Visible to Intel only — GUID: GUID-5A3F7F99-BD41-4520-91A9-8022AD31375D
Visible to Intel only — GUID: GUID-5A3F7F99-BD41-4520-91A9-8022AD31375D
ATTRIBUTES Directive Options
Standard Fortran provides a syntax for interoperating with C, including applying the appropriate naming conventions. You can use BIND(C) for interoperability with C, instead of specifying ATTRIBUTES options.
The ATTRIBUTES options (also known as properties) C, STDCALL (Windows only), REFERENCE, VALUE affect the calling convention of routines. You can specify:
The C, STDCALL, and properties for an entire routine.
The VALUE and REFERENCE properties for individual arguments.
By default, Fortran passes all data by reference (except the hidden length argument of strings, which is passed by value). If the C (or STDCALL) option is used, the default changes to passing almost all data except arrays by value. However, in addition to the calling-convention options C and STDCALL, you can specify argument options, VALUE and REFERENCE, to pass arguments by value or by reference, regardless of the calling convention option. Arrays can only be passed by reference.
Different Fortran calling conventions can be specified by declaring the Fortran procedure to have certain attributes.
The following table summarizes the effect of the most common Fortran calling convention directives.
Argument |
Default |
C |
C, REFERENCE |
STDCALL (Windows IA-32 Architecture) |
STDCALL, REFERENCE (Windows IA-32 Architecture) |
---|---|---|---|---|---|
Scalar |
Reference |
Value |
Reference |
Value |
Reference |
Scalar [value] |
Value |
Value |
Value |
Value |
Value |
Scalar [reference] |
Reference |
Reference |
Reference |
Reference |
Reference |
String |
Reference, either Len:End or Len:Mixed |
String(1:1) |
Reference, either Len:End or Len:Mixed |
String(1:1) |
String(1:1) |
String [value] |
Error |
String(1:1) |
String(1:1) |
String(1:1) |
String(1:1) |
String [reference] |
Reference, either No Len or Len:Mixed |
Reference, No Len |
Reference, No Len |
Reference, No Len |
Reference, No Len |
Array |
Reference |
Reference |
Reference |
Reference |
Reference |
Array [value] |
Error |
Error |
Error |
Error |
Error |
Array [reference] |
Reference |
Reference |
Reference |
Reference |
Reference |
Derived Type |
Reference |
Value, size dependent |
Reference |
Value, size dependent |
Reference |
Derived Type [value] |
Value, size dependent |
Value, size dependent |
Value, size dependent |
Value, size dependent |
Value, size dependent |
Derived Type [reference] |
Reference |
Reference |
Reference |
Reference |
Reference |
F90 Pointer |
Descriptor |
Descriptor |
Descriptor |
Descriptor |
Descriptor |
F90 Pointer [value] |
Error |
Error |
Error |
Error |
Error |
F90 Pointer [reference] |
Descriptor |
Descriptor |
Descriptor |
Descriptor |
Descriptor |
The naming conventions are shown in the following table.
Argument |
Default |
C |
C, REFERENCE |
STDCALL (Windows IA-32 Architecture) |
STDCALL, REFERENCE (Windows IA-32 Architecture) |
---|---|---|---|---|---|
Prefix |
_ (Windows operating systems using IA-32 architecture, macOS operating systems) none for all others |
_ (Windows operating systems using IA-32 architecture, macOS operating systems) none for all others |
_ (Windows operating systems using IA-32 architecture, macOS operating systems) none for all others |
_ |
_ |
Suffix |
none (Windows) _ (Linux, macOS) |
none |
none |
@n |
@n |
Case |
Upper Case (Windows) Lower Case (Linux, macOS) |
Lower Case |
Lower Case |
Lower Case |
Lower Case |
Stack Cleanup |
Caller |
Caller |
Caller |
Callee |
Callee |
The terms in the previous table mean the following:
Term | Term Meaning |
---|---|
[value] |
Argument assigned the VALUE attribute. |
[reference] |
Argument assigned the REFERENCE attribute. |
Value |
The argument value is pushed to the stack. All values are padded to the next 4-byte boundary. |
Reference |
On systems using IA-32 architecture, the 4-byte argument address is pushed to the stack. On systems using Intel® 64 architecture, the 8-byte argument address is pushed to the stack. |
Len:End or Len:Mixed |
For certain string arguments:
|
No Len or Len:Mixed |
For certain string arguments:
|
No Len |
For string arguments, the length of the string is not available to the called procedure. |
String(1:1) |
For string arguments, the first character is converted to INTEGER(4) as in ICHAR(string(1:1)) and pushed to the stack by value. |
Error |
Produces a compiler error. |
Descriptor |
On systems using IA-32 architecture, the 4-byte address of the array descriptor. On systems using Intel® 64 architecture, the 8-byte address of the array descriptor. |
@n |
On systems using IA-32 architecture, the at sign (@) followed by the number of bytes (in decimal) required for the argument list. |
Size dependent |
On systems using IA-32 architecture, derived-type arguments specified by value are passed as follows:
|
Upper Case |
Procedure name in all uppercase. |
Lower Case |
Procedure name in all lowercase. |
Callee |
The procedure being called is responsible for removing arguments from the stack before returning to the caller. |
Caller |
The procedure doing the call is responsible for removing arguments from the stack after the call is over. |
You can use the BIND(C, name=<string>) attribute to resolve discrepancies with C. Alternatively, the ALIAS option can be used with any other Fortran calling-convention option to preserve mixed-case names.
For Windows operating systems, the compiler option /iface also establishes some default argument passing conventions. The /iface compiler option has the following choices:
Option |
Argument Is Passed By |
Appends @n to Names on Systems Using IA-32 Architecture? |
Who Cleans the Stack? |
Varargs Support? |
---|---|---|---|---|
/iface:cref |
By reference |
No |
Caller |
Yes |
/iface:stdref |
By reference |
Yes |
Callee |
No |
/iface:default |
By reference |
No |
Caller |
Yes |
/iface:c |
By value |
No |
Caller |
Yes |
/iface:stdcall |
By value |
Yes |
Callee |
No |
/iface:cvf |
By reference |
Yes |
Callee |
No |
The following applies to Windows: When interfacing to the Windows GUI or making API calls, you will typically use STDCALL.