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Fortran Programming Guide

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.