Check if the user input an empty string in C using char array

Question

I'm trying to have the program check, that, if a user inputs nothing the print statement will say it cant find the file name, but the issue I'm having is that the command line will just go to a new line after hitting enter instead of saying the print statement. This is the code here. I was told that Null is the place holder for if nothing is put in so I thought it would work.

int main()
{
FILE *fin;
FILE *fout;
char fInName[50];
char fOutName[50];
printf("pleas type input file, and output file please type legibly\n ");

scanf("%s %s", &fInName, &fOutName);
fin = fopen(fInName, "r");
fout = fopen(fOutName, "r");

if (fInName == NULL && fOutName == NULL)
{
    printf("Error: Cannot open input file %s.", fInName);
}
else if (fInName != NULL && fOutName == NULL)
 {
    printf("file found");
 }
}

What im trying to test is if a first file name is entered and the second isnt then print the statement. If both arent entered then print file does not exist. there is more to the code to see if the file exists or not, but thst would be a bit much, now Im just trying to understand why it wont read unentered data. Ive tried looking at examples such as: How to detect empty string from fgets and tried to alter the code to fit that type of style but it didnt work for me so Im giving you the code it was originally so that anything helpful wouldnt confuse me more.

Edit: okay so I tried to do a simple code in order to see what may be the cause of this issue:

   int main()
{
    char firstname[50];
    char lastname[50];
    char nothing [0];


    printf("pleas type input file, and output file please type legibly pwease\n ");
    scanf("%s" "%s", firstname, lastname);


 if (firstname == lastname )
{
    printf("Error: Cannot open input file %s.", firstname);
}
else
{
    printf("file found");
}

}

I ran the code using adam and either if I typed adam (space) adam or adam(enter) adam the program thinks that the input is not the same, I feel like that would help identify why it doesnt know why nothing is typed in.

Answer 1

The problem is occurring when you try to check if fInName == NULL.

The problem is that fInName is just a variable that you're using to store the name of the file that you want to open. What you actually want to check is that the user gave you a valid filename, and to do so you will want to understand what the return value of functions are.

For example, when you try to open a file using fopen(), if fopen() is unable to successfully open the file, say because the user didn't input anything or misspelled the filename, then fopen() will return NULL, storing it in whatever variable you assigned it to (in your case, *fin and *fout).

Also, scanf() is not recommended for char arrays because if the user inputs more data than you allocated for the array, which in this case is enough space for 50 characters, then scanf() will try to write data to memory that's not yours, causing a buffer overflow.

A much safer option is to use fgets() because you can choose exactly how much data is written into your char array, with the only downside being that fgets() will write newline characters \n (caused by hitting the enter key) into the array, though the simple solution is to overwrite the newline character with '\0'.

Therefore, I would propose:

int main(void)
{
    char fInName[50];
    char fOutName[50];

    // ensure proper usage
    do
    {
        printf("What file would you like to open? ");
        // get infile from user and remove trailing newline '\n' character
        fgets(fInName, 50, stdin);
        fInName[strcspn(fInName, "\n")] = '\0';
    }
    // prompt for input until user enters something
    while (strlen(fInName) < 1);

    do
    {
        printf("What file would you like to output to? ");
        // get outfile from user and remove trailing newline '\n' character
        fgets(fOutName, 50, stdin);
        fOutName[strcspn(fOutName, "\n")] = '\0';
    }
    // prompt for input until user enters something
    while (strlen(fOutName) < 1);

    FILE *fin = fopen(fInName, "r");

    if (fin == NULL)
    {
        printf("Error: Cannot open input file %s.", fInName);
        return 1;
    }
    else
    {
        printf("file found");
    }
}