Question

I am trying to open file in function that loads arguments and then save it into struct containing args for later use in other functions. However I cant seem to be able to save it properly (FILE *A,*B equals 0x0). Here’s whan I’ve got so far:

struct Args
{
       int action;      /* holds action token (see teacts)*/
       int error;
       FILE *A,*B;      /* files containing matrixes*/
       int r,s;         /* start coordinates*/
       int power,dir;   /* used in some other function */
} Args;

void getArgs(int argc, char *argv[],struct Args * args)
{
....
testopen(argv[4],args->A);
....
}

int openFile (const char *arg, FILE *input)
{
        input = fopen(arg,"r");

        if (input == NULL)
                return (1);
        else
                return (0);
}

int main (int argc, char **argv)
{
     struct Args args = {.action = A_ERROR};
     getArgs(argc,argv,&args);
     ....
}

Could anyone tell me what am I doing wrong?

Answer 1

Pointers are passed by value in C, like everything else. The line:

input = ....;

in openFile only changes the value of the local variable (argument) input in that function, it doesn’t change args->A.

To fix that, you could:

• pass a pointer to args into openFile.

  int openFile (const char *arg, struct Args *args)
  {
      args->A = fopen(arg,"r");
      ...
  

• pass a pointer to args->A (making the argument a FILE**):

  int openFile (const char *arg, FILE **input)
  {
      *input = fopen(arg,"r");
      ...
  

• not pass a FILE* at all, but return one and do:

  args->A = openFile(argv[4]);
  

  Make sure you return NULL on error and you can do error checking in the caller.