gets() does not append '\0'?

[StarKid]

Whenever I use the c function gets() to input a string, then try to parse the string with a pointer in a loop using pointer != '\0' as the loop termination condition, the pointer always goes way out of the bounds of the string anyhow.

#include <stdio.h>

#define string_terminator    '\0'
#define MAX_CHARS            80

void main()
{
    char input[MAX_CHARS];
    char *ptr_to_string  = input;

    gets ( input );

    for (input; input != string_terminator; input++)
        puts ( input );

This code outputs all kinds of stuff outside of the array of chars. It does the exact same thing if I use

for (ptr_to_string; ptr_to_string != string_terminator; ptr_to_string++)

What am I doing wrong?

[MadCow108]

from its own manpage:

Never use gets()

its so bad it has even been removed from the C standard!

use fgets instead

as for your problem ptr_to_string != string_terminator compares a pointer and will thus always be true
you have to dereference input to get the value it points to:

*ptr_to_string != string_terminator

[trent.josephsen]

The pointer `input` never becomes 0. The referent of input, i.e. the character it points to, will become 0 when input points 1 past the end of the string. You need to change the test to `*input` instead of just `input`. A C-idiomatic way to do this might be more like

while (*input++) {
    puts(input);
}

However you should really not use gets in the first place.

[Bachstelze]

Incidentally, you should probably stop using whatever learning material you are using right now which gave you the idea to use gets().

[StarKid]

Incidentally, you should probably stop using whatever learning material you are using right now which gave you the idea to use gets().

I would, but the library doesn't have any other books on c available, and I don't have internet access at home. I read in the documentation of gets() that it should not be used, but I haven't learned how to manipulate streams yet so I don't know how to use fgets.

[trent.josephsen]

It's easy.

fgets(buf, sizeof buf, stdin);

Nothing magic about it, `stdin` just represents that you want to read from standard input (usually a terminal). You can replace it with other things later when you want to read from files.

(Edit: In case it wasn't clear, `buf` here should be the array you want to read into.)

[Lux Perpetua]

fgets(buf, sizeof buf, stdin);

This is partially correct, but it's important to note that it will only work if buf is allocated statically; it will fail horribly if buf is allocated dynamically. This trips up beginners all the time.

StarKid - Instead of

gets(buf);

use

fgets(buf, buf_size, stdin);

buf_size is however many characters the buffer can hold. (In your example, it's MAX_CHARS.) stdin is stdin; don't mess with it. Later, you'll learn about reading from files, which is what that third argument lets you do.

Seriously, though, don't ever use gets.

[trent.josephsen]

This is partially correct, but it's important to note that it will only work if buf is allocated statically; it will fail horribly if buf is allocated dynamically. This trips up beginners all the time.

It would be more accurate to say that it works only when buf is an array object and not when buf is a pointer object. Whether the memory itself is statically or dynamically allocated is irrelevant, as sizeof is evaluated at compile time and the first argument will decay to a pointer anyway.

You're quite right to warn about it, though.