Fortran & C - Mixing Fortran & C

The following are examples that demonstrate how to call Fortran from C or vice versa. This has also been worked into individual slides (or a collected version).

• Fortran & C differences
• Cray Fortran & C data types
• IBM Fortran & C data types
• Fortran to C:
  • f2c.f
  • f2cfn.c
  • compilation steps
  • output
• C to Fortran:
  • c2f.c
  • c2ffn.f
  • compilation steps
  • output

Fortran & C differences

• Fortran :
  • Passes by address
  • Routine names are case insensitive
  • Arrays are column-major
  • Strings contain length
• C :
  • Passes by value
  • Names are case sensitive
  • Arrays are row-major
  • "Strings" are null terminated

These demo codes use the Cray C predefined macro _CRAY to conditionally compile any Cray dependent parts, _AIX for any IBM xlc dependent parts, and __hpux for HP machines. Most everything else are satisfied by the defaults.

Note that for the Cray that the C parts need to convert Fortran character strings to C char pointers. On workstations this is usually no problem. In addition, since Fortran is case insensitive Cray Fortran defaults subroutine names to UPPERCASE. Workstations default to lowercase with an underscore (_) usually appended to the name.

Cray Fortran & C data types:

Fortran	C	size(bits)
INTEGER	int	46/64
cft77 -i64	cc -nofastmd
INTEGER	long	64
REAL	double or float	64
DOUBLE PRECISION	long double	128
COMPLEX	float complex or double complex	128
SUBROUTINE SUBR( )	void SUBR( )
FUNCTION FN( )	double FN( )
COMMON /CB/data(10)	struct common { double data[10]; } CB;

The demo codes use the IBM C predefined macro _AIX to conditionally compile any IBM dependent parts.

In addition, since Fortran is case insensitive IBM Fortran defaults subroutine names to lowercase and does not append or prepend an underscore (_) to the name.

IBM Fortran & C data types:

Fortran	C	size(bits)
INTEGER*1	signed char	8
INTEGER*2	short	16
INTEGER	int	32
REAL	float	32
DOUBLE PRECISION	double	64
COMPLEX	2 float struct	64
DOUBLE COMPLEX	2 double struct	128
SUBROUTINE SUBR( )	void subr( )
FUNCTION FN( )	float fn( )
COMMON /CB/data(10)	?

---

f2c.f calls f2cfn.c

---

      PROGRAM F2C
C
      CHARACTER*32 NAME
      INTEGER AGE
      REAL TEMP
C
      NAME = "Knut"
C add null character at end for portable & safe handling by C
      NAME(LEN(NAME):LEN(NAME)) = CHAR(0)
C note that LEN(NAME) = 32 in this case
      AGE = 4
      TEMP = 98.6
      CALL NAMEAGE(NAME, AGE, TEMP)
      END

---

f2cfn.c called by f2c.f

---

 
#include <string.h>
#ifdef _CRAY
#  include <fortran.h>
#  define nameage       NAMEAGE
#else
#  if !defined(_AIX) && !defined(__hpux)
#    define nameage     nameage_
#  endif
#  define _fcd          char *
#  define _fcdtocp(a)   (a)
#  define _fcdlen(a)    strlen(a)
#endif

void nameage(_fcd name, int *age, float *temp) {
        char *cp;
        size_t len;

        cp = _fcdtocp(name);    /* convert to C char* */
        len = _fcdlen(name);

        /* strip trailing blanks */
        while (cp[len-1] == ' ' || cp[len-1] == '\0') --len;
        printf("Hello %.*s, who is %d years old, "
                "has a temperature of %4.1f\n", len, cp, *age, *temp);
}
 

Compilation Steps for the Above Example

• Cray - PVPs - UNICOS
  • cc -c f2cfn.c
  • cf77 f2c.f f2cfn.o
  • (or substitute f90 for cf77)
• IBM - SP2 - AIX
  • xlc -c f2cfn.c
  • xlf f2c.f f2cfn.o
• SGI - IRIX
  • gcc -c f2cfn.c
  • f77 f2c.f f2cfn.o
• SUN - SunOS
  • gcc -c f2cfn.c
  • f77 f2c.f f2cfn.o
• HP - UX
  • gcc -c f2cfn.c
  • f77 f2c.f f2cfn.o
• PC - Linux
  • gcc -c f2cfn.c
  • g77 f2c.f f2cfn.o

Output for the Above Example

Hello Knut, who is 4 years old, has a temperature of 98.6

---

c2f.c calls c2ffn.f

---

 
#include <string.h>
#ifdef _CRAY
#  include <fortran.h>
#  define nameage      NAMEAGE
#else
#  if !defined(_AIX) && !defined(__hpux)
#    define nameage    nameage_
#  endif
#  define _fcd          char *
#  define _cptofcd(a,b) (a)
#  define _fcdlen(a)    strlen(a)
#endif

void nameage(_fcd name, int *nlen, int *age, float *temp);

int main() {
        char *name = "Knut";
        _fcd fp;
        int nlen,age = 4;
        float temp = 98.6;

        nlen = strlen(name);
        fp = _cptofcd(name, nlen);      /* convert to Fortran string */

        nameage(fp, &nlen, &age, &temp);
        return 0;
}

---

c2ffn.f called by c2f.c

---

      SUBROUTINE NAMEAGE(NAME, NLEN, AGE, TEMP)
      CHARACTER*(*) NAME
      INTEGER NLEN,AGE
      REAL TEMP
C
      WRITE(6,1000) NAME(1:NLEN),AGE,TEMP
 1000 FORMAT(1X,'Hello ',A,', who is ',I2,
     .       ' years old, has a temperature of ', f4.1)
      RETURN
      END

Compilation Steps for the Above Example

To discover which libraries are necessary for Fortran modules, compile a short Fortran test program and add the -V or -v option (or whatever) to get a verbose execution listing to show what arguments are being passed to each compilation stage.

• Cray - PVPs - UNICOS
  • cf77 -c c2ffn.f
  • cc -c c2f.c
  • cf77 c2f.o c2ffn.o
  • (or substitute f90 for cf77)
• IBM - SP2 - AIX
  • xlf -c c2ffn.f
  • cc c2f.c c2ffn.o -lxlf90 -lxlf -lm
• SGI - IRIX
  • f77 -c c2ffn.f
  • gcc c2f.c c2ffn.o -lF77 -lm -lU77 -lI77 -lisam -lmpc -lc
• SUN - SunOS
  • f77 -c c2ffn.f
  • gcc c2f.c c2ffn.o -L/usr/lang/SC0.0 -lF77 -lm -lc
• HP - UX
  • f77 -c c2ffn.f
  • gcc c2f.c c2ffn.o -lcl -lisamstub -lc
• PC - Linux
  • g77 -c c2ffn.f
  • gcc -c c2f.c
  • g77 c2f.o c2ffn.o

Output for the Above Example

 Hello Knut, who is  4 years old, has a temperature of 98.6