Issues passing string parameters from C to a Fortran subroutine

Question

I am developing a Linux program that receives some user parameters by the command line.

The user interface was made in C and data processing is in Fortran, so the main function in C passes some parameters to the Fortran subroutine. Some of these parameters are strings and it is with them that I am having problems. Here are some parts of the codes:

The C main function:

extern void anaconv_(char *FileName,int *n,char *PlanName,int *m,char *TipoDados,char *FrontName,int *l,char *Dirdat, int *h, bool *kvuge, bool *ch_serie, bool *ch_shuntb, bool *simulacao);
void ajuda_anaconv(void);

int main (int argc, char **argv)
{
  int  i;
  int  n,m,l,h;
  bool flagk     = 0;
  bool flags     = 0;
  bool flagp     = 0;
  bool flagsimul = 0;
  char *dirpwf     = malloc(sizeof(char)*80);
  char *dirbar     = malloc(sizeof(char)*80);
  char *dirfro     = malloc(sizeof(char)*80);
  char *dirdat     = malloc(sizeof(char)*500);
  char *tpdados    = malloc(sizeof(char));
  ...
  dirpwf  = argv[i+1];
  i++;
  ...
  dirbar  = argv[i+1];
  i++;
  ...
  dirfro  = argv[i+1];
  i++;
  ...
  dirdat  = argv[i+1];
  i++;
  ...
  tpdados = argv[i+1];
  i++;
  ...
  n = strlen(dirpwf)
  m = strlen(dirbar)
  l = strlen(dirfro)
  h = strlen(dirdat)
  ...
  anaconv_(dirpwf,&n,dirbar,&m,tpdados,dirfro,&l,dirdat,&h,&flagk,&flags,&flagp,&flagsimul);
  ...
  free(dirpwf);
  free(dirbar);
  free(dirfro);
  free(dirdat);
  free(dirdatcomp);
  free(tpdados);
  ...
  return 1;
}

And them, the Fortran subroutine is something like:

SUBROUTINE ANACONV
 +(FileName,FileNameSize,PlanName,PlanNameSize,TpData,FrontName,FrontNameSize,Dirdat,DirdatSize,kVUGE,CHSERIE,CHSHUNTB,SIMUL)

    IMPLICIT NONE
    CHARACTER*80  FileName,PlanName,FrontName
    CHARACTER*500 Dirdat        
    INTEGER*4     FileNameSize,PlanNameSize,FrontNameSize,DirdatSize
    CHARACTER*1   TpData
    LOGICAL*1     kVUGE,CHSERIE,CHSHUNTB,SIMUL
    ...
END SUBROUTINE ANACONV

The compiler is gcc version 4.8.5 20150623 and I'm running the program in a CentOS Linux release 7.3.1611.

Although the code is compiling without problems, when I run the program passing some parameters, I get the following message:

Fortran runtime error: Actual string length is shorter than the declared one for dummy argument 'filename' (-136420716/80)

I tried to find the problem debugging with gdb, but I could not find the error. The FileName string gets the correct value from the dirpwf pointer of the main function in C, and the string size stored in the FileNameSize variable is also correct, whereas Fortran understands that the received size (apparently -136420716) is less than expected (80 ).

How can I fix this?

Answer 1

Fortran and C keep track of string lengths in different ways. C uses a NUL character at the end as a terminator, while Fortran keeps the length separately. Just how Fortran does that is implementation-dependent. For calling Fortran procedures, most compilers look for the length passed as additional arguments after the named ones, as an address-sized integer passed by value. But this is not universal and not portable. You have passed these lengths, but have done so immediately after the string addresses - that's not where gfortran wants them.

Fortran 2003 introduced C interoperability features that allow you to mix Fortran and C in a portable manner. Fortran 2008 extended that somewhat and Fortran 2018 extends it more. Unfortunately, passing strings from C to Fortran is still messy, even in F2018.

One might think that the simple solution is to add BIND(C) to the Fortran subroutine header - this tells Fortran to not use hidden arguments, among other things. But your use of CHARACTER*80, etc., is then not allowed.

The simplest fix is to move n, m, l and h to the end of the argument list in the C call, remove the Fortran arguments for the sizes, and make n, m, l and h size_t rather than int. Also, don't use & to pass them. This is still non-portable, but it will get you going for now.