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

ID 767251
Date 11/07/2023
Public

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Document Table of Contents

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)

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 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:

  • 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.

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

NOTE:

The following applies to Windows: When interfacing to the Windows GUI or making API calls, you will typically use STDCALL.