Issues ignoring spaces with scanf when using multiple scanf's in C

Question

I'm trying to use scanf multiple times in a small program to grab inputs that are guaranteed to have spaces. From the multiple threads I've browsed though it seems like scanf("%[^\n]", string); is the way to get it to ignore spaces. This works, for one line, but any other scanf's after that line don't go through and their respective strings put out the following:

Action: J���J
 Resolution: J:F�J�B�J

Here is a bit of example code that I thought would work, but does not.

#include <stdio.h>

int main(void)
{   
    char str1[100];
    char str2[100];

    printf("Situation?\n");
    scanf("%[^\n]", str1);

    printf("Action Taken?\n");
    scanf("%[^\n]", str2);

    printf("Situation: %s\n",str1);
    printf("Action: %s\n",str2);
}

If I input "Just a test" when prompted for the situation the following happens:

Situation?
just a test
Action Taken?
Situation: just a test
Action: ��_��?�J.N=��J�J�d�����J0d���8d��TJ�J

Any suggestions or solutions (excluding fgets)? An explanation of what's happening would be great as well.

Edit: The solution over at scanf: "%[^\n]" skips the 2nd input but " %[^\n]" does not. why?

Adding in the char* fmt = "%[^\n]%*c"; worked 100%.

char* fmt = "%[^\n]%*c";

  printf ("\nEnter str1: ");
  scanf (fmt, str1);
  printf ("\nstr1 = %s", str1);

  printf ("\nEnter str2: ");
  scanf (fmt, str2);
  printf ("\nstr2 = %s", str2);

  printf ("\nEnter str3: ");
  scanf (fmt, str3);
  printf ("\nstr2 = %s", str3);

  printf ("\n");

http://stackoverflow.com/questions/6083045/scanf-n-skips-the-2nd-input-but-n-does-not-why? — Retired Ninja, Aug 19 '14 at 00:02
The `[` specifier must find at least 1 character else the match fails. You will find that `scanf` returns `0` in this case (you should always check the return value of scanf) and then you cause undefined behaviour by using uninitialized variable `str2`. — M.M, Aug 19 '14 at 01:27

score 3 · Accepted Answer · answered Aug 19 '14 at 00:15

3

change

scanf("%[^\n]", str1);

to

scanf("%[^\n]%*c", str1);//consume a newline at the end of the line

answered Aug 19 '14 at 00:15

BLUEPIXY

39,699
7
33
70

chux - Reinstate Monica · Answer 2 · 2014-08-19T00:39:11.617

2

Number of approaches:

Rather than the following which does not consume the Enter or '\n' (which is the problem):

scanf("%[^\n]",str1);

Consume the trailing newline. "%*1[\n]" will only consume 1 '\n', but not save it.
```
scanf("%99[^\n]%*1[\n]" ,str1);
```
Consume the trailing newline on the next scanf(). The " " consume previous and leading white-space.
```
scanf(" %99[^\n]", str1);
```
Use fgets(), but of course, this is not scanf(). The best method.
```
fgets(str1, sizeof str1, stdin);
```

Whatever solution, limit the maximum charcters read and check the function's return value.

    if (fgets(str1, sizeof str1, stdin) == NULL) Handle_EOForIOError();

edited Aug 19 '14 at 00:39

answered Aug 19 '14 at 00:32

chux - Reinstate Monica

143,097
13
135
256

One question: why add `99`? – Tony Aug 19 '14 at 01:52
1

@Tony 99 limits the numbers of characters (width) scanned and stored in `str1`. After storing up to 99 `char`, the `\0' is appended. – chux - Reinstate Monica Aug 19 '14 at 01:55
Thanks. I thought `99` would be some special number here and It seems fine for me to use other number. – Tony Aug 19 '14 at 02:00
1

@Tony As with other `scanf()` specifier, `"%5d"` says to scan up to 5 characters, so values -9999 to 99999 could be scanned and saved. The 99 is a width parameter here for `str1[100]`. – chux - Reinstate Monica Aug 19 '14 at 02:21

score 1 · Answer 3 · answered Aug 18 '14 at 23:55

1

I don't have an immediate answer for you problem, why don't you simply use fgets (or even gets) if you want a line of input ?

answered Aug 18 '14 at 23:55

NoDataFound

11,381
33
59

score 0 · Answer 4 · edited Dec 24 '17 at 01:26

0

Solution one: Using scanf

If you still want to read it by scanf, the answer provided by @chux and @BLUEPLXY is good enough. Like:

 scanf(" %[^\n]", str);  //notice a space is in the formatted string

or

 scanf("%[^\n]%*c", str);

Solution two: Using getline()(although it is a POSIX extension)

Because using gets() and 'fgets()` are unreliable sometimes.

edited Dec 24 '17 at 01:26

Jonathan Leffler

730,956
141
904
1,278

answered Aug 19 '14 at 01:37

Tony

5,972
2
39
58

Issues ignoring spaces with scanf when using multiple scanf's in C

4 Answers4