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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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DBSYM(8)						    BSD System Manager's Manual 						  DBSYM(8)

NAME
dbsym -- copy kernel symbol table into db_symtab space SYNOPSIS
dbsym [-v] [-b bfdname] kernel DESCRIPTION
dbsym is used to copy the symbol table in a newly linked kernel into the db_symtab array (in the data section) so that the ddb(4) kernel debugger can find the symbols. This program is only used on systems for which the boot program does not load the symbol table into memory with the kernel. The space for these symbols is reserved in the data segment using a config option like: options SYMTAB_SPACE=72000 The size of the db_symtab array (the value of SYMTAB_SPACE) must be at least as large as the kernel symbol table. If insufficient space is reserved, dbsym will refuse to copy the symbol table. To recognize kernel executable format, the -b flag specifies BFD name of kernel. If the -v flag is given, dbsym will print out status information as it is copying the symbol table. Note that debugging symbols are not useful to the ddb(4) kernel debugger, so to minimize the size of the kernel, one should either compile the kernel without debugging symbols (no -g flag) or use the strip(1) command to strip debugging symbols from the kernel before dbsym is used to copy the symbol table. The command strip -d netbsd will strip out debugging symbols. SEE ALSO
strip(1), ddb(4) BSD
November 9, 2001 BSD
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