do-while doesn't work

Can anyone help with my do-while statement?
It doesn't work when input file name is blank (empty return).

void
main(int argc, char *argv[])
{
char fileNameIn[20];
char fileNameOut[20] = { "\0" };


if(argc == 1) // 0 & 1; only the program name is input
{
do
{
printf("\n Enter the input file name: ");
fflush(stdin);
gets(fileNameIn);
} while (fileNameIn == NULL);

printf("\n Enter the output file name: ");
fflush(stdin);
gets(fileNameOut);
} ;

if (argc == 2 || fileNameOut == NULL) // two strings input
// (pgm10 and input fileName);
{ // output fileName is missing
printf("\n Please enter output file name: ");
fflush(stdin);
gets(fileNameOut);
} ;

if (argc == 3) // user inputs all 3 variables pgm10 fileNameIn
// fileNameOut
{
strcpy (fileNameIn, argv[1]);
strcpy (fileNameOut, argv[2]);
} ;

printf("\n argc = %d\n", argc);
printf(" Program name = %s\n", argv[0]);

if (argc == 2) { strcpy (fileNameIn, argv[1]); }
printf(" file name in = %s\n", fileNameIn);
if (argc == 3)
printf(" file name out = %s\n", argv[2]);
else
printf(" file name out = %s\n", fileNameOut);
}

Quote:

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Can anyone help with my do-while statement?
It doesn't work when input file name is blank (empty return).

Code:

---

if(argc == 1) // 0 & 1; only the program name is input
{
do
{
printf("\n Enter the input file name: ");
fflush(stdin);
gets(fileNameIn);
} while (fileNameIn == NULL);

printf("\n Enter the output file name: ");
fflush(stdin);
gets(fileNameOut);
} ;

---

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Your compiler realizes that you are brutalizing the C language, so it refuses to allow the code to work properly. Either that, or you're just doing some things horribly wrong. In the event it is the latter, allow me to help:

Code:

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/**
*** Point #1: 'main' returns an 'int'. Period.
*** Anyone who tells you otherwise is wrong.
*** End of discussion.
***/

int main( int argc, char *argv[])
{
    /**
    *** Slight adjustment on your declaration. You can
    *** just use 0 here, or '\0', and while "\0" may work,
    *** it isn't the preferred way of doing it. Just remember
    *** this: single quotes for a single character.
    **/
    char fileNameIn[20] ={0};
    char fileNameOut[20] = {0};

    /**
    *** This will work also:
    **/
    if( argc < 2 )
    {
        do
        {
            printf("\n Enter the input file name: ");
            /**
            *** 'fflush( inputStreamHere )' will not work the same
            *** on every compiler. Just because it works on one,
            *** doesn't mean it will work on another. It is not ANSI
            *** defined.
            ***
            *** Never, ever, ever, use 'gets'. Use 'fgets' instead.
            *** 'gets' is unsafe because i could enter 80 characers in
            *** your buffer and trash your program.
            ***
            *** 'fileNameIn' will never equal NULL. fileNameIn[0] may
            *** be null, but that 'fileNameIn' still points to allocated
            *** memory. It is not a NULL poitner.
            **/
        } while( fgets( fileNameIn, 20, stdin ) == NULL );
    /**
    *** The ; you had here is not needed. Neither is this gets for
    *** the input file name. See below.
    **/
    }

    if ( argc == 2 || fileNameOut[0] == NULL )
    {
        printf("\n Please enter output file name: ");
        fgets( fileOutName, 20, stdin );
        strncpy( fileNameIn, argv[1], 20 );
        fileNameIn[19] = NULL;
    }

    /**
    *** I prefer strncpy over strcpy due to it being safer.
    *** I manually make sure the last element is NULL.
    if ( argc == 3 )
    {   
        strncpy( fileNameIn, argv[1], 20 );
        strncpy( fileNameOut, argv[2], 20 );
        fileNameIn[19] = fileNameOut[19] = NULL;
    }

    printf("\n argc = %d\n", argc);
    printf(" Program name = %s\n", argv[0]);
    printf(" file name in = %s\n", fileNameIn);
    printf(" file name out = %s\n", fileNameOut);

    /**
    *** Since 'main' always returns an int, we return one.
    **/
    return 0;
}

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Try that on for size. If you've any more questions, fire away.

Quzah.

Thanks for all your tips. I am going to incorporate your inputs. I especially like your thoughts on writing safe programs!