The C Standard requires: The reference to the above list is the list of the ten decimal digits starting with 0 and followed in order the by the digits 1 through 9. In a C source program an integer valued decimal digit will never appear in quotes; the code representing the character zero will be in single quotes (e.g., '0') and the code representing the character zero when it is part of a string of characters will be in double quotes (e.g., "0369").

In ASCII the character '0' has decimal value 48 (as you already know). In EBCDIC the character '0' has decimal value 240. If you want to write code that will only work when using code sets with an ASCII base, you can convert convert a character c that represents one of the decimal digits to the corresponding decimal value using c - 48. If you want to write code that will only work when using an EBCDIC code set, you can use c - 240 for the same conversion. If you want to right an expression that will work with portably with ASCII, EBCDIC, or any other codeset supported for use by C on your system, you can use c - '0'.

Note that in C, '0' is a character but is also an integral value that can be used in an arithmetic expression. The integer constant 3 always has decimal value 3. The integer constant '3' has a decimal value that varies depending on what character set is being used. The integer value of the byte in a string that contains the character '6' has the same decimal value of the integer constant '6' which will alway be 6 more than the decimal value of the integer constant '0'.