GCC -g vs -g3 GDB Flag: What is the Difference?
tl;dr: To answer your specific question, -g3 "includes extra information such as macro definitions... Some debuggers support macro expansion when you use -g3", while -g does not include this extra information.
The broader answer is that gcc supports four levels of debug information, from -g0 (debug information disabled) through -g3 (maximum debug information).
Specifying -g is equivalent to -g2. Curiously, the gcc docs say little about what information -g/-g2 includes or excludes:
Request debugging information and also use level to specify how much information. The default level is 2. Level 0 produces no debug information at all. Thus, -g0 negates -g.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.
From the docs:
-g
Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF 2). GDB can work with this debugging information. On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
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-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF 2, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.
-gvmslevel
Request debugging information and also use level to specify how much information. The default level is 2. Level 0 produces no debug information at all. Thus, -g0 negates -g.
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Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.