Why this c program fall into infinity loop? - c

Question

#include <stdio.h>
int main()
{
int sum=0;
char s[10];
while(scanf("%[^\n]s", s)!=EOF)
{
printf("%s", s);
}
return 0;
}
This while loop fall into infinity loop for any string input.

Answer 1

Given
while(scanf("%[^\n]s", s)!=EOF)
{
printf("%s", s);
}
you ask "Why this c program fall into infinity loop?"
Therefore, you have to understand how scanf() works. scanf() uses your format string to parse input from stdin, which is a stream that delivers character by character.
Assume you have the following in stdin:
foo\n
bar\n
<<EOF>>
Now your format string has this conversion %[^\n] which means "match anything but newline and copy into a char buffer". The s following it is just a literal s because it doesn't have a % in front of it ... so it would match a literal s if there was one. In other words, it doesn't matter here.
Now, calling your scanf() the first time, it WILL match foo and consume it. it returns 1 because it matched one element. After that, stdin looks like this:
\n
bar\n
<<EOF>>
Note the unmatched newline is still there. Your next call again starts with a format string to match anything but newline, but the next character is a newline. scanf() matches nothing and therefore returns 0. It cannot return EOF because, as you can see, EOF is not reached yet. Here is your infinite loop.
So, you have to fix your format string. First, remove the s that never matches anything. It doesn't really hurt here, but it's still wrong. Then, you can take advantage of the whitespace matching of scanf(). Whitespace is either space, tab or newline. If your format string contains whitespace, scanf() will match as much whitespace as is there (this can be none as well). Therefore, an easy fix would be to just start your format string with a space, this will "eat" the left-over newline:
while(scanf(" %[^\n]", s)!=EOF)
{
printf("%s", s);
}
Now this is still dangerous code, because [^\n] matches any amount of characters as long as there is no newline, but your buffer has only room for 9 character plus terminating 0. So you MUST tell scanf() not to match more than that limit, and this can be done by putting the number between the % and the conversion specification:
while(scanf(" %9[^\n]", s)!=EOF)
{
printf("%s", s);
}
This code will do what you want in a safe manner, but assume you wanted to match something more specific, e.g. numbers, and you get some unexpected input: By waiting for EOF, you'd have your infinite loop again, because there is input that doesn't match, and so you can never reach EOF (as above with the unmatched newline). Therefore, always check for the number of successful matches instead. Here you expect exactly one match, so the loop should look like this:
while (scanf(" %9[^\n]", s) == 1)
{
printf("%s", s);
}
In practice, for reading whole lines, you don't need scanf(), just using fgets() would be easier. Your code could look as simple as this (the newlines will be read into s as well):
while (fgets(s, 10, stdin))
{
printf("%s", s);
}
Note the second parameter 10 here: fgets() automatically accounts for the terminating 0 character, so you just give it the size of your buffer.
Just a final hint: this could be even simpler without using printf() when there's no formatting to do:
while (fgets(s, 10, stdin))
{
fputs(s, stdout);
}

Answer 2

int main()
{
int sum = 0;
char s[10];
int ret = scanf("%[^\n]s", s);
while (ret != EOF)
{
printf("%s", s);
memset(s,0,10);
ret = scanf("%[^\n]s", s);
}
return 0;
}
with the debug, you can see the second ret value in the while block is 0, that's , and not equal EOF, so infinity loop.
man scanf:
RETURN VALUE
Upon successful completion, these functions shall return the number of successfully matched and assigned input items; this number can be zero in the event of an early matching failure.
That mean the other scanf is matching failure, except the first one.
But why?
The stdio has cache, and the matching only read "\n" from the stdin, but when you input the "Enter", it input "\n\r", so, the "\r" will in the stdin cache and become the input value for other scanf, but cannot match the input, so matching failure. And scanf return 0, and goto infinity loop.
But if you only input "\n" for each input, it's OK!
You can press "Alt" and at same time press "0" + "1" + "0", and the release the "Alt", the console will input "\n" only and you will get the right result , and the program will be OK, not into infinity loop!
And this's my test result:enter image description here