End of File EOF in C

Question

I am currently reading the book C Programming Language by Ritchie  amp  Kernighan  And I am pretty confused about the usage of EOF in the getchar   function    First  I want to know why the value of EOF is -1 and why the value of getchar    EOF is 0  Pardon me for my question but I really don t understand  I really tried but I can t    Then I tried to run the example on the book that can count the number of characters using the code below but it seems that I never get out of the loop even if I press enter so I am wondering when would I reach the EOF   main       long nc     nc   0     while  getchar      EOF           nc     printf   ld n   nc       Then  I read the same problem at Problem with EOF in C  Most people advised that instead of using EOF  use the terminator  n or the null terminator   0  which makes a lot of sense   Does it mean that the example on the book serves another purpose

User · Accepted Answer

EOF indicates  end of file   A newline  which is what happens when you press enter  isn t the end of a file  it s the end of a line  so a newline doesn t terminate this loop   The code isn t wrong     it just doesn t do what you seem to expect  It reads to the end of the input  but you seem to want to read only to the end of a line   The value of EOF is -1 because it has to be different from any return value from getchar that is an actual character  So getchar returns any character value as an unsigned char  converted to int  which will therefore be non-negative   If you re typing at the terminal and you want to provoke an end-of-file  use CTRL-D  unix-style systems  or CTRL-Z  Windows   Then after all the input has been read  getchar   will return EOF  and hence getchar      EOF will be false  and the loop will terminate       well  it has undefined behavior if the input is more than LONG MAX characters due to integer overflow  but we can probably forgive that in a simple example

User · Answer

EOF is -1 because that s how it s defined  The name is provided by the standard library headers that you  include  They make it equal to -1 because it has to be something that can t be mistaken for an actual byte read by getchar    getchar   reports the values of actual bytes using positive number  0 up to 255 inclusive   so -1 works fine for this   The    operator means  not equal   0 stands for false  and anything else stands for true  So what happens is  we call the getchar   function  and compare the result to -1  EOF   If the result was not equal to EOF  then the result is true  because things that are not equal are not equal  If the result was equal to EOF  then the result is false  because things that are equal are not  not equal    The call to getchar   returns EOF when you reach the  end of file   As far as C is concerned  the  standard input   the data you are giving to your program by typing in the command window  is just like a file  Of course  you can always type more  so you need an explicit way to say  I m done   On Windows systems  this is control-Z  On Unix systems  this is control-D   The example in the book is not  wrong   It depends on what you actually want to do  Reading until EOF means that you read everything  until the user says  I m done   and then you can t read any more  Reading until   n  means that you read a line of input  Reading until   0  is a bad idea if you expect the user to type the input  because it is either hard or impossible to produce this byte with a keyboard at the command prompt

User · Answer

That s a lot of questions     Why EOF is -1  usually -1 in POSIX system calls is returned on error  so i guess the idea is  EOF is kind of error  any boolean operation  including     returns 1 in case it s TRUE  and 0 in case it s FALSE  so getchar      EOF is 0 when it s FALSE  meaning getchar   returned EOF  in order to emulate EOF when reading from stdin press Ctrl D

[c] End of File (EOF) in C

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