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Top Forums UNIX for Dummies Questions & Answers Compiling with debugger flag -g fixes SEGfault (fortran90) Post 302481526 by Corona688 on Friday 17th of December 2010 07:57:01 PM
Old 12-17-2010
Quote:
Originally Posted by drbones
So the title kinda says it all. I was getting a SEGfault, so I decided to compile with the -g option to find where, and low and behold the SEGfault doesn't occur.

I suppose the answer is "Problem solved! You fixed yet another SEGfault." But I am very curious how this could have happened.
Enabling debugging likely disabled some optimizations, altering the way the code was generated and perhaps the pattern of memory use.

Imagine you're using an uninitialized variable. It may just happen to always be zero when the code isn't optimized since it wasn't used before. But optimize it and it decides it doesn't need a variable at all and puts it in a register! Suddenly you're using an uninitialized register, which could be anything, and it crashes...

Or you're overflowing the end of an array, and the arrangement of memory is different when the executable's optimized. When it's not optimized, you might be corrupting empty space and not care. When it's optimized, you could be munging something important, like pointers to somewhere else, or even your stack frame...

Can you debug it when it's not compiled for debugging? The information you get might be limited, but limited's better than nothing.
 

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OCAMLDEBUG(1)						      General Commands Manual						     OCAMLDEBUG(1)

NAME
ocamldebug - the OCaml source-level replay debugger. SYNOPSIS
ocamldebug [ options ] program [ arguments ] DESCRIPTION
ocamldebug is the OCaml source-level replay debugger. Before the debugger can be used, the program must be compiled and linked with the -g option: all .cmo and .cma files that are part of the program should have been created with ocamlc -g, and they must be linked together with ocamlc -g. Compiling with -g entails no penalty on the running time of programs: object files and bytecode executable files are bigger and take longer to produce, but the executable files run at exactly the same speed as if they had been compiled without -g. OPTIONS
A summary of options are included below. For a complete description, see the html documentation in the ocaml-doc package. -c count Set the maximum number of simultaneously live checkpoints to count. -cd dir Run the debugger program from the working directory dir, instead of the current working directory. (See also the cd command.) -emacs Tell the debugger it is executed under Emacs. (See The OCaml user's manual for information on how to run the debugger under Emacs.) -I directory Add directory to the list of directories searched for source files and compiled files. (See also the directory command.) -s socket Use socket for communicating with the debugged program. See the description of the command set socket in The OCaml user's manual for the format of socket. -version Print version string and exit. -vnum Print short version number and exit. -help or --help Display a short usage summary and exit. SEE ALSO
ocamlc(1) The OCaml user's manual, chapter "The debugger". AUTHOR
This manual page was written by Sven LUTHER <luther@debian.org>, for the Debian GNU/Linux system (but may be used by others). OCAMLDEBUG(1)
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