C：您如何处理没有用户输入的scanf？

Question

Dear fellow Stackoverflowers,

How do you handle for 0 user input?

For example, if the user enters " " or just presses ENTER, how do you handle for that?

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

int main() {

   printf("> \n");
   char string[129];
   int i = 0, length = 0, flag = 0;

   printf("Input a string: ");
   scanf("%128s", string);
   if(strlen(string) != 0) {
      printf("%s\n", string);
   } else {
      printf("Please enter at least one argument.");
   }
}

Answer 1

Quoting C11 , chapter §7.21.6.2, fscanf() , regarding the %s conversion specifier,

s Matches a sequence of non-white-space characters.

and regarding the steps for execution of a conversion specifier

Input white-space characters (as specified by the isspace function) are skipped, unless the specification includes a [ , c , or n specifier.

So, unless a non-whitespace character is there in the input stream, it will wait. No matching will take place.

Also, it's very important that you check the return value of scanf() and family to ensure that the scanning is success.

That said, int main() should be int main(void) at least to conform to the standards.

Answer 2

Achieving this with scanf() is probably impossible and I have no interest in finding out whether it's possible because this solution

#include <stdio.h> /* For fgets(), fprintf() and printf() */
#include <stdlib.h> /* For EXIT_FAILURE */
#include <ctype.h> /* For isspace() */

int
please_enter_at_least_one_argument()
{
    fprintf(stderr, "Please enter at least one argument\n");
    return EXIT_FAILURE;
}

int
main(void)
{
    char string[130];

    printf("> \n");
    printf("Input a string: ");
    if (fgets(string, sizeof(string), stdin) == NULL)
        return please_enter_at_least_one_argument();
    else
    {
        char *pointer;

        pointer = string;
        while (isspace((unsigned char) *pointer) != 0)
            pointer++;
        if (*pointer == '\0')
            return please_enter_at_least_one_argument();
        printf("%s\n", string);
    }
    return 0;
}

solves the problem and is simple very easy to understand.

Please note that the first please_enter_at_least_one_argument() might not be correct because fgets() might return NULL if you press Ctrl + D ( Or on windows Ctrl + Z ) and also when an error occurs. But to find out how to handle that you should probably read man fgets(3) .