Why is my program got into an infinite loop? - c

Question

I want to scan a line until newline is pressed. I'm aware of gets() function, but I wanted to learn it with scanf(). The problem is, that my program falls into an infinite loop, where it scans the input from user and then infinitely prints it out, where it should print once after each scan. Can anybody explain why is it behaving like this?
#include<stdio.h>
int main()
{
char str[100];
while(str[0]!='\0')
{
scanf("%[^\n]",str);
printf("%s\n",str);
}
}

Answer 1

If you insist on using scanf, then change format specifier:
" %[^\n]"
The space in front will skip any previous "dangling" \n
Also you should initialize the str array before checking the contents of it, better yet use a do-while-loop instead .
Something like this should work
char str[100] = {0};
do
{
scanf(" %[^\n]",str);
printf("%s\n",str);
}
while(str[0]!='q');
Personally I prefer to use fgets(...) in combination with sscanf(...)
Also it is good practice to check the return value of scanf, there is a return value for a purpose.
added another while condition, loops until "q" or "quit"

Answer 2

Since %[^\n] does not accept newline, input is not accepted in the second loop.
Probably, this will do what you want.
#include<stdio.h>
int main(void){
char str[100];
while(1== scanf("%99[^\n]%*c",str)){//%*c consumes newline. Also In case of only newline terminates the loop
printf("%s\n",str);
}
}

Answer 3

BLUEPIXY is absolutely correct. The first return from scanf() is leaving the \n in the input buffer. Subsequent scanf() calls will return right away without reading any characters from stdin because scanf() stops reading on a \n. So a loop ensues. One way to avoid the loop is to read the \n from the input after the call to scanf() as shown below:
#include <stdio.h>
int main()
{
char str[100] = {0};
do
{
scanf("%[^\n]",str);
getchar();
printf("%s\n",str);
}while( str[0] != '\0' );
}
You seem to be under some false beliefs regarding how input works. So I'm going to explain below.
You don't need to do this in a loop because scanf() doesn't read and return one character at a time when you specify a string format. The input is buffered by the terminal. When you ask scanf() to return a string, the terminal will only send the input string to scanf() when it a newline is received. When that happens scanf() returns the string without the newline.
You'll need to do extra work to turn off terminal line buffering. The below example code shows how to turn off terminal I/O buffering.
#include <stdio.h>
#include <unistd.h>
#include <termios.h>
int main()
{
struct termios old_tio, new_tio;
unsigned char c;
/* get the terminal settings for stdin */
tcgetattr(STDIN_FILENO,&old_tio);
/* we want to keep the old setting to restore them a the end */
new_tio=old_tio;
/* disable canonical mode (buffered i/o) and local echo */
new_tio.c_lflag &=(~ICANON & ~ECHO);
/* set the new settings immediately */
tcsetattr(STDIN_FILENO,TCSANOW,&new_tio);
do {
c=getchar();
printf("%c ",(char)c);
} while(c!='q');
/* restore the former settings */
tcsetattr(STDIN_FILENO,TCSANOW,&old_tio);
return 0;
}