How to generate gcc debug symbol outside the build target? - gcc

Question

I know I can generate debug symbol using -g option. However the symbol is embeded in the target file. Could gcc generate debug symbol outside the result executable/library? Like .pdb file of windows VC++ compiler did.

Answer 1

You need to use objcopy to separate the debug information:
objcopy --only-keep-debug "${tostripfile}" "${debugdir}/${debugfile}"
strip --strip-debug --strip-unneeded "${tostripfile}"
objcopy --add-gnu-debuglink="${debugdir}/${debugfile}" "${tostripfile}"
I use the bash script below to separate the debug information into files with a .debug extension in a .debug directory. This way I can tar the libraries and executables in one tar file and the .debug directories in another. If I want to add the debug info later on I simply extract the debug tar file and voila I have symbolic debug information.
This is the bash script:
#!/bin/bash
scriptdir=`dirname ${0}`
scriptdir=`(cd ${scriptdir}; pwd)`
scriptname=`basename ${0}`
set -e
function errorexit()
{
errorcode=${1}
shift
echo $#
exit ${errorcode}
}
function usage()
{
echo "USAGE ${scriptname} <tostrip>"
}
tostripdir=`dirname "$1"`
tostripfile=`basename "$1"`
if [ -z ${tostripfile} ] ; then
usage
errorexit 0 "tostrip must be specified"
fi
cd "${tostripdir}"
debugdir=.debug
debugfile="${tostripfile}.debug"
if [ ! -d "${debugdir}" ] ; then
echo "creating dir ${tostripdir}/${debugdir}"
mkdir -p "${debugdir}"
fi
echo "stripping ${tostripfile}, putting debug info into ${debugfile}"
objcopy --only-keep-debug "${tostripfile}" "${debugdir}/${debugfile}"
strip --strip-debug --strip-unneeded "${tostripfile}"
objcopy --add-gnu-debuglink="${debugdir}/${debugfile}" "${tostripfile}"
chmod -x "${debugdir}/${debugfile}"

Answer 2

Compile with debug information:
gcc -g -o main main.c
Separate the debug information:
objcopy --only-keep-debug main main.debug
or
cp main main.debug
strip --only-keep-debug main.debug
Strip debug information from origin file:
objcopy --strip-debug main
or
strip --strip-debug --strip-unneeded main
debug by debuglink mode:
objcopy --add-gnu-debuglink main.debug main
gdb main
You can also use exec file and symbol file separatly:
gdb -s main.debug -e main
or
gdb
(gdb) exec-file main
(gdb) symbol-file main.debug
For details:
(gdb) help exec-file
(gdb) help symbol-file
Ref:
https://sourceware.org/gdb/onlinedocs/gdb/Files.html#Files
https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html

Answer 3

NOTE: Programs compiled with high-optimization levels (-O3, -O4) cannot generate many debugging symbols for optimized variables, in-lined functions and unrolled loops, regardless of the symbols being embedded (-g) or extracted (objcopy) into a '.debug' file.
Alternate approaches are
Embed the versioning (VCS, git, svn) data into the program, for compiler optimized executables (-O3, -O4).
Build a 2nd non-optimized version of the executable.
The first option provides a means to rebuild the production code with full debugging and symbols at a later date. Being able to re-build the original production code with no optimizations is a tremendous help for debugging. (NOTE: This assumes testing was done with the optimized version of the program).
Your build system can create a .c file loaded with the compile date, commit, and other VCS details. Here is a 'make + git' example:
program: program.o version.o
program.o: program.cpp program.h
build_version.o: build_version.c
build_version.c:
#echo "const char *build1=\"VCS: Commit: $(shell git log -1 --pretty=%H)\";" > "$#"
#echo "const char *build2=\"VCS: Date: $(shell git log -1 --pretty=%cd)\";" >> "$#"
#echo "const char *build3=\"VCS: Author: $(shell git log -1 --pretty="%an %ae")\";" >> "$#"
#echo "const char *build4=\"VCS: Branch: $(shell git symbolic-ref HEAD)\";" >> "$#"
# TODO: Add compiler options and other build details
.TEMPORARY: build_version.c
After the program is compiled you can locate the original 'commit' for your code by using the command: strings -a my_program | grep VCS
VCS: PROGRAM_NAME=my_program
VCS: Commit=190aa9cace3b12e2b58b692f068d4f5cf22b0145
VCS: BRANCH=refs/heads/PRJ123_feature_desc
VCS: AUTHOR=Joe Developer joe.developer#somewhere.com
VCS: COMMIT_DATE=2013-12-19
All that is left is to check-out the original code, re-compile without optimizations, and start debugging.

Answer 4

Check out the "--only-keep-debug" option of the strip command.
From the link:
The intention is that this option will be used in conjunction with --add-gnu-debuglink to create a two part executable. One a stripped binary which will occupy less space in RAM and in a distribution and the second a debugging information file which is only needed if debugging abilities are required.

Answer 5

No answer so far mentions eu-strip --strip-debug -f <out.debug> <input>.
This is provided by elfutils package.
The result will be that <input> file has been stripped of debug symbols which are now all in <out.debug>.