12.6 Character Constants

A character constant is written with single quotes, as in 'c'. In the simplest case, c is a single ASCII character that the constant should represent. The constant has type int, and its value is the character code of that character. For instance, 'a' represents the character code for the letter ‘a’: 97, that is.

To put the ‘'’ character (single quote) in the character constant, escape it with a backslash (‘\’). This character constant looks like '\''. The backslash character here functions as an escape character, and such a sequence, starting with ‘\’, is called an escape sequence.

To put the ‘\’ character (backslash) in the character constant, escape it with ‘\’ (another backslash). This character constant looks like '\\'.

Here are all the escape sequences that represent specific characters in a character constant. The numeric values shown are the corresponding ASCII character codes, as decimal numbers.

'\a' ⇒ 7       /* alarm, CTRL-g */
'\b' ⇒ 8       /* backspace, BS, CTRL-h */
'\t' ⇒ 9       /* tab, TAB, CTRL-i */
'\n' ⇒ 10      /* newline, CTRL-j */
'\v' ⇒ 11      /* vertical tab, CTRL-k */
'\f' ⇒ 12      /* formfeed, CTRL-l */
'\r' ⇒ 13      /* carriage return, RET, CTRL-m */
'\e' ⇒ 27      /* escape character, ESC, CTRL-[ */
'\\' ⇒ 92      /* backslash character, \ */
'\'' ⇒ 39      /* single quote character, ' */
'\"' ⇒ 34      /* double quote character, " */
'\?' ⇒ 63      /* question mark, ? */

‘\e’ is a GNU C extension; to stick to standard C, write ‘\33’. (The number after ‘backslash’ is octal.) To specify a character constant using decimal, use a cast; for instance, (unsigned char) 27.

You can also write octal and hex character codes as ‘\octalcode’ or ‘\xhexcode’. Decimal is not an option here, so octal codes do not need to start with ‘0’.

The character constant’s value has type int. However, the character code is treated initially as a char value, which is then converted to int. If the character code is greater than 127 (0177 in octal), the resulting int may be negative on a platform where the type char is 8 bits long and signed.