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No matter what method the operating system actually uses to monitor the end of the file , stay C In language , use getchar() When reading the file and monitoring the end of the file, a special value will be returned , namely EOF( end of file Abbreviation ).scanf() The function also returns... When it detects the end of the file EOF.

The key is to remember EOF It's a value , It indicates that the end of the file is monitored , Not the symbol found in the file .

Pay attention to some points. ：

1. Don't define EOF, because stdio.h Has been defined in .

2. Never mind EOF The real value of , because EOF stay stdio.h of use #define Preprocessing instruction definition , Can be used directly , There is no need to write code to assume EOF For a certain value .

3. Variable ch Type from char Turn into int , because char Variables of type can only represent 0~255 Of unsigned integers , however EOF The value of is -1. not so bad ,getchar() The type of the actual return value of the function is int, So it can read EOF character .

4. because getchar() The return type of the function is int, If you put getchar() The return value of is assigned to char Variable of type , Some compilers warn of possible data loss .

5.ch Being an integer does not affect putchar(), This function will still print equivalent characters .

6. Use this program for keyboard input , Try to enter EOF character . You can't just enter characters EOF, You can't just enter -1( Input -1 Two characters will be transmitted : A hyphen and a number 1). The right way is , We must find out the requirements of the current system .

see ecoh.c and ecoh_eof.c The difference between ：

#include <stdio.h>
int main(void)
{
    char ch;
​
    while ((ch = getchar()) != '#')
        putchar(ch);
​
    return 0;
}

ecoh.c Output result of :

Hello,there.I would
Hello,there.I would
like a #3 bag of potatoes.
like a 

Here are ecoh_eof.c The program :

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

ecoh_eof.c Output result of

hello!
hello!
How are you?
How are you?
zzzz
zzzz
^Z

In the above output results ^Z( It's on the keyboard Ctrl+Z),UNIX and Linux Press... At the beginning of a line in the system Ctrl+D The end of file signal will be transmitted . Many microcomputer systems start with Ctrl+Z Recognized as a signal at the end of the file .

The concept of flow

flow (stream) Is an idealized data flow of actual input or output mapping . This means that different attributes and different kinds of inputs , Represented by a more uniform flow of attributes . The process of opening a file is to associate the stream with the file , And reading and writing are done through streams .