我正在阅读“The C Programming Language”（2ed），但我不明白一个概念：

Question

I am reading "The C programming Language" (2ed) and I didn't understand a concept.

There is this code to copy input to output

#include <stdio.h>

main()
{
    int c;
    c = getchar();
    while (c != EOF) {
        putchar(c);
        c = getchar();
    }
}

In the beginning, c is declared as an int type variable, but if I write something like "test line" the output is the expected "test line". My question is: how is c declared as an int type variable but the program stores space enough to store a lot of characters? "Inside" the computer, it registers as an array of characters? How does this process work?

Answer 1

but if i write something like "test line" the output is the expected "test line".

how is "c" declared as a int type variabel but the program stores space enough to a lot of characters ?

All that input is not stored in variable c at the same time, but in input and output buffers. Variable c handles those characters one at a time.

Lets walk though the sequence of events.

1. main() starts and runs to getchar() .

2. getchar() waits for input.

3. User types t , the OS buffers the character.

4. User types e s ...., e the OS buffers those characters.

5. User types Enter , the OS buffers the character '\\n' and gives the buffer to stdin for getchar() to start using.

6. getchar() returns t and saves in c .

7. Code continues to putchar(c); and puts the t in the stdout buffer.

8. Code continues to getchar(); , returns the buffered e and saves in c .

9. Code continues to putchar(c); and puts the e in the stdout buffer.

10. 8 - 9 repeat for st line .

11. Code continues to getchar(); , returns the buffered \\n and saves in c .

12. Code continues to putchar(c); and puts the \\n in the stdout buffer. The buffer is now flushed to the OS output terminal. Output now visible.

13. Back to step 2 until no input available.

Answer 2

First, some background.

A character is an element of a string. A character is a number .

The glyphs you see on the screen ( a , b , etc) are simply how your terminal interprets the numbers it receives (97, 98, etc).

In a C program, there is absolutely no difference between

'a'

97

Both are integer literals of type int with a value of ninety-seven. (I'm assuming ASCII-based machines here. They're actually different on an EBCDIC-based machine.)

---

Now for your question.

but the program stores space enough to a lot of characters ?

It does not. c only contains a single character at any given time. The first time through the loop, 116 aka 't' is assigned to it. The second time through the loop, 101 aka 'e' is assigned to it. etc.

Answer 3

how is "c" declared as a int type variabel but the program stores space enough to a lot of characters ?

The source code shown does not store a lot of characters. It processes one character at a time.

c = getchar(); puts the code for one input character in c . Then putchar(c); writes that code to output. Then the program can forget about the character. It does not need it any more, and it can get a new character code and reuse c for that.

The output stream may hold a lot of characters in a buffer before writing them to the output device, but that is in separate source code that is not directly in the source code shown. It is part of the C standard libraries.

Answer 4

Suppose I'm mixing drinks for my friends. I have four glasses in front of me. I want to put 1/8 cup (1 shot) of gin in each glass. But I don't have a 1/8 cup measuring cup. So I get a 1/4 cup measuring cup, and I fill it halfway, and that way I measure 1/8 cup of liquor for each glass.

Is it a problem that I used a 1/4 cup measure to measure 1/8 cup of liquid? What about all the empty space at the top? Well, obviously, it won't cause any problem at all.

Similarly, if you use an int (which is probably a 16- or 32-bit variable) to hold the return value from calling getchar() , this won't cause any problem at all even if most of the character values that come back from getchar are just 8 bits.

But it will make a difference when it comes time for getchar to return the code EOF (end of file) to you! EOF is a value that will not fit in 8 bits, because it is (by definition) not an 8-bit character value. That's why it's important to use an int variable to handle values coming back from getchar , not char as might seem obvious.

Answer 5

• something is called io buf,other called return value
• Specifically, it can be understood as getchar read chars to io buffer, and return one by one to api user