getchar gives the same output as the input string

Question

I am confused by a program mentioned in K amp R that uses getchar    It gives the same output as the input string    include  lt stdio h gt   main        int c      c   getchar        while c    EOF            putchar c            c   getchar              Why does it print the whole string  I would expect it to read a character and ask again for the input   And  are all strings we enter terminated by an EOF

User · Answer

Strings  by C definition  are terminated by   0   You have no  C strings  in your program   Your program reads characters  buffered till ENTER  from the standard input  the keyboard  and writes them back to the standard output  the screen   It does this no matter how many characters you type or for how long you do this   To stop the program you have to indicate that the standard input has no more data  huh   how can a keyboard have no more data     You simply press Ctrl D  Unix  or Ctrl Z  Windows  to pretend the file has reached its end  Ctrl D  or Ctrl Z  are not really characters in the C sense of the word   If you run your program with input redirection  the EOF is the actual end of file  not a make belief one   a out  lt  source c

User · Answer

There is an underlying buffer stream that getchar   and friends read from  When you enter text  the text is stored in a buffer somewhere  getchar   can stream through it one character at a time  Each read returns the next character until it reaches the end of the buffer  The reason it s not asking you for subsequent characters is that it can fetch the next one from the buffer   If you run your script and type directly into it  it will continue to prompt you for input until you press CTRL D  end of file   If you call it like   program  lt  myInput where myInput is a text file with some data  it will get the EOF when it reaches the end of the input  EOF isn t a character that exists in the stream  but a sentinel value to indicate when the end of the input has been reached   As an extra warning  I believe getchar   will also return EOF if it encounters an error  so you ll want to check ferror    Example below  not tested  but you get the idea    main         int c      do           c   getchar            if  c    EOF  amp  amp  ferror                  perror  getchar                      else               putchar c                       while c    EOF

User · Answer

According to the definition of getchar    it reads a character from the standard input  Unfortunately stdin is mistaken for keyboard which might not be the case for getchar  getchar uses a buffer as stdin and reads a single character at a time  In your case since there is no EOF  the getchar and putchar are running multiple times and it looks to you as it the whole string is being printed out at a time  Make a small change and you will understand    putchar c   printf   n         c   getchar      Now look at the output compared to the original code   Another example that will explain you the concept of getchar and buffered stdin     void main    int c  printf  Enter character    c   getchar    putchar    c   getchar    putchar        Enter two characters in the first case  The second time when getchar is running are you entering any character  NO but still putchar works   This ultimately means there is a buffer and when ever you are typing something and click enter this goes and settles in the buffer  getchar uses this buffer as stdin

User · Answer

getchar   reads a single character of input and returns that character as the value of the function  If there is an error reading the character  or if the end of input is reached  getchar   returns a special value  represented by EOF

User · Answer

In the simple setup you are likely using  getchar works with buffered input  so you have to press enter before getchar gets anything to read  Strings are not terminated by EOF  in fact  EOF is not really a character  but a magic value that indicates the end of the file  But EOF is not part of the string read  It s what getchar returns when there is nothing left to read

[c] `getchar()` gives the same output as the input string

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