C language: getchar () and cache

When we use the keyboard to type in character information , In fact, the typed characters are stored in the cache , When pressing a series of characters on the screen （ Formed a string ） With the Enter key , There will also be characters and enter keys in the cache （\n） Appear as a character . and getchar The function reads characters one by one from the cache .

For example, press on the screen abc Back carriage return , What appears in the cache is abc\n.

Every time the program is called getchar（） function ,getchar Just read a character from the cache and store it in the corresponding variable . Again ,\n It will also be used as a character getchar Read .

about putchar（） In terms of functions , The function is to print character variables on the screen , If putchar（） The parameter of is just \n when , Then there will be no display on the screen , Carriage return character \n Also will be getchar Read a character .

in general ：getchar Read the characters one by one , But the input characters are stored in the cache first , When enter is pressed getchar Just start reading data in the cache , Every getchar Functions only read one character .putchar Display characters , If you give putchar Yes. \n, That's the carriage return , that putchar Will not be displayed .

Here is a program to illustrate ：

#include <stdio.h>
 
int main()
{
int c;
while((c=getchar())!=EOF)
{
	putchar(c);
	c=getchar();
	
	
}
	return 0;
} 

Run the program , Input 123, And then go back . result ：13

analysis ： When while At the beginning of the program , As while The judgment condition of cycle , here getchar The function starts waiting for the keyboard to enter character information , When the input 123, And then go back . When getchar Function determines that enter has been pressed , That is, the buffer has been flushed , Start reading data from the cache , At this time, the cache is ：123\n

Get into while loop , As a circular judgment condition ,getchar Read the first character 1, accord with while The loop condition , Enter loop execution putchar, The output characters 1.

Go down ,getchar The function is called again , Because there are still 23\n, because getchar Function has not read \n, So continue to read characters in the cache 2, here c=2. Go back to while loop , As a condition of judgment ,getchar Continue reading cache characters , here c=3, Into the loop ,putchar Print character 3, Keep going down ,getchar Read \n. Go back to the cycle condition ,getchar It is found that there are no characters in the cache to read , here getchar（）=-1, That is, it no longer meets the cycle condition , Now jump out of the loop , So the end result is 13.

in general ,getchar When will a value of EOF（-1） In the case? , That is when getchar Function finds that there are no characters in the buffer to read , Even if it's \n It has also been called before getchar Read go , That is, when the buffer is empty ,getchar The return value of will be -1.