standard input / output check

This is a discussion on standard input / output check within the C Programming forums, part of the General Programming Boards category; I'm trying to check the command line arguments for a -i input_filename flag. If it's present I want to read ...

  1. #1
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    standard input / output check

    I'm trying to check the command line arguments for a -i input_filename flag. If it's present I want to read the input stream from the file named there. If it's not I want to read the input from standard input.

    Also, I want to check the command line arguments for a -o output_filename, if it's present it'll write the output stream named there, if not it'll write the input to standard output.

    My question is whats the best way to go about doing these checks for my program? Thanks, let me know.

    So far this is what I have:

    FILE *fp;

    if (!strcmp(argv[1], "-i") && !strcmp(argv[3], "-o")) { // Ex: -i input.txt -o output.txt

    /* Input File */ fp = fopen(argv[2],"r");
    // Output File output = fopen(argv4], "w");
    }
    Last edited by benjammin5; 10-26-2011 at 12:21 AM.

  2. #2
    and the hat of int overfl Salem's Avatar
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    Something like
    Code:
    #include <stdio.h>
    #include <string.h>
    
    int main(int argc, char *argv[])
    {
      FILE *fp;
      if ( argc == 2 ) {
        printf("Input from file\n");
        fp = fopen(argv[1],"r");
      } else {
        printf("Input from stdin\n");
        fp = stdin;
      }
      int ch;
      while ( (ch=fgetc(fp)) != EOF ) fputc(ch,stdout);
      fclose(fp);
      return 0;
    }
    
    
    $ ./a.out 
    Input from stdin
    hello
    hello
    $ ./a.out < input.txt 
    Input from stdin
    123
    $ ./a.out input.txt 
    Input from file
    123
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    how do I use fopen on the file name if they use standard input < filename.txt? my program uses f open if they put in the -i filename.txt

  4. #4
    and the hat of int overfl Salem's Avatar
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    < redirections are handled by the shell, not your program.
    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.

  5. #5
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    For a.out < filename.txt or cat filename.txt | a.out
    no filename is needed in your program, use stdin like:

    Code:
    char line[BUFSIZ];
    while( fgets(line,BUFSIZ,stdin) )
    {
      ...
    }
    Last edited by BillyTKid; 10-26-2011 at 02:16 AM.

  6. #6
    msh
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    Quote Originally Posted by benjammin5 View Post
    how do I use fopen on the file name if they use standard input < filename.txt? my program uses f open if they put in the -i filename.txt
    You don't. It's their choice to use redirection instead of `-i` option, and your program shouldn't care.
    Disclaimer: This post shows my ignorance at the time of its making. I claim ownership of but not responsibility for all errors in it. Reference at your own peril.

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