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" ).

Did you read documentation of scanf(3) and of every other function you are using? — Basile Starynkevitch
– Basile Starynkevitch, Commented Oct 23, 2014 at 17:02
How scanf works will vary from operating system to operating system. For example here's how it works on a Mac, chosen just by virtue of being the easiest version to find. — Dan
– Dan, Commented Oct 23, 2014 at 17:03

rodrigo · Accepted Answer · 2014-10-23 16:57:06Z

2

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?

answered Oct 23, 2014 at 16:57

rodrigo

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5 Comments

Basile Starynkevitch Over a year ago

You might even have some C implementations which won't compile that assignment (e.g. because FILE would be typedef-ed to some private struct).

Santi Santichaivekin Over a year ago

YEP! But I thought that was how I should do it if I want to know the data in stdin, I mean, how to actually look at the current stdin?

rodrigo Over a year ago

@SantiSantichaivekin: You do not look into stdin. You read from stdin. And you write to stdout, BTW. FILE does not have any public field, so you can use it only through the library functions.

Dan Over a year ago

If you just want to see what characters are coming next in stdin without reading them, someone has a StackOverflow question about that

rodrigo Over a year ago

@Dan: Yes, but IME, peeking that way is not worth the trouble.

ryyker · Accepted Answer · 2014-10-23 19:16:20Z

0

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;
}

edited Oct 23, 2014 at 19:16

answered Oct 23, 2014 at 18:07

ryyker

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2 Comments

user3629249 Over a year ago

the function fscanf() returns the number of parameters set, never EOF, though it can return 0 if no parameters are set.

ryyker Over a year ago

@user3629249 - It may be you are using an old implementation compiler?. From my documentation - fscanf Reads input from the specified stream and converts it into a series of values according to the specifications of the formatting string. If an input failure occurs before any conversion, the function returns EOF (-1); otherwise, the function returns the number of input items successfully read. From MSDN, From Linux man pages. (look under Return Value)

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