Scanf input carries over to next scanf

Question

I have a code here which has some unwanted behaviour when inputting strings with spaces. eg print illegal_argument. When an input with multiple words comes in I would like it to only register the first word.

What currently happens is:

christian@oblig2 $ print 0
Enter router ID for information > ID: 0, flag: f7, model_length: 13, model: D-Link y6000

It registers the first print for the comparison in the command-loop. But instead of scanf prompting the user for the next input, it instead ignores that and takes the second word/number input by the user from the previous scanf.

What I want to happen is:

christian@oblig2 $ print 0
Enter router ID for information > 0
ID: 0, flag: f7, model_length: 13, model: D-link y6000.

Everything works as intended otherwise, but I wonder if there's a simpler way to do this. I've tried using Strtok, without much luck.

char input[256];
printf("%s@oblig2 $ ", getenv("USER"));
scanf("%s", input);

if(strcmp(input, "print") == 0) {
    print();
}

The above code is how I get input from the user.

Anyone got any ideas?

When you want the whole input line, don't use `scanf`, use `fgets` instead. — Olaf Dietsche, Oct 03 '16 at 20:53
@OlafDietsche Trying `fgets(input, 256, stdin)` doesn't work, unless I'm doing something wrong. Trying `$ print` produces `: command not found.`. — Christian M, Oct 03 '16 at 21:02
@Christian S `fgets(input, 256, stdin)` works. Other things in code need to change too. See http://stackoverflow.com/questions/2693776/removing-trailing-newline-character-from-fgets-input — chux - Reinstate Monica, Oct 03 '16 at 21:49

ad absurdum · Answer 1 · 2016-10-03T23:59:52.693

2

After the scanf() takes the first input value, it leaves the rest in the input stream. Try this to discard the remaining characters:

int ch;
while ((ch = getchar()) != '\n' && ch != EOF)
    continue;             // discard remaining characters

If you put these lines before the next read from stdin, the extra characters won't corrupt your input.

edited Oct 03 '16 at 23:59

answered Oct 03 '16 at 21:46

ad absurdum

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Note: On end-of-file `while (getch() != '\n')` is an infinite loop. – chux - Reinstate Monica Oct 03 '16 at 21:50
True @chux, but a line of user input from the terminal always ends with a '\n', doesn't it? – ad absurdum Oct 03 '16 at 21:53
"input from the terminal always ends with a '\n'," --> not _always_. The behavior is implementation specific. – chux - Reinstate Monica Oct 03 '16 at 21:54
OK, @chux-- edited the above code to handle such cases. – ad absurdum Oct 03 '16 at 21:57
`getch` is not a standard function. Did you mean `getc`? – jamesdlin Oct 03 '16 at 23:57

Scanf input carries over to next scanf

1 Answers1