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Character sets and identifiers

The following points apply to the character sets and identifiers expected by the compiler:

  • Uppercase and lowercase characters are distinct in all internal and external identifiers. An identifier can also contain a dollar ($) character unless the --strict compiler option is specified. To permit dollar signs in identifiers with the --strict option, also use the --dollar command-line option.

  • Calling setlocale(LC_CTYPE, "ISO8859-1") makes the isupper() and islower() functions behave as expected over the full 8-bit Latin-1 alphabet, rather than over the 7-bit ASCII subset. The locale must be selected at link time.

  • Source files are compiled according to the currently selected locale. You might have to select a different locale, with the --locale command-line option, if the source file contains non-ASCII characters. See Compiler command-line options listed by group in Using the Compiler for more information.

  • The compiler supports multibyte character sets, such as Unicode.

  • Other properties of the source character set are host-specific.

The properties of the execution character set are target-specific. The ARM C and C++ libraries support the ISO 8859-1 (Latin-1 Alphabet) character set with the following consequences:

  • The execution character set is identical to the source character set.

  • There are eight bits in a character in the execution character set.

  • There are four characters (bytes) in an int. If the memory system is:


    The bytes are ordered from least significant at the lowest address to most significant at the highest address.


    The bytes are ordered from least significant at the highest address to most significant at the lowest address.

  • In C all character constants have type int. In C++ a character constant containing one character has the type char and a character constant containing more than one character has the type int. Up to four characters of the constant are represented in the integer value. The last character in the constant occupies the lowest-order byte of the integer value. Up to three preceding characters are placed at higher-order bytes. Unused bytes are filled with the NULL (\0) character.

  • Table 31 lists all integer character constants, that contain a single character or character escape sequence, are represented in both the source and execution character sets.

    Table 31. Character escape codes
    Escape sequenceChar valueDescription
    \a 7Attention (bell)
    \b 8Backspace
    \t 9Horizontal tab
    \n10New line (line feed)
    \v11Vertical tab
    \f12Form feed
    \r13Carriage return
    \xnn0xnnASCII code in hexadecimal
    \nnn0nnnASCII code in octal

  • Characters of the source character set in string literals and character constants map identically into the execution character set.

  • Data items of type char are unsigned by default. They can be explicitly declared as signed char or unsigned char:

    • the --signed_chars option can be used to make the char signed

    • the --unsigned_chars option can be used to make the char unsigned.


    Care must be taken when mixing translation units that have been compiled with and without the --signed_chars and --unsigned_chars options, and that share interfaces or data structures.

    The ARM ABI defines char as an unsigned byte, and this is the interpretation used by the C++ libraries supplied with the ARM compilation tools.

  • No locale is used to convert multibyte characters into the corresponding wide characters for a wide character constant. This is not relevant to the generic implementation.