ATTRIBUTES Directive Properties

Intel® Fortran Compiler Classic and Intel® Fortran Compiler Developer Guide and Reference

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Date 6/24/2024

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ATTRIBUTES Directive Properties

Standard Fortran provides a syntax for interoperating with C, including applying the appropriate naming conventions. You should use BIND(C) for interoperability with C, instead of specifying ATTRIBUTES properties. Do not specify both BIND(C) and ATTRIBUTES C for the same procedure. Specifying ATTRIBUTES STDCALL has no effect if BIND(C) is also specified for the same procedure.

The ATTRIBUTES properties C, STDCALL (Windows only), REFERENCE, and VALUE affect the calling convention of routines. You can specify:

The C and STDCALL 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) property is used, the default changes to passing almost all data except arrays by value. However, in addition to the calling-convention properties C and STDCALL, you can specify properties 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) `none` for all others	`_` (Windows operating systems using IA-32 architecture) `none` for all others	`_` (Windows operating systems using IA-32 architecture) `none` for all others	_	_
Suffix	none (Windows) `_` (Linux)	`none`	`none`	@`n`	@`n`
Case	Upper Case (Windows) Lower Case (Linux)	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: `Len:End` applies when -nomixed-str-len-arg (Linux) or /iface:nomixed_str_len_arg (Windows) is set. The length of the string is pushed (by value) on the stack after all of the other arguments. This is the default. `Len:Mixed` applies when -mixed-str-len-arg (Linux) or /iface:mixed_str_len_arg (Windows) is set. The length of the string is pushed (by value) on the stack immediately after the address of the beginning of the string.
`No Len` or `Len:Mixed`	For certain string arguments: `No Len` applies when -nomixed-str-len-arg (Linux) or /iface:nomixed_str_len_arg (Windows) is set. The length of the string is not available to the called procedure. This is the default. `Len:Mixed` applies when -mixed-str-len-arg (Linux) or /iface:mixed_str_len_arg (Windows) is set. The length of the string is pushed (by value) on the stack immediately after the address of the beginning of the string.
`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 argument 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 ifort systems using IA-32 architecture, derived-type arguments specified by value are passed as follows: Arguments from 1 to 4 bytes are passed by value. Arguments from 5 to 8 bytes are passed by value in two registers (two arguments). Arguments more than 8 bytes provide value semantics by passing a temporary storage address by reference. On Intel® 64 architecture: Linux Arguments from 1 to 8 bytes are passed by register. Arguments from 9 to 16 bytes are passed in two registers. Arguments more the 16 bytes provide value semantics by passing a temporary storage address by register. Windows Arguments from 1 to 8 bytes are passed by register. Arguments greater than 8 bytes provide value semantics by passing a temporary storage address by reference.
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 property can be used with any other Fortran calling-convention option to preserve mixed-case names.

On Windows, the compiler option /iface also establishes some default argument passing conventions. Option /iface 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

NOTE:

On Windows, when interfacing to the Windows GUI or making API calls, you will typically use STDCALL.

Parent topic: Legacy Extensions

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ATTRIBUTES Directive Properties