Octal representation inside a string in C

问题

In the given program:

int main() {
  char *p = "\0777";
  printf("%d %d %d\n",p[0],p[1],p[2]);
  printf("--%c-- --%c-- --%c--\n",p[0],p[1],p[2]);
  return 0;  
}

It is showing the output as:

63 55 0
--?-- --7-- ----

I can understand that it is converting the first two characters after \0 (\077) from octal to decimal but can any one explain me why 2 characters, why not 1 or 3 or any other ?

Please explain the logic behind this.

回答1:

char *p = "\07777";

Here a string literal assigned to a pointer to a char.

"\07777"

In this string literal octal escape sequence is used so first three digits represents a octal number.because rules for octal escape sequence is---

You can use only the digits 0 through 7 in an octal escape sequence. Octal escape sequences can never be longer than three digits and are terminated by the first character that is not an octal digit. Although you do not need to use all three digits, you must use at least one. For example, the octal representation is \10 for the ASCII backspace character and \101 for the letter A, as given in an ASCII chart.

SO your string literal stored in memory like

1st byte as a octal number 077 which is nothing but 63 in decimal and '?' in character

2nd and 3rd byte as a characters '7' and '7' respectively

and a terminating character '\0' in last.

so your answer are as expected 1st,2nd,3d byte of the string literal.

for more explanation you can visit this web site

http://msdn.microsoft.com/en-us/library/edsza5ck.aspx

回答2:

It's just the way the language defines octal escape sequences.

An octal escape sequence, which can be part of a character constant or string literal, consists of a \ followed by exactly 1, 2, or 3 octal digits ('0' .. '7').

In "\07777", the backslash is followed by 3 octal digits (0, 7, 7), which represents a character with the value 077 in octal, or 63 in decimal. In ASCII or an ASCII-derived encoding, that happens to be a question mark '?'.

So the literal represents a string with a length of 3, consisting of '?', '7', '7'.

But there must be a typo in your question. When I run your program, the output I get is:

63 55 55
--?-- --7-- --7--

If I change the declaration of p to

char *p = "\0777";

I get the output you describe. Note that the final ---- is really two hyphens, followed by a null character, followed by two hyphens. If you're on a Unix-like system, try piping the program's output through cat -v or cat -A.

When you post code, it's very important to copy-and-paste it, not retype it.

(And you're missing the #include <stdio.h> at the top.)

来源：https://stackoverflow.com/questions/11815076/octal-representation-inside-a-string-in-c

标签

octal

number-systems