How to read white spaces with scanf

Question

Even though I use this condition in scanf("[^\n]s", x), or "%34[^\n]", or %127s, I'm unable to get answers correctly. Is there any problem with the scanf area or in some other part....

#include <stdio.h>

int main() 
{
    int i = 4;
    double d = 4.0;
    char s[] = "hello ";
    int a;
    double b;
    unsigned char string_2[100];
    scanf("%d",&a);
    scanf("%lf",&b);
    scanf("%[^\n]s",string_2);
    printf("%d",a+i);
    printf("\n%lf",d+b);
    printf("\n%s",s);
    printf("%s",string_2);
    return(0);
}

Answer 1

Don't use scanf like that.

In this:

scanf("%lf",&b);
scanf("%[^\n]s",string_2);

The first scanf reads a number from the input, but has to wait for your terminal to give the program a complete line of input first. Assume the user 123, so the program reads 123\n from the OS.

scanf sees the newline that is not part of the number any more, and stops at that leaving the newline in the input buffer (within stdio). The second scanf tries to read something that is not newlines, but can't do that, since the first thing it sees is a newline. If you check the return value of the scanf calls, you'll see that the second scanf returns a zero, i.e. it couldn't complete the conversion you asked for.

Instead, read full lines at a time, with fgets or getline:

#include <stdio.h>
int main(void)
{
    char *buf = NULL;
    size_t n = 0;
    double f;
    getline(&buf, &n, stdin);
    if (sscanf(buf, "%lf", &f) == 1) {
        printf("you gave the number %lf\n", f);
    }
    getline(&buf, &n, stdin);
    printf("you entered the string: %s\n", buf);
    return 0;
}  

For a longer discussion, see: http://c-faq.com/stdio/scanfprobs.html