Using the Gnu debugger (gdb) to debug your C++ programs

Here are a few useful gdb commands. You can get even more information about g++ and gdb from the Cygnus Support web page(follow the link to the technical library).

Before trying gdb, make sure you have used both "-Wall" and "-g" in the CFLAGS variable in Makefile. Look at any warnings produced when compiling, and see if they give you any insight into the problem (as a rule, you should not ignore a warning unless I said you could or you understand why the compiler produced it and know you don't have to worry about it).

To start gdb in emacs, type M-x gdb (return) and the program name

To start gdb in an xterm, type "gdb program-name"

gdb commands:

• help gets help
• run runs the program
• quit exits gdb (this is a good idea when you recompile)
• up & down get to calling or called function
• print var prints variable var (this may not work for local variables that are not declared at the start of the function)
• set var = value sets variable value
• info breakpoints gets info about breakpoints
• delete & disable get rid of breakpoints
• cont continues
• step single step (debugger goes into func. calls)
• next steps to next line
• finish finishes the current function

• C-x space sets a breakpoint on current line
• M-p previous command

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