How input works in C

Question

I'm quite confused about the function scanf and how the input works in C.

See this code :

#include<stdio.h>

int main()
{
    FILE pt = *stdin;
    char b[100];
    scanf("%s", b);
    scanf("%s", b); //breakpoint here
    scanf(" ");     //breakpoint here
}

When the code runs, I input 12345 into the console. I found that pt _ptr(which I don't actually know what it is) has the value "12345\n\n" and b[] has the value "12345".

Then I continue the program and input 23456. Now pt _ptr is "23456\n\n" and b[] is "23456".

My question :

• How does the input work in C? Why does pt _ptr have the value of "12345\n\n" not "12345\n" since I pressed enter only one time(and it seems like the function scanf skips those two \n after successfully read "12345" ).

Answer 1

You are looking under the carpet... You are not supposed to copy FILE structs so the first line

FILE pt = *stdin;

results are actually undefined. Do not look inside, unless you are willing to read and understand the source of you standard C library!

The rest of the code is pretty easy to understand, as b has always the expected value, isn't it?

Answer 2

It is not apparent what it is you are really trying to do besides learn scanf, but you are also invoking stdin and the FILE struct.

Using fscanf instead, (read link for details on `fscanf) it will be easier to use all three.

Here is a simple example of how you can combine fscanf, FILE, and stdin:

int main(void)
{
    FILE *fp={0};
    int res =0 //use to check return value of fscanf 
    char buf[80]; //use 80 for demonstration only
    fp = fopen("c:\\dev\\play\\playtext.txt", "w");//create file and stream
    res = fscanf(stdin, "%s", buf);//stream input from stdin
    if(res == EOF) 
    {
        printf("Exiting...  input error");//notify and exit upon error
        return -1;
    }
    while (strstr(buf, "quit") == NULL)//allow exit by typing quit
    {
        if(fputs(buf, fp) == EOF) return -1;
        buf[0]=0;//reset buffer
        res = fscanf(stdin, "%s", buf);
        if(res == EOF) 
        {
            printf("Exiting...  input error");//notify and exit upon error
            return -1;
        }
    }
    fclose(fp);
    return 0;
}