Debugging with GDB (September 2007)

86 Debugging with GDB

9.4.1.2 C and C++ constants

GDB allows you to express the constants of C and C++ in the following ways:

• Integer constants are a sequence of digits. Octal constants are speciﬁed by a leading

‘0’ (i.e. zero), and hexadecimal constants by a leading ‘0x’ or ‘0X’. Constants can also

end with a letter ‘l’, specifying that the constant should be treated as a long value.

• Floating point constants are a sequence of digits, followed by a decimal point, followed

by a sequence of digits, and optionally followed by an exponent. An exponent is of the

form: ‘e[[+]|-]nnn ’, where nnn is a sequence of digits. The ‘+’ is optional for positive

exponents. A ﬂoating-point constant may also end with a letter ‘f’ or ‘F’, specifying

that the constant should be treated as being of the float (as opposed to the default

double) type; or with a letter ‘l’ or ‘L’, which speciﬁes a long double constant.

• Enumerated constants consist of enumerated identiﬁers, or their integral equivalents.

• Character constants are a single character surrounded by single quotes (’), or a number

or the ordinal value of the corresponding character (usually its ascii value). Within

quotes, the single character may be represented by a letter or by escape sequences,

which are of the form ‘\nnn ’, where nnn is the octal representation of the character’s

ordinal value; or of the form ‘\x ’, where ‘x ’ is a predeﬁned special character—for

example, ‘\n’ for newline.

• String constants are a sequence of character constants surrounded by double quotes (").

Any valid character constant (as described above) may appear. Double quotes within

the string must be preceded by a backslash, so for instance ‘"a\"b’c"’ is a string of

ﬁve characters.

• Pointer constants are an integral value. You can also write pointers to constants using

the C operator ‘&’.

• Array constants are comm a-se parated lists surrounded by braces ‘{’ and ‘}’; for e x-

ample, ‘{1,2,3}’ is a three-element array of integers, ‘{{1,2}, {3,4}, {5,6}}’ is a

three-by-two array, and ‘{&"hi", &"there", &"fred"}’ is a three-element array of

pointers.

9.4.1.3 C++ expressions

GDB expression handling can interpret most C++ expressions.

Warning: GDB can only debug C++ code if you use the proper compiler. Typ-

ically, C++ debugging depends on the use of additional debugging information

in the symb ol table, and thus requires special support. In particular, if your

compiler generates a.out, MIPS ecoff, RS/6000 xcoff, or elf with stabs ex-

tensions to the symbol table, these facilities are all available. (With gnu CC,

you can use the ‘-gstabs’ option to request stabs debugging extensions explic-

itly.) Where the object code format is standard coff or dwarf in elf, on the

other hand, most of the C++ support in GDB do e s not work.

1. Member function calls are allowed; you can use expressions like

count = aml->GetOriginal(x, y)