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Fortran Programming Guide

Chapter 11 C-Fortran Interface

This chapter treats issues regarding Fortran and C interoperability.

The discussion is inherently limited to the specifics of the Sun FORTRAN 77, Fortran 90, and C compilers.

Note -

Material common to both Sun FORTRAN 77 and Fortran 90 is presented in examples that use FORTRAN 77.

Compatibility Issues

Most C-Fortran interfaces must agree in all of these aspects:

Some C-Fortran interfaces must also agree on:

Function or Subroutine?

The word function has different meanings in C and Fortran. Depending on the situation, the choice is important:

When a Fortran routine calls a C function:

When a C function calls a Fortran subprogram:

Data Type Compatibility

The tables below summarize the data sizes and default alignments for FORTRAN 77 and Fortran 90 data types. In both tables, note the following:

FORTRAN 77 and C Data Types

Table 11-1 shows the sizes and allowable alignments for FORTRAN 77 data types. It assumes no compilation options affecting alignment or promoting default data sizes are applied. (See also the FORTRAN 77 Language Reference Manual).

Table 11-1 Data Sizes and Alignments--(in Bytes) Pass by Reference (f77 and cc)

FORTRAN 77 Data Type 

C Data Type 

Size  

Default Alignment SPARC     x86  

BYTE X

CHARACTER X

CHARACTER*n X

char x

unsigned char x

unsigned char x[n]

1 

1 

n 

1  

1  

1  

1 

1 

1 

COMPLEX X

COMPLEX*8 X

DOUBLE COMPLEX X

COMPLEX*16 X

COMPLEX*32 X

struct {float r,i;} x;

struct {float r,i;} x;

struct {double dr,di;}x;

struct {double dr,di;}x;

struct {long double dr,di;} x;

8 

8 

16 

16 

32 

4  

4  

4/8  

4/8 

4/8/16  

4 

4 

4 

4 

-- 

DOUBLE PRECISION X

REAL X

REAL*4 X

REAL*8 X

REAL*16 X

double x

float x

float x

double x

long double x

8 

4 

4 

8 

16 

4/8  

4  

4  

4/8 

4/8/16 

4 

4 

4 

4 

-- 

INTEGER X

INTEGER*2 X

INTEGER*4 X

INTEGER*8 X

int x

short x

int x

long long int x

4 

2 

4 

8 

4  

2  

4  

4 

4 

2 

4 

4 

LOGICAL X

LOGICAL*1 X

LOGICAL*2 X

LOGICAL*4 X

LOGICAL*8 X

int x

char x

short x

int x

long long int x

4 

1 

2 

4 

8 

4  

1  

2  

4  

4 

4 

1 

2 

4 

4 

SPARC: Fortran 90 and C Data Types

The following table similarly compares the Fortran 90 data types with C.

Table 11-2 Data Sizes and Alignment--(in Bytes) Pass by Reference (f90 and cc)

Fortran 90 Data Type  

C Data Type 

Size 

Alignment  
 CHARACTER x

unsigned char x ;

 1 1
  CHARACTER (LEN=n) x

unsigned char x[n] ;

n 1
 COMPLEX x

struct {float r,i;} x;

 8 4
 

COMPLEX (KIND=4) x

COMPLEX (KIND=8) x

COMPLEX (KIND=16) x

struct {float r,i;} x;

struct {double dr,di;} x;

struct {long double, dr,di;} x;

8 

16 

32 

4  

4/8 

4/8/16 
 DOUBLE COMPLEX x struct {double dr, di;} x;  16 4/8
 DOUBLE PRECISION x

double x ;

 8 4
 REAL x

float x ;

 4 4
 

REAL (KIND=4) x

REAL (KIND=8) x

REAL (KIND=16) x

float x ;

double x ;

long double x ;

4 

8 

16 

4  

4/8 

4/8/16 
 INTEGER x

int x ;

 4 4
 

INTEGER (KIND=1) x

INTEGER (KIND=2) x

INTEGER (KIND=4) x

INTEGER (KIND=8) x

signed char x ;

short x ;

int x ;

long long int x;

1 

2 

4 

8 

4 

4 

4  

4 
 LOGICAL x

int x ;

 4 4
 

LOGICAL (KIND=1) x

LOGICAL (KIND=2) x

LOGICAL (KIND=4) x

LOGICAL (KIND=8) x

signed char x ;

short x ;

int x ;

long long int x;

1 

2 

4 

8 

4 

4 

4 

4 

Case Sensitivity

C and Fortran take opposite perspectives on case sensitivity:

The f77 and f90 default is to ignore case by converting subprogram names to lowercase. It converts all uppercase letters to lowercase letters, except within character-string constants.

There are two usual solutions to the uppercase/lowercase problem:

Use one of these two solutions, but not both.

Most examples in this chapter use all lowercase letters for the name in the C function, and do not use the f77 -U compiler option. (f90 does not have an equivalent option.)

Underscores in Routine Names

The Fortran compiler normally appends an underscore (_) to the names of subprograms appearing both at entry point definition and in calls. This convention differs from C procedures or external variables with the same user-assigned name. If the name has exactly 32 characters, the underscore is not appended. All Fortran library procedure names have double leading underscores to reduce clashes with user-assigned subroutine names.

There are three usual solutions to the underscore problem:

Use only one of these solutions.

The examples in this chapter could use the FORTRAN 77 C() compiler pragma to avoid underscores. The C() pragma directive takes the names of external functions as arguments. It specifies that these functions are written in the C language, so the Fortran compiler does not append an underscore as it ordinarily does with external names. The C()directive for a particular function must appear before the first reference to that function. It must also appear in each subprogram that contains such a reference. The conventional usage is: 

      EXTERNAL ABC, XYZ      !$PRAGMA C( ABC, XYZ )

If you use this pragma, the C function does not need an underscore appended to the function name.

Fortran 90 does not have equivalent methods for avoiding underscores. Trailing underscores are required in the names of C routines called from Fortran 90 routines.

Argument-Passing by Reference or Value

In general, Fortran routines pass arguments by reference. In a call, if you enclose an argument with the f77 and f90 nonstandard function %VAL(), the calling routine passes it by value.

In general, C passes arguments by value. If you precede an argument by the ampersand operator (&), C passes the argument by reference using a pointer. C always passes arrays and character strings by reference.

Argument Order

Except for arguments that are character strings, Fortran and C pass arguments in the same order. However, for every argument of character type, the Fortran routine passes an additional argument giving the length of the string. These are long int quantities in C, passed by value.

The order of arguments is:

Example: 

This Fortran code fragment:
Is equivalent to this in C:
      CHARACTER*7 S
      INTEGER B(3)
       ...
      CALL SAM( S, B(2) )
      char s[7];
      long b[3];
        ...
      sam_( s, &b[1], 7L ) ;

Array Indexing and Order

Array indexing and order differ between Fortran and C.

Array Indexing

C arrays always start at zero, but by default Fortran arrays start at 1. There are two usual ways of approaching indexing.

Array Order

Fortran arrays are stored in column-major order: A(3,2) 

A(1,1) A(2,1) A(3,1) A(1,2) A(2,2) A(3,2) A(1,3) A(2,3) A(3,3)

C arrays in row-major order: A[3][2] 

A[0][0] A[0][1] A[0][2] A[1][0] A[1][1] A[1][2] A[2][0] A[2][1] A[2][2]

For one-dimensional arrays, this is no problem. For two-dimensional and higher arrays, be aware of how subscripts appear and are used in all references and declarations--some adjustments might be necessary.

For example, it may be confusing to do part of a matrix manipulation in C and the rest in Fortran. It might be preferable to pass an entire array to a routine in the other language and perform all the matrix manipulation in that routine to avoid doing part in C and part in Fortran.

File Descriptors and stdio

Fortran I/O channels are in terms of unit numbers. The I/O system does not deal with unit numbers but with file descriptors. The Fortran runtime system translates from one to the other, so most Fortran programs do not have to recognize file descriptors.

Many C programs use a set of subroutines, called standard I/O (or stdio). Many functions of Fortran I/O use standard I/O, which in turn uses operating system I/O calls. Some of the characteristics of these I/O systems are listed in in the following table.

Table 11-3 Comparing I/O Between Fortran and C
 

Fortran Units 

Standard I/O File Pointers 

File Descriptors 

Files Open 

Opened for reading and writing  

Opened for reading; or Opened for writing; or Opened for both; or Opened for appending; See OPEN(3S)

Opened for reading; or Opened for writing; or Opened for both 

Attributes 

Formatted or unformatted 

Always unformatted, but can be read or written with format-interpreting routines 

Always unformatted 

Access 

Direct or sequential  

Direct access if the physical file representation is direct access, but can always be read sequentially  

Direct access if the physical file representation is direct access, but can always be read sequentially 

Structure 

Record 

Byte stream 

Byte stream 

Form 

Arbitrary nonnegative integers from 0-2147483647 

Pointers to structures in the user's address space  

Integers from 0-1023  

File Permissions

C programs typically open input files for reading and output files for writing or for reading and writing. A f77 program can OPEN a file READONLY or with READWRITE='READ' or 'WRITE' or 'READWRITE'. f90 supports the READWRITE specifier, but not READONLY.

Fortran tries to open a file with the maximum permissions possible, first for both reading and writing, then for each separately.

This event occurs transparently and is of concern only if you try to perform a READ, WRITE, or ENDFILE operation but you do not have permission. Magnetic tape operations are an exception to this general freedom, since you can have write permissions on a file, but not have a write ring on the tape.

Libraries and Linking With the f77 or f90 Command

To link the proper Fortran and C libraries, use the f77 or f90 command to invoke the linker.

Example 1: Use f77 to link: 

demo% cc -c RetCmplxmain.c 
demo% f77 RetCmplx.f RetCmplxmain.o       
¨ This command line does the linking.
demo% a.out 
 4.0 4.5 
 8.0 9.0 
demo%

Passing Data Arguments by Reference

The standard method for passing data between Fortran routines and C procedures is by reference. To a C procedure, a Fortran subroutine or function call looks like a procedure call with all arguments represented by pointers. The only peculiarity is the way Fortran handles character strings and functions as arguments and as the return value from a CHARACTER*n function.

Simple Data Types

For simple data types (not COMPLEX or CHARACTER strings), define or pass each associated argument in the C routine as a pointer:.

Table 11-4 Passing Simple Data Types

Fortran calls C 

C calls Fortran 

integer i

real r

external CSim

i = 100

call CSim(i,r)

...

----------------------------

void csim_(int *i, float *r)

{

*r = *i;

}

int i=100;

float r;

extern void fsim_(int *i, float *r);

fsim_(&i, &r);

...

------------------------------

subroutine FSim(i,r)

integer i

real r

r = i

return

end

COMPLEX Data

Pass a Fortran COMPLEX data item as a pointer to a C struct of two float or two double data types:

Table 11-5 Passing COMPLEX Data Types

Fortran calls C 

C calls Fortran 

complex w

double complex z

external CCmplx

call CCmplx(w,z)

...

------------------------------

struct cpx {float r, i;};

struct dpx {double r,i;};

void ccmplx_(

struct cpx *w,

struct dpx *z)

{

w -> r = 32.;

w -> i = .007;

z -> r = 66.67;

z -> i = 94.1;

}

struct cpx {float r, i;};

struct cpx d1;

struct cpx *w = &d1;

struct dpx {double r, i;};

struct dpx d2;

struct dpx *z = &d2;

fcmplx_( w, z );

...

---------------

subroutine FCmplx( w, z )

complex w

double complex z

w = (32., .007)

z = (66.67, 94.1)

return

end

In 64-bit environments and compiling with -xarch=v9, COMPLEX values are returned in registers.

Character Strings

Passing strings between C and Fortran routines is not recommended because there is no standard interface. However, note the following:

A Fortran call with a character string argument is shown in the next example with its C equivalent:

Table 11-6 Passing a CHARACTER string

Fortran call: 

C equivalent: 

CHARACTER*7 S

INTEGER B(3)

...

CALL CSTRNG( S, B(2) )

...

char s[7];

long b[3];

...

cstrng_( s, &b[1], 7L );

...

If the length of the string is not needed in the called routine, the extra arguments may be ignored. However, note that Fortran does not automatically terminate strings with the explicit null character that C expects. This must be added by the calling program.

One-Dimensional Arrays

Array subscripts in C start with 0.

Table 11-7 Passing a One-Dimensional Array

Fortran calls C 

C calls Fortran 

integer i, Sum

integer a(9)

external FixVec

...

call FixVec ( a, Sum )

...

------------------------------

void fixvec_ (

int v[9], int *sum )

{

int i;

*sum = 0;

for ( i = 0; i <= 8; i++ )

*sum = *sum + v[i];

}

extern void vecref_

( int[], int * );

...

int i, sum;

int v[9] = ...

vecref_( v, &sum );

...

------------------------------

subroutine VecRef( v, total)

integer i, total, v(9)

total = 0

do i = 1,9

total = total + v(i)

end do

...

Two-Dimensional Arrays

Rows and columns between C and Fortran are switched.

Table 11-8 Passing a Two-Dimensional Array

Fortran calls C 

C calls Fortran 

REAL Q(10,20)

...

Q(3,5) = 1.0

CALL FIXQ(Q)

...

------------------------------

void fixq_( float a[20][10] )

{

...

a[5][3] = a[5][3] + 1.;

...

}

extern void

qref_( int[][10], int *);

...

int m[20][10] = ... ;

int sum;

...

qref_( m, &sum );

...

------------------------------

SUBROUTINE QREF(A,TOTAL)

INTEGER A(10,20), TOTAL

DO I = 1,10

DO J = 1,20

TOTAL = TOTAL + A(I,J)

END DO

END DO

...

Structures

C and FORTRAN 77 structures and Fortran 90 derived types can be passed to each other's routines as long as the corresponding elements are compatible.

Table 11-9 Passing FORTRAN 77 STRUCTURE Records

Fortran calls C 

C calls Fortran 

STRUCTURE /POINT/

REAL X, Y, Z

END STRUCTURE

RECORD /POINT/ BASE

EXTERNAL FLIP

...

CALL FLIP( BASE )

...

------------------------------

struct point {

float x,y,z;

};

void flip_( struct point *v; )

{

float t;

t = v -> x;

v -> x = v -> y;

v -> y = t;

v -> z = -2.*(v -> z);

}

struct point {

float x,y,z;

};

void fflip_ ( struct point *) ;

...

struct point d;

struct point *ptx = &d;

...

fflip_ (ptx);

...

------------------------------

SUBROUTINE FFLIP(P)

STRUCTURE /POINT/

REAL X,Y,Z

END STRUCTURE

RECORD /POINT/ P

REAL T

T = P.X

P.X = P.Y

P.Y = T

P.Z = -2.*P.Z

...

Table 11-10 Passing Fortran 90 Derived Types

Fortran 90 calls C 

C calls Fortran 90 

TYPE point

SEQUENCE

REAL :: x, y, z

END TYPE point

TYPE (point) base

EXTERNAL flip

...

CALL flip( base)

...

------------------------------

struct point {

float x,y,z;

};

void flip_( struct point *v; )

{

float t;

t = v -> x;

v -> x = v -> y;

v -> y = t;

v -> z = -2.*(v -> z);

}

struct point {

float x,y,z;

};

extern void fflip_ (

struct point *) ;

...

struct point d;

struct point *ptx = &d;

...

fflip_ (ptx);

...

------------------------------

SUBROUTINE FFLIP( P )

TYPE POINT

REAL :: X, Y, Z

END TYPE POINT

TYPE (POINT) P

REAL :: T

T = P%X

P%X = P%Y

P%Y = T

P%Z = -2.*P%Z

...

Pointers

A FORTRAN 77 pointer can be passed to a C routine as a pointer to a pointer because the Fortran routine passes arguments by reference.

Table 11-11 Passing a FORTRAN 77 POINTER

Fortran calls C 

C calls Fortran 

REAL X

POINTER (P2X, X)

EXTERNAL PASS

P2X = MALLOC(4)

X = 0.

CALL PASS(X)

...

------------------------------

void pass_(x)

int **x;

{

**x = 100.1;

}

extern void fpass_;

...

float *x;

float **p2x;

fpass_(p2x) ;

...

------------------------------

SUBROUTINE FPASS (P2X)

REAL X

POINTER (P2X, X)

X = 0.

...

C pointers are compatible with Fortran 90 (release 2.0) scalar pointers, but not array pointers.

Passing Data Arguments by Value

Call by value is available only for simple data with FORTRAN 77, and only by Fortran routines calling C routines. There is no way for a C routine to call a Fortran routine and pass arguments by value. It is not possible to pass arrays, character strings, or structures by value. These are best passed by reference.

Use the nonstandard Fortran function %VAL(arg) as an argument in the call.

In the following example, the Fortran routine passes x by value and y by reference. The C routine incremented both x and y, but only y is changed.

Table 11-12 Passing Simple Data Arguments by Value: FORTRAN 77 Calling C

Fortran 77 calls C 

REAL x, y

x = 1.

y = 0.

PRINT *, x,y

CALL value( %VAL(x), y)

PRINT *, x,y

END

-----------------------------------------------------------

void value_( float x, float *y)

{

printf("%f, %f\n",x,*y);

x = x + 1.;

y = y + 1.;

printf("%f, %f\n",x,*y);

}

-----------------------------------------------------------

Compiling and running produces output:

1.00000 0. x and y from Fortran

1.000000, 0.000000 x and y from C

2.000000, 1.000000 new x and y from C

1.00000 1.00000 new x and y from Fortran

Functions That Return a Value

A Fortran function that returns a value of type BYTE (FORTRAN 77 only), INTEGER, REAL, LOGICAL, DOUBLE PRECISION, or REAL*16 (SPARC only) is equivalent to a C function that returns a compatible type (see Table 11-1 and Table 11-2). There are two extra arguments for the return values of character functions, and one extra argument for the return values of complex functions.

Returning a Simple Data Type

The following example returns a REAL or float value. BYTE, INTEGER, LOGICAL, DOUBLE PRECISION, and REAL*16 are treated in a similar way:

Table 11-13 Functions Returning a REAL or float Value

Fortran calls C 

C calls Fortran 

real ADD1, R, S

external ADD1

R = 8.0

S = ADD1( R )

...

------------------------------

float add1_( pf )

float *pf;

{

float f ;

f = *pf;

f++;

return ( f );

}

float r, s;

extern float fadd1_() ;

r = 8.0;

s = fadd1_( &r );

...

------------------------------

real function fadd1 (p)

real p

fadd1 = p + 1.0

return

end

Returning COMPLEX Data

A Fortran function returning COMPLEX or DOUBLE COMPLEX is equivalent to a C function with an additional first argument that points to the return value in memory. The general pattern for the Fortran function and its corresponding C function is:

Fortran function 

C function 

COMPLEX FUNCTION CF(a1, a2, ..., an)

cf_ ()return, a1, a2, ..., an

struct { float r, i; } *;return

Table 11-14 Function Returning COMPLEX Data

Fortran calls C 

C calls Fortran 

COMPLEX U, V, RETCPX

EXTERNAL RETCPX

U = ( 7.0, -8.0)

V = RETCPX(U)

...

------------------------------

struct complex { float r, i; };

void retcpx_( temp, w )

struct complex *temp, *w;

{

temp->r = w->r + 1.0;

temp->i = w->i + 1.0;

return;

}

struct complex { float r, i; };

struct complex c1, c2;

struct complex *u=&c1, *v=&c2;

extern retfpx_();

u -> r = 7.0;

u -> i = -8.0;

retfpx_( v, u );

...

------------------------------

COMPLEX FUNCTION RETFPX(Z)

COMPLEX Z

RETFPX = Z + (1.0, 1.0)

RETURN

END

In 64-bit environments and compiling with -xarch=v9, COMPLEX values are returned in floating-point registers: COMPLEX and DOUBLE COMPLEX in %f0 and %f1, and COMPLEX*32 in %f0, %f1, %f2, and %f3.

Returning a CHARACTER String

Passing strings between C and Fortran routines is not encouraged. However, a Fortran character-string-valued function is equivalent to a C function with two additional first arguments--data address and string length. The general pattern for the Fortran function and its corresponding C function is:

Fortran function 

C function 

CHARACTER*n FUNCTION C(a1, ..., an)

void c_ (result, length, a1, ..., an)

char result[ ];

long length;

Here is an example:

Table 11-15 A Function Returning a CHARACTER String

Fortran calls C 

C calls Fortran 

CHARACTER STRING*16, CSTR*9

STRING = ' '

STRING = '123' // CSTR('*',9)

...

------------------------------

void cstr_( char *p2rslt,

int rslt_len,

char *p2arg,

int *p2n,

int arg_len )

{ /* return n copies of arg */

int count, i;

char *cp;

count = *p2n;

cp = p2rslt;

for (i=0; i<count; i++) {

*cp++ = *p2arg ;

}

}

void fstr_( char *, int,

char *, int *, int );

char sbf[9] = "123456789";

char *p2rslt = sbf;

int rslt_len = sizeof(sbf);

char ch = '*';

int n = 4;

int ch_len = sizeof(ch);

/* make n copies of ch in sbf

*/

fstr_( p2rslt, rslt_len,

&ch, &n, ch_len );

...

------------------------------

FUNCTION FSTR( C, N)

CHARACTER FSTR*(*), C

FSTR = ''

DO I = 1,N

FSTR(I:I) = C

END DO

FSTR(N+1:N+1) = CHAR(0)

END

In this example, the C function and calling C routine must accommodate two initial extra arguments (a pointer to the result string and the length of the string) and one additional argument at the end of the list (length of character argument). Note that in the Fortran routine called from C, it is necessary to explicitly add a final null character.

Labeled COMMON

Fortran labeled COMMON can be emulated in C by using a global struct.

Table 11-16 Emulating Labeled COMMON

Fortran COMMON Definition 

C "COMMON" Definition 

COMMON /BLOCK/ ALPHA,NUM

...

extern struct block {

float alpha;

int num;

};

extern struct block block_ ;

main ()

{

...

block_.alpha = 32.;

block_.num += 1;

...

}

Note that the external name established by the C routine must end in an underscore to link with the block created by the Fortran program. Note also that the C directive #pragma pack may be needed to get the same padding as with Fortran. Both f77 and f90 align data in COMMON blocks to at most 4-byte boundaries.

Sharing I/O Between Fortran and C

Mixing Fortran I/O with C I/O (issuing I/O calls from both C and Fortran routines) is not recommended. It is better to do all Fortran I/O or all C I/O, not both.

The Fortran I/O library is implemented largely on top of the C standard I/O library. Every open unit in a Fortran program has an associated standard I/O file structure. For the stdin, stdout, and stderr streams, the file structure need not be explicitly referenced, so it is possible to share them.

If a Fortran main program calls C to do I/O, the Fortran I/O library must be initialized at program startup to connect units 0, 5, and 6 to stderr, stdin, and stdout, respectively. The C function must take the Fortran I/O environment into consideration to perform I/O on open file descriptors.

However, if a C main program calls a Fortran subprogram to do I/O, the automatic initialization of the Fortran I/O library to connect units 0, 5, and 6 to stderr, stdin, and stdout is lacking. This connection is normally made by a Fortran main program. If a Fortran function attempts to reference the stderr stream (unit 0) without the normal Fortran main program I/O initialization, output will be written to fort.0 instead of to the stderr stream.

The C main program can initialize Fortran I/O and establish the preconnection of units 0, 5, and 6 by calling the f_init() FORTRAN 77 library routine at the start of the program and, optionally, f_exit() at termination.

Remember: even though the main program is in C, you should link with f77.

Alternate Returns

Fortran's alternate returns mechanism is obsolescent and should not be used if portability is an issue. There is no equivalent in C to alternate returns, so the only concern would be for a C routine calling a Fortran routine with alternate returns.

The Sun Fortran implementation returns the int value of the expression on the RETURN statement. This is implementation dependent and its use should be avoided.

Table 11-17 Alternate Returns

C calls Fortran 

Running the Example 

int altret_ ( int * );

main ()

{

int k, m ;

k =0;

m = altret_( &k ) ;

printf( "%d %d\n", k, m);

}

------------------------------

SUBROUTINE ALTRET( I, *, *)

INTEGER I

I = I + 1

IF(I .EQ. 0) RETURN 1

IF(I .GT. 0) RETURN 2

RETURN

END

demo% cc -c tst.c

demo% f77 -o alt alt.f tst.o

alt.f:

altret:

demo% alt

1 2

The C routine receives the return value 2 from

the Fortran routine because it executed the

RETURN 2 statement.