Why can't I get the argv[1] value?

[jsrig88]

Why won't this let the user type in the argv[1] value?

#include <stdio.h>

int main ( int argc, char *argv[] )
{
	gets(argv[1]);
    if ( argc != 2 ) /* argc should be 2 for correct execution */
    {
        /* We print argv[0] assuming it is the program name */
        printf( "usage: %s filename", argv[0] );
    }
    else 
    {
        // We assume argv[1] is a filename to open
        FILE *file = fopen( argv[1], "r" );

        /* fopen returns 0, the NULL pointer, on failure */
        if ( file == 0 )
        {
            printf( "Could not open file\n" );
        }
        else 
        {
            unsigned char x;
            /* read one character at a time from file, stopping at EOF, which
               indicates the end of the file.  Note that the idiom of "assign
               to a variable, check the value" used below works because
               the assignment statement evaluates to the value assigned. */
            while  ( ( x = fgetc( file ) ) != EOF )
            {
                printf( "%c", x );
            }
        }
        fclose( file );
    }
return 0;
}

Yes, I pulled much of this from the tutorial, but I don't know how to get a program to LET a user PUT IN command line arguments. I'm just trying to learn how to program in C. Thanks!

[fnoyan]

...but I don't know how to get a program to LET a user PUT IN command line arguments. I'm just trying to learn how to program in C. Thanks!

Have you ever used command line? For example in DOS what does "dir /w" mean or in any UNIX system what does "ls -l" mean? Those options used after command name are the command line arguments and passed to your program by operating system (actually by the OS shell).

You don't need an extra gets() (which is unsafe!, use fgets() instead). You can find lots of exmaples about "how to pass parameter to main". I am sure that oyur C book has such a section!

[jsrig88]

YES, I have used the command line before. And yes, I have used other, more C++-related methodologies for obtaining command line arguments from the user. And no, I do NOT have a C book.

[waltr]

And no, I do NOT have a C book.

says you

[Prelude]

>gets(argv[1]);
That's about the coolest thing I've seen someone try to do...this week at least.

[jsrig88]

"Says you"!? I've been using the tutorials on this website. I've also learned some C++ from a C++ book. But I need to do stuff that is not involved with C++. I want to know the C way of doing it.

[Prelude]

>I want to know the C way of doing it.
The C way of doing it is identical to the C++ way of doing it. Your approach is largely correct, with the difference that you don't seem to be passing your arguments from the command line itself and expecting to somehow prompt the user from within your program. Here's a quick example. You get command line arguments from the program's startup code, not from any code you write. In other words, the system calls main and passes the command line arguments:

#include <stdio.h>

int main ( int argc, char *argv[] )
{
  int i;

  for ( i = 0; i < argc; i++ )
    printf ( "argv[%d] = %s\n", i, argv[i] );

  return 0;
}

Then on the command line, you invoke the program like so:

$ myprog This is a test

And the output would be something like:

argv[0] = myprog
argv[1] = This
argv[2] = is
argv[3] = a
argv[4] = test

[quzah]

I think most users get lost at the phrase "command line", having never encountered one before.


Quzah.

[Prelude]

>I think most users get lost at the phrase "command line", having never encountered one before.
That's probably true. It's scary how the most common question when talking about command line parameters is "where do I click?".

[jsrig88]

Thanks, prelude. But I put that code in, and when I build it and debug it in MS Visual Studio .NET 2003, it still won't let the user put in anything. Is there something up with the compiler in that case, or is there another piece of code I need to put in?

[Dave_Sinkula]

But I put that code in, and when I build it and debug it in MS Visual Studio .NET 2003, it still won't let the user put in anything. Is there something up with the compiler in that case, or is there another piece of code I need to put in?

Run it from the command line. Or find where to tell the debugger what the command line arguments should be. For example, MSVC6 had this: