How to do scanf for single char in C

Question

In C: I'm trying to get char from the user with scanf and when I run it the program don't wait for the user to type anything...

This is the code:

char ch;
printf("Enter one char");
scanf("%c", &ch);
printf("%c\n",ch);

Why is not working?

Solution 1

The %c conversion specifier won't automatically skip any leading whitespace, so if there's a stray newline in the input stream (from a previous entry, for example) the scanf call will consume it immediately.

One way around the problem is to put a blank space before the conversion specifier in the format string:

scanf(" %c", &c);

The blank in the format string tells scanf to skip leading whitespace, and the first non-whitespace character will be read with the %c conversion specifier.

Solution 2

First of all, avoid scanf(). Using it is not worth the pain.

See: Why does everyone say not to use scanf? What should I use instead?

Using a whitespace character in scanf() would ignore any number of whitespace characters left in the input stream, what if you need to read more inputs? Consider:

#include <stdio.h>

int main(void)
{
   char ch1, ch2;

   scanf("%c", &ch1);  /* Leaves the newline in the input */
   scanf(" %c", &ch2); /* The leading whitespace ensures it's the
                          previous newline is ignored */
   printf("ch1: %c, ch2: %c\n", ch1, ch2);

   /* All good so far */

   char ch3;
   scanf("%c", &ch3); /* Doesn't read input due to the same problem */
   printf("ch3: %c\n", ch3);

   return 0;
}

While the 3rd scanf() can be fixed in the same way using a leading whitespace, it's not always going to that simple as above. Another major problem is, scanf() will not discard any input in the input stream if it doesn't match the format. For example, if you input abc for an int such as: scanf("%d", &int_var); then abc will have to read and discarded. Consider:

#include <stdio.h>

int main(void)
{
    int i;

    while(1) {
        if (scanf("%d", &i) != 1) { /* Input "abc" */
            printf("Invalid input. Try again\n");
        } else {
            break;
        }
    }

    printf("Int read: %d\n", i);
    return 0;
}

Another common problem is mixing scanf() and fgets(). Consider:

#include <stdio.h>

int main(void)
{
    int age;
    char name[256];
    printf("Input your age:");
    scanf("%d", &age); /* Input 10 */
    printf("Input your full name [firstname lastname]");
    fgets(name, sizeof name, stdin); /* Doesn't read! */
    return 0;
}

The call to fgets() doesn't wait for input because the newline left by the previous scanf() call is read and fgets() terminates input reading when it encounters a newline.

There are many other similar problems associated with scanf(). That's why it's generally recommended to avoid it.

So, what's the alternative? Use fgets() function instead in the following fashion to read a single character:

#include <stdio.h>

int main(void)
{
    char line[256];
    char ch;

    if (fgets(line, sizeof line, stdin) == NULL) {
        printf("Input error.\n");
        exit(1);
    }

    ch = line[0];
    printf("Character read: %c\n", ch);
    return 0;
}

One detail to be aware of when using fgets() will read in the newline character if there's enough room in the inut buffer. If it's not desirable then you can remove it:

char line[256];

if (fgets(line, sizeof line, stdin) == NULL) {
    printf("Input error.\n");
    exit(1);
}

line[strcpsn(line, "\n")] = 0; /* removes the trailing newline, if present */

Solution 3

This works for me try it out

int main(){
char c;
scanf(" %c",&c);
printf("%c",c);
return 0;
}

Solution 4

Here is a similiar thing that I would like to share,

while you're working on Visual Studio you could get an error like:

'scanf': function or variable may be unsafe. Consider using scanf_s instead. To disable deprecation, use _CRT_SECURE_NO_WARNINGS

To prevent this, you should write it in the following format

A single character may be read as follows:

char c;
scanf_s("%c", &c, 1);

When multiple characters for non-null terminated strings are read, integers are used as the width specification and the buffer size.

char c[4];
scanf_s("%4c", &c, _countof(c));

Solution 5

neither fgets nor getchar works to solve the problem. the only workaround is keeping a space before %c while using scanf scanf(" %c",ch); // will only work

In the follwing fgets also not work..

char line[256];
char ch;
int i;

printf("Enter a num : ");
scanf("%d",&i);
printf("Enter a char : ");

if (fgets(line, sizeof line, stdin) == NULL) {
    printf("Input error.\n");
    exit(1);
}

ch = line[0];
printf("Character read: %c\n", ch);

View more solutions

Comments

[Ben Voigt]

This is the same suggestion John Bode gave two years ago.

[ForceBru]

This doesn't answer the question because the OP wanted to scanf a single character, not a series of them.

[P.P]

fflush(stdin); is undefined behaviour per C standard.

[danglingpointer]

This is not correct, fflush(stdin) is UB, mentioned in lots of question at SO.

[Hoppeduppeanut]

"ch is not a valid variable for this program" Except it is? It's the first line of code in the given example.

[tripleee]

How exactly is this attempting to add value over older, existing answers?

[edmanicom]

Actually after reading the whole thread I would have to conclude there is no definitive answer here... just workarounds

[Max Coplan]

Now that's wild! Would you explain why the space in front of %c makes a difference?

[Max Coplan]

Solved: check these out

[domsson]

This doesn't seem to add any more information than the accepted answer from 2012.

[Besuglov Sergey]

It seems like it doesn't work for me. Unhandled exception at 0x799AF2F6 (ucrtbased.dll) in Deitel0805.exe: An invalid parameter was passed to a function that considers invalid parameters fatal. #include<stdio.h> #include<ctype.h> int main() { char c; printf("%s", "Please enter a character:"); scanf_s("%с", &c, 1); return 0; }

[WENDYN]

It works with Microsoft compiler but that's about it

[FaranAiki]

This answer is very useful. I did not know that C compiler is not case-sensitive. For example: THIS_ANSWER_IS_BAD can be detected by C compiler as this_answer_is_BAD. Oh, it should be mychar, not ch, tested it and it works.