |
| (<code>dimension(*)</code>). As <code>TYPE(*)</code>, the <code>NO_ARG_CHECK</code> attribute |
| requires an explicit interface. |
| </p> |
| <ul> |
| <li> <code>NO_ARG_CHECK</code> – disable the type, kind and rank checking |
| </li></ul> |
| |
| |
| <p>The attributes are specified using the syntax |
| </p> |
| <p><code>!GCC$ ATTRIBUTES</code> <var>attribute-list</var> <code>::</code> <var>variable-list</var> |
| </p> |
| <p>where in free-form source code only whitespace is allowed before <code>!GCC$</code> |
| and in fixed-form source code <code>!GCC$</code>, <code>cGCC$</code> or <code>*GCC$</code> shall |
| start in the first column. |
| </p> |
| <p>For procedures, the compiler directives shall be placed into the body |
| of the procedure; for variables and procedure pointers, they shall be in |
| the same declaration part as the variable or procedure pointer. |
| </p> |
| |
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