gdb.info: Source

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Examining Source Files

GDB can print parts of your program's source, since the debugging
information recorded in the program tells GDB what source files were
used to build it.  When your program stops, GDB spontaneously prints
the line where it stopped.  Likewise, when you select a stack frame
(*note Selecting a frame: Selection.), GDB prints the line where
execution in that frame has stopped.  You can print other portions of
source files by explicit command.
   If you use GDB through its GNU Emacs interface, you may prefer to
use Emacs facilities to view source; see *Note Using GDB under GNU
Emacs: Emacs.