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EOF & precedence of !=

 
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Old 04-28-2004
Question EOF & precedence of !=
Gurus,

I am teaching myself C and have a question.

I wrote a small prog that reads characters as entered at the prompt and checks the value for EOF.

Unless I am 100% wrong, the value will be '1' until getchar() has anything to read in my stream.

/* PROG 1 */
#include <stdio.h>

main()
{
int c;

while (c = getchar() != EOF)
printf("%d\n", c);
}

I compile this code, I run it at the prompt and type a string "1234<enter>" and get this output
1
1
1
1
1

So far, so good, right? For grins, I changed the number of brackets in my while() clause like below:

/* PROG 2 */
...
while ((c = getchar()) != EOF)
...

I re-compile this code and run it: at the prompt, I type the same string "1234<enter>" and get this output
49
50
51
52
10

Now, I obtain the decimal values for the stream I entered (line feed included).

Can someone please explain what is going on?

Thanks in advance! Al.
 
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LEARN ABOUT REDHAT

fgets

GETS(3) 						     Linux Programmer's Manual							   GETS(3)

NAME

       fgetc, fgets, getc, getchar, gets, ungetc - input of characters and strings

SYNOPSIS

       #include <stdio.h>

       int fgetc(FILE *stream);
       char *fgets(char *s, int size, FILE *stream);
       int getc(FILE *stream);
       int getchar(void);
       char *gets(char *s);
       int ungetc(int c, FILE *stream);

DESCRIPTION

       fgetc() reads the next character from stream and returns it as an unsigned char cast to an int, or EOF on end of file or error.

       getc() is equivalent to fgetc() except that it may be implemented as a macro which evaluates stream more than once.

       getchar() is equivalent to getc(stdin).

       gets()  reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF, which it replaces with ''.	No
       check for buffer overrun is performed (see BUGS below).

       fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s.  Reading stops after an
       EOF or a newline.  If a newline is read, it is stored into the buffer.  A '' is stored after the last character in the buffer.

       ungetc()  pushes  c  back to stream, cast to unsigned char, where it is available for subsequent read operations.  Pushed - back characters
       will be returned in reverse order; only one pushback is guaranteed.

       Calls to the functions described here can be mixed with each other and with calls to other input functions from the stdio library  for  the
       same input stream.

       For non-locking counterparts, see unlocked_stdio(3).

RETURN VALUE

       fgetc(), getc() and getchar() return the character read as an unsigned char cast to an int or EOF on end of file or error.

       gets() and fgets() return s on success, and NULL on error or when end of file occurs while no characters have been read.

       ungetc() returns c on success, or EOF on error.

CONFORMING TO

       ANSI - C, POSIX.1

BUGS

       Never  use  gets().  Because it is impossible to tell without knowing the data in advance how many characters gets() will read, and because
       gets() will continue to store characters past the end of the buffer, it is extremely dangerous to use.  It has been used to break  computer
       security.  Use fgets() instead.

       It is not advisable to mix calls to input functions from the stdio library with low - level calls to read() for the file descriptor associ-
       ated with the input stream; the results will be undefined and very probably not what you want.

SEE ALSO

       read(2), write(2), ferror(3), fopen(3), fread(3), fseek(3), puts(3), scanf(3), unlocked_stdio(3)

GNU
								    1993-04-04								   GETS(3)