Null termination of C string

Question

Is it right to say that the null terminating C string is automatically added by the compiler in general?

So in the following example:

char * str = "0124";
printf("%x", str[str[3] - str[2] + str[4]]);

the output is always 32?

Thanks.

-1 Why are you interested in that weirdo `printf` output? This just distracts from your question, doesn't it? Please reformulate. — Jens Gustedt, Jul 18 '13 at 13:03
It gives me a warning "deprecated conversion from string constant to 'char*' ". You should use char const* str = "0124" — poorvank, Jul 18 '13 at 13:05
@DanielFischer But (I believe) `printf("%c", str[str[3] - str[2] + str[4]]);` should print "2" even with EBCDIC. — Bernhard Barker, Jul 18 '13 at 13:11
@Dukeling Indeed (modulo the glyphs used, but that's beyond the scope of the language). That is mandated by the standard. — Daniel Fischer, Jul 18 '13 at 13:13
@Dukeling: it would because the EBCDIC terminal yould recognize 0xf2 as '2'. If you redirected the output and examine the binary contents you would find 0xf2 — Ingo Leonhardt, Jul 18 '13 at 13:13

score 3 · Answer 1 · answered Jul 18 '13 at 13:05

3

First question: yes

Second question: yes on a ASCII system: you calculate '4' - '2' + '\0' which is in integers: 0x34 - 0x32 + 0 = 2 so you get str[2] which is '2' which is 0x32. '4' - '2' to be 2 is defined in C, but if you ran your code on an EBCDIC system, '2' was 0xf2

answered Jul 18 '13 at 13:05

Ingo Leonhardt

9,435
2
24
33

1

FWIW: `'4' - '2' + '\0'` would be 2 regardless if ASCII, EBCDIC, or any other conforming C character set as digit characters are required to be consecutive. – chux - Reinstate Monica Jul 25 '13 at 23:17

score 1 · Accepted Answer · answered Jul 18 '13 at 13:07

Yes, the compiler does add the null terminator. Thus there is 5 bytes of memory allocated to str off the stack.

By the looks of it, with that string literal, (str[3] - str[2] + str[4]) evaluates to (52 - 50 + 0), so you are acessing str[2], which will print 0x32 in hex.

score 1 · Answer 3 · answered Jul 18 '13 at 13:08

The terminating null character is added by the compiler; 6.4.5p6:

6 - In translation phase 7, a byte or code of value zero is appended to each multibyte character sequence that results from a string literal or literals. The multibyte character sequence is then used to initialize an array of static storage duration and length just sufﬁcient to contain the sequence. [...]

The printf output will be the character code of the 2 character on your system. The characters 0 to 9 are guaranteed to have contiguous codes (5.2.1p3), but not to have any particular value.

Null termination of C string

3 Answers3