printing the octal number through printf command

Question

I have been trying to understand the printf functionality for octal numbers. if I write the code as:

int main()
{
  char *s = "\123";
  printf("%s",s);
}

It gives me an output as S which actually is correct since ASCII of S is 123 in octal. But my question is, how does the compiler identifies the sequence of numbers to convert from octal. for example:

char *s = "\123456" 

would give an output as S456

Is it that it takes max 3 numbers for octal conversion???

Is there any max limit within which the octal should be given (the max 3 digit octal number would be 777)

Now since there are max 255 ascii characters (octal 377) then when i try to print 777 it prints a typical � ascii character, which is presume may be since there is no such ascii assigned to this number. Also is this functionality a compiler/os dependent???

I hope am pretty much clear since its my first question here :-)

Answer 1

1. Yes. Three digits are the maximum for an octal character literal. From the spec 6.4.4.4 Character constants:

   octal-escape-sequence:
   \ octal-digit
   \ octal-digit octal-digit
   \ octal-digit octal-digit octal-digit

   hexadecimal-escape-sequence:
   \x hexadecimal-digit
   hexadecimal-escape-sequence hexadecimal-digit

2. The maximum octal escape sequence is \777 as you mention. There is no maximum limit for a hexadecimal escape sequence as you can see from the spec quote above.

3. There are only 128 ASCII characters (0-127). That means you can use octal \000 through \177 for ASCII. If you use a different character set, you might be able to go to \377 in an 8-bit char, and all the way to \777 (or higher, using hex escape sequences) for a wchar_t. The spec says:

   The value of an octal or hexadecimal escape sequence shall be in the range of representable values for the type unsigned char for an integer character constant, or the unsigned type corresponding to wchar_t for a wide character constant.

   On most machines, unsigned char is an 8-bit type, limiting your octal escape sequence to \377 in that context and the hex sequence to \xff. In the case of a 32-bit wchar_t context, the hex sequence could be as high as \xffffffff.

Answer 2

The C99 standard (the one I can look at) defines octal-escape-sequence for strings as this:

octal-escape-sequence:
    \ octal-digit
    \ octal-digit octal-digit
    \ octal-digit octal-digit octal-digit

Thus, any octal-escape-sequence has at most 3 octal digits (0 to 7).

The explanation simply states that:

The octal digits that follow the backslash in an octal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant. The numerical value of the octal integer so formed specifies the value of the desired character or wide character.

Also,

Each octal or hexadecimal escape sequence is the longest sequence of characters that can constitute the escape sequence.

And the following constraint:

The value of an octal or hexadecimal escape sequence shall be in the range of representable values for the type unsigned char for an integer character constant, or the unsigned type corresponding to wchar_t for a wide character constant.

Thus, a value of \777 violates the constraint if at most 8 bits are used (CHAR_BIT < 9). I couldn't find something about this in the spec, I guess it is undefined behavior, thus compiler dependent.