Strcmp — loop while no input

Question

This program does the following:

1. Scans a string of text char input[15];
2. Compares it to char password[ ] = "1sure";
3. Loops if the strings do not match.
4. Terminates if the strings match.

The program loops while the strings do not match. However, I also want the program to loop if nothing is input and the user simply presses enter. I tried using an isgraph function but this causes the program to crash. I commented that section out in the code. Can someone suggest how to get the program to loop if there is no input?

#include <stdio.h>
#include <string.h>

int main()
{
    char password[] = "1sure";
    char input[15];

    do
    {
        printf("Password: ");
        scanf("%s", input);

        if(strcmp(password,input)==0)
        {
            printf("Password accepted.");
            putchar('\n');
            return(0);
        }
        /*else if(isgraph(input)==0)
        {
            printf("No input detected."); //Program crashes with this segment.
            continue;
        }*/
        else
        {
            printf("\nInvalid password.\n");
            continue;
        }
    }
    while(1);
}

Answer 1

The program could look the following way

#include <stdio.h>
#include <string.h>
#include <ctype.h>

int main( void )
{
    char password[] = "1sure";
    char input[15];

    do
    {
        printf("\nPassword: ");

        if ( fgets( input, sizeof( input ), stdin ) == NULL )
        {
            printf( "An error occured or input was interrupted\n" );
            return 0;
        }

        size_t n = strlen( input );

        while ( n && isspace( input[n-1] ) ) input[--n] = '\0';

        if ( input[0] == '\0' )
        {
            printf("No input detected.\n");
            continue;
        }
        else if( strcmp( password, input ) == 0 )
        {
            printf("Password accepted.\n");
            return(0);
        }
        else
        {
            printf("\nInvalid password.\n");
            continue;
        }
    } while(1);
}

Answer 2

The scanf function skips leading whitespace when used with %s and most other specifiers. Whitespace includes newlines, so it is not possible to detect a blank line with scanf.

Instead you can read a line with fgets. Note that I have added error checking:

if ( ! fgets(input, sizeof input, stdin) )
    break;

Unfortunately fgets has a quirk in that it puts the newline character into the buffer, so you have to remove that; one way to do that is:

char *newline = strchr(input, '\n');
if ( newline )
    *newline = '\0';

Then you can go on to strcmp and the rest of your loop.

---

Other notes: If you didn't get compiler errors for isgraph(input) then you need to figure out how to invoke your compiler properly. That code is illegal and if errors are not shown then you may be missing out on other useful information that the compiler could be telling you.

Also, there is no point having else after return. A return statement cannot fail. A continue as the last line of the loop is similarly redundant.

Answer 3

Check the return value of scanf (it should return 1) to continue the rest of the code

printf("Password: ");
char line[15];
if(fgets(line,15,stdin)!=NULL) {
   if(scanf("%s", input)==1) {
      if(strcmp(password,input)==0) {
   ...
   }
}

Answer 4

Simply check the return value of scanf.

if ( scanf("%s",input) != 1 )
       continue;
...
...

Or else you can do like this.

while( scanf("%s",input) != 1 )// It will continue the loop until the correct input come.
       continue;