What is EOF in the C programming language?

问题:

How do you get to see the last print? In other words what to put in for EOF? I checked the definitions and it says EOF is -1.

And if you enter Ctrl-D you won\'t see anything.

#include <stdio.h>

int main() {
 int c;
 while((c = getchar() != EOF)) {
  printf(\"%d\\n\", c);
 }
 printf(\"%d - at EOF\\n\", c);
}

回答1:

On Linux systems and OS X, the character to input to cause an EOF is Ctrl-D. For Windows, it\'s Ctrl-Z.

Depending on the operating system, this character will only work if it\'s the first character on a line, i.e. the first character after an Enter. Since console input is often line-oriented, the system may also not recognize the EOF character until after you\'ve followed it up with an Enter.

And yes, if that character is recognized as an EOF, then your program will never see the actual character. Instead, a C program will get a -1 from getchar().

回答2:

You should change your parenthesis to

while((c = getchar()) != EOF)

Because the \"=\" operator has a lower precedence than the \"!=\" operator. Then you will get the expected results. Your expression is equal to

while (c = (getchar()!= EOF))

You are getting the two 1\'s as output, because you are making the comparison \"c!=EOF\". This will always become one for the character you entered and then the \"\\n\" that follows by hitting return. Except for the last comparison where c really is EOF it will give you a 0.

EDIT about EOF: EOF is typically -1, but this is not guaranteed by the standard. The standard only defines about EOF in section 7.19.1:

EOF which expands to an integer constant expression, with type int and a negative value, that is returned by several functions to indicate end-of-file, that is, no more input from a stream;

It is reasonable to assume that EOF equals -1, but when using EOF you should not test against the specific value, but rather use the macro.

回答3:

The value of EOF is a negative integer to distinguish it from \"char\" values that are in the range 0 to 255. It is typically -1, but it could be any other negative number ... according to the POSIX specs, so you should not assume it is -1.

The ^D character is what you type at a console stream on UNIX/Linux to tell it to logically end an input stream. But in other contexts (like when you are reading from a file) it is just another data character. Either way, the ^D character (meaning end of input) never makes it to application code.

As @Bastien says, EOF is also returned if getchar() fails. Strictly speaking, you should call ferror or feof to see whether the EOF represents an error or an end of stream. But in most cases your application will do the same thing in either case.

回答4:

EOF means end of file. It\'s a sign that the end of a file is reached, and that there will be no data anymore.

Edit:

I stand corrected. In this case it\'s not an end of file. As mentioned, it is passed when CTRL+d (linux) or CTRL+z (windows) is passed.

回答5:

Couple of typos:

while((c = getchar())!= EOF)

in place of:

while((c = getchar() != EOF))

Also getchar() treats a return key as a valid input, so you need to buffer it too.EOF is a marker to indicate end of input. Generally it is an int with all bits set.

#include <stdio.h>
int main()
{
 int c;
 while((c = getchar())!= EOF)
 {
  if( getchar() == EOF )
    break;
  printf(\" %d\\n\", c);
 }
  printf(\"%d %u %x- at EOF\\n\", c , c, c);
}

prints:

49
50
-1 4294967295 ffffffff- at EOF

for input:

1
2
<ctrl-d>

回答6:

nput from a terminal never really \"ends\" (unless the device is disconnected), but it is useful to enter more than one \"file\" into a terminal, so a key sequence is reserved to indicate end of input. In UNIX the translation of the keystroke to EOF is performed by the terminal driver, so a program does not need to distinguish terminals from other input files. By default, the driver converts a Control-D character at the start of a line into an end-of-file indicator. To insert an actual Control-D (ASCII 04) character into the input stream, the user precedes it with a \"quote\" command character (usually Control-V). AmigaDOS is similar but uses Control-\\ instead of Control-D.

In Microsoft\'s DOS and Windows (and in CP/M and many DEC operating systems), reading from the terminal will never produce an EOF. Instead, programs recognize that the source is a terminal (or other \"character device\") and interpret a given reserved character or sequence as an end-of-file indicator; most commonly this is an ASCII Control-Z, code 26. Some MS-DOS programs, including parts of the Microsoft MS-DOS shell (COMMAND.COM) and operating-system utility programs (such as EDLIN), treat a Control-Z in a text file as marking the end of meaningful data, and/or append a Control-Z to the end when writing a text file. This was done for two reasons:

1. Backward compatibility with CP/M. The CP/M file system only recorded the lengths of files in multiples of 128-byte \"records\", so by convention a Control-Z character was used to mark the end of meaningful data if it ended in the middle of a record. The MS-DOS filesystem has always recorded the exact byte-length of files, so this was never necessary on MS-DOS.

2. It allows programs to use the same code to read input from both a terminal and a text file.

回答7:

#include <stdio.h>

int main() {
    int c;
    while((c = getchar()) != EOF) { //precedence of != is greater than =, so use braces
        printf(\"%d\\n\", c);
    }
    printf(\"%d - at EOF\\n\", c);
}

I think this is right way to check value of EOF. And I checked the output.

For INPUT: abc and Enter I got OUTPUT: 97 98 99 10. ( the ASCII values)

For INPUT Ctrl-D I got OUTPUT: -1 - at EOF. So I think -1 is the value for EOF.

Try other inputs instead of Ctrl-D, like Ctrl-Z. I think it varies from compiler to compiler.

回答8:

#include <stdio.h>

int main() {
    int c;
    while((c = getchar()) != EOF) { 
        putchar(c);
    }    
    printf(\"%d  at EOF\\n\", c);
}

modified the above code to give more clarity on EOF, Press Ctrl+d and putchar is used to print the char avoid using printf within while loop.

回答9:

int c;

while((c = getchar())!= 10)
{
    if( getchar() == EOF )
        break;

     printf(\" %d\\n\", c);
}