Q:

GCC -g vs -g3 GDB Flag: What is the Difference?

When compiling C source code with either gcc or Clang, I always use the -g flag to generate debugging information for gdb.
gcc -g -o helloworld helloworld.c
I noticed that some people recommend -g3 instead. What is the difference between the -g and -g3 flags? Also is there a difference between -g and -ggdb?

A:

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).
...

-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.
....
Level 3 includes extra information, such as all the macro definitions
present in the program. Some debuggers support macro expansion when
you use -g3.

A:

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.

