File pointers as function parameters

[Metalix]

Hey,

I'm trying to pass some file pointers as function parameters but I'm having a little trouble with it.

Say, I pass the pointer to a function to open my file:

openF(&FilePntr, argv);


...


OpenF(FilePntr, argv)
FILE FilePntr;
argv[];
{
*FilePntr = fopen(argv[1], "r");
}


Assuming the function's as simple as this, this is pretty much what I have done.

The compiler generates an error and says that on line 5 there's a type mis match.

How can I get around this? It's important that the program alters the variables used in the other functions by passing pointers as parameter functions, but how can I do it?

Thanks alot,

Lewis

[Thantos]

well it appears you are using a very old style of declaring parameters. Try this:

void OpenF(FILE **ptr_to_Fileptr, const char **argv)
{
  *ptr_to_FilePtr = fopen(argv[1], "r");
}

And call with:

int main(int argc, char *argv[])
{
  FILE *fptr=NULL;
  OpenF(&fptr, argv);
  /* blah blah blah */
  fclose(fptr);
  return 0;
}

[Sebastiani]

the FILE structure wasn't intended to be used directly - you should treat it as an opaque handle to an open file.

FILE * fp = fopen(fname, "rb");
if(fp) {
  /* do some work */
 fclose(fp);
 fp = NULL; /* a good idea if you're reusing the pointer */
 }

if you want to change the value of the file pointer from within a function, you'll need to pass a pointer to it:

void foo(FILE ** fp, char * fname) {
 if(*fp) {
  fclose(*fp);
  }
 *fp = fopen(fname, "rb");
 / * etc */
 }

[Thantos]

the FILE structure wasn't intended to be used directly

I'm sorry but I don't see where this is being done.

[Sebastiani]

>> I'm sorry but I don't see where this is being done.

me neither - nevermind.

[Thantos]

Ok just wanted to make sure I wasn't going more insane

[Metalix]

Okay, thanks alot.

I know this sound's very stupid, but I'm not quite aware what two stars mean. Is that a pointer to a pointer? or what?

Thanks.

[bithub]

Is that a pointer to a pointer?

sure is.

[Metalix]

So, in theory it's used as:

int Variable;
int *Pntr;
int **Pntr2Pntr;

Pntr = &Variable;
Pntr2Pnr = &Pntr;

[Metalix]

Thanks for the help. The functions for opening my files now run as sweet as a nut!

I've encountered a similar problem though. When I try to use a double pointer for a function that reads the file, I get an incompatable pointer error.

ReadGroup(&InfilePtr);


...


void ReadGroup(FILE **PtrfilePtr)
{
char toWrite;

while (toWrite != EOF)
{
toWrite = getc(PtrfilePtr);
printf("%c", toWrite);
}
}


This is pretty much exactly what I'm doing. Can anyone shed some light?

[andyhunter]

Here try reading this post. It might answer your question. BTW, ignore my comment, I was being stupid that day but Prelude kindly pointed that out for me

[Kleid-0]

So, in theory it's used as:

int Variable;
int *Pntr;
int **Pntr2Pntr;

Pntr = &Variable;
Pntr2Pnr = &Pntr;

MISSPELLING ALERT!!! ERROR CODE 79287349816716874
AT LINE 6!!!

Pntr2Pnr = &Pntr; <------ AAHHH!!!

SHOULD BE

Pntr2Pntr = &Pntr;

Phew! *sips some coffee*

[Metalix]

It doesn't seem to answer my questions. See, I can't see what I'm doing wrong. I passed the parameter as a pointer to a pointer, I gave the function the address, why would it be giving me an incompatabilty error. The code worked in the main function.

[andyhunter]

alright, just a simple misunderstanding. Hopefully I will be able to clear it up:

void ReadGroup(FILE **PtrfilePtr)

You are passing ReadGroup a pointer to a pointer to a file. Thus the pointer to a file is obtained by dereferencing it once, aka *PtrfilePtr. So the rest of your code should look like:

{
        char toWrite;

        while (toWrite != EOF)
        {
                toWrite = getc(*PtrfilePtr);
                printf("%c", toWrite);
        }
}

Let me know if this helps.

[Metalix]

I haven't tried it yet (on another machine), but I think you've hit it spot on.

Thanks!