Resetting pointer to the start of file

Question

How would I be able to reset a pointer to the start of a commandline input or file. For example my function is reading in a line from a file and prints it out using getchar(

Answer 1

If you have a FILE* other than stdin, you can use:

rewind(fptr);

or

fseek(fptr, 0, SEEK_SET);

to reset the pointer to the start of the file.

You cannot do that for stdin.

If you need to be able to reset the pointer, pass the file as an argument to the program and use fopen to open the file and read its contents.

int main(int argc, char** argv)
{
   int c;
   FILE* fptr;

   if ( argc < 2 )
   {
      fprintf(stderr, "Usage: program filename\n");
      return EXIT_FAILURE;
   }

   fptr = fopen(argv[1], "r");
   if ( fptr == NULL )
   {
      fprintf(stderr, "Unable to open file %s\n", argv[1]);
      return EXIT_FAILURE;
   }

    while((c=fgetc(fptr))!=EOF)
    {
       // Process the input
       // ....
    }

    // Move the file pointer to the start.
    fseek(fptr, 0, SEEK_SET);

    // Read the contents of the file again.
    // ...

    fclose(fptr);

    return EXIT_SUCCESS;
}

Answer 2

Piped / redirected input doesn't work like that. Your options are:

• Read the input into an internal buffer (which you already seem to be doing); or
• Pass the file name as a command-line argument instead, and do with it as you please.