Problem involving a pointer/passing a pointer through a function

This is a discussion on Problem involving a pointer/passing a pointer through a function within the C Programming forums, part of the General Programming Boards category; edit: took out the code since I posted updated code a few posts down...

  1. #1
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    Problem involving a pointer/passing a pointer through a function

    edit: took out the code since I posted updated code a few posts down
    Last edited by Sholcomb1; 11-05-2006 at 01:40 PM.

  2. #2
    and the hat of wrongness Salem's Avatar
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    > FileOpen(&fileinp, dataFile, &length);
    You need an extra * in several places then.

    Code:
    void FileOpen(FILE **fileptr, char *file_name, int *lengthptr)
    {
       printf("Please enter the filename you wish to open : ");
       scanf("%s", file_name);
    
       *fileptr = fopen(file_name, "r");
       
       if (*fileptr == NULL)
       {
          printf("An error has occurred while attempting to open the file\n");
          exit(-1);
       }
    
       while(fgetc(*fileptr) != EOF)
       {
          length++;
       }
      
      
    }
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
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    ooooh, so that extra * in the prototype is what was needed, Ok. Thanks alot!

  4. #4
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    Ok, so the extra * allowed the file input to work out well, but I'm still having issues with the length pointer counter actually incrementing.

    Code:
    void FileOpen(FILE *fileptr, char *file_name, int *lengthptr)
    {
       printf("Please enter the filename you wish to open : ");
       scanf("%s", file_name);
    
       fileptr = fopen(file_name, "r");
       
       if (fileptr == NULL)
       {
          printf("An error has occurred while attempting to open the file\n");
          exit(-1);
       }
    
       while(fgetc(inp1) != EOF)
       {
          length++;
       }
      
      
    }
    I pass the variable by reference "length" to the function, and the value of length is always zero. However, when I copy/paste that exact same code into main and swap out the pointers for the regular variables it works perfectly fine. Am I not allowed to do an increment to lengthptr? Am I missing something that would keep lengthptr from incrementing?

    Code:
    while ( fgetc(fileinput) != EOF
    {
        length++;
    }
    (this is what I would put into main, which would work out fine)

  5. #5
    and the hat of wrongness Salem's Avatar
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    Same thing - use a *

    Though watch out for
    ++*p
    is different from
    *p++
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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