Hi all, To fully understand the purpose of my e-mail, you shall know what my goal is: "produce reproducibly identical binaries and libraries on all my development computers" (ie.: produce binaries and libraries which MD5 checksum will be the same on all my development computers). Here is a light description of my environment: Computer 1: - hostname: dev1 - user: userX - home: /home/userX - OS: RHEL4-U4 Computer 2: - hostname: dev2 - user: userY - home: /home/userY - OS: RHEL4-U4 On both computers, packages set installed are equivalent (cloned). Only user names used for login differs from one computer to the other (will be important, considering static libraries checksum). Now the full version of my e-mail object: "Why does gcc output (MD5 checksum) differs when I build a binary using the project object files (*.o) or the project convenience libraries (*.a)?". In my peregrinations, first of all, I realized that the first source of variable checksum was the non-systematic use of "-fPIC -DPIC" in gcc options for object files generation. Therefore, I configured my project with "--with-pic" too force the use of PIC on each object generated. The result was as expected: all object files produced, whether on dev1 or on dev2 are identical (same md5sum). Nevertheless, the generated binary is finally still different on dev1 and dev2. I looked at the convenience libraries I produced and discovered that none of them was identical between dev1 and dev2 one. A quick edit of the produced ar archives and a little reading on http://en.wikipedia.org/wiki/Ar_%28Unix%29 gave me the explanation of all that. Ar, as tar also do, stores various info on objects archived: such as its modification timestamp, its uid, its gid, etc. Therefore, AFAK, in any case, convenience libraries could be produced two times successively with the same MD5 checksum (i.e.: if object files are also re-generated, timestamps will change MD5 checksum). Therefore, to understand what was going on in there, I extracted all ar archive contents in one directory and I checked all my object files. All object files extracted from convenience libraries were the same as the one spread over my project directory. Good point, but it was also what I expected. Therefore I tried a little workaround to reach my goal: extract all object files contained in my convenience libraries and build my binary from these extracted object files. And here are the results obtained on both computers: dev1: - Checksum of binary built from *.a: fda85686a499d273596d33b661a01913 - Checksum of binary built from *.o: 9e1525d404db45bf920e54a8728313eb dev2: - Checksum of binary built from *.a: 5f8c6a658343741f1a251f00ab60a9c3 - Checksum of binary built from *.o: 9e1525d404db45bf920e54a8728313eb Then, when building my binary with object files, both generated binaries are identical on dev1 and dev2. But if I use convenience libraries as input, generated binaries are different. So, to put it in a nutshell, all my generated objects file are identical on dev1 and dev2 and object files contained in my convenience libraries are all identical. The only difference remaining, before I generate my binary, resides in the generated convenience libraries which are not identical, but their contents are. So AFAK, this slight difference shall not make the difference. So "why does gcc output (MD5 checksum) differs when I build a binary using the project object files (*.o) or the project convenience libraries (*.a)?" and "what can I do to fix that?". Here are the commands I executed on dev1 and dev2 to generate my binaries, respectively, with object files (*.o) or convenience libraries (*.a) as inputs: gcc -Wall -Wextra -Ulinux -Dlinux=linux -D_GNU_SOURCE -DNDEBUG \ -o my_binary *.o -L/opt/snmp/lib /opt/snmp/lib/libnetsnmpmibs.so \ /usr/lib/librpm.so -L/usr/src/build/757196-i386/install/usr/lib \ -L/usr/lib /usr/lib/librpmdb.so -L/usr/local/lib -lelf -lselinux \ /usr/lib/librpmio.so /usr/lib/libbeecrypt.so -lbz2 /usr/lib/libpopt.so \ /opt/snmp/lib/libnetsnmphelpers.so /opt/snmp/lib/libnetsnmpagent.so \ /opt/snmp/lib/libnetsnmp.so -lcrypto /usr/lib/libccgnu2.so \ /usr/lib/libccext2.so -lrt -lxerces-c -ldl /usr/lib/libxml2.so -lpthread \ -lz -lm -lcib -lplumb -lcrmcommon -lhbclient -lc -Wl,--rpath \ -Wl,/opt/snmp/lib -Wl,--rpath -Wl,/opt/snmp/lib gcc -Wall -Wextra -Ulinux -Dlinux=linux -D_GNU_SOURCE -DNDEBUG \ -o my_binary -L/opt/snmp/lib /opt/snmp/lib/libnetsnmpmibs.so \ /usr/lib/librpm.so -L/usr/src/build/757196-i386/install/usr/lib \ -L/usr/lib /usr/lib/librpmdb.so -L/usr/local/lib -lelf -lselinux \ /usr/lib/librpmio.so /usr/lib/libbeecrypt.so -lbz2 /usr/lib/libpopt.so \ /opt/snmp/lib/libnetsnmphelpers.so /opt/snmp/lib/libnetsnmpagent.so \ /opt/snmp/lib/libnetsnmp.so -lcrypto /usr/lib/libccgnu2.so \ /usr/lib/libccext2.so -lrt -lxerces-c -ldl /usr/lib/libxml2.so -lpthread \ -lz -lm -lcib -lplumb -lcrmcommon -lhbclient *.a -lc -Wl,--rpath \ -Wl,/opt/snmp/lib -Wl,--rpath -Wl,/opt/snmp/lib Rem: These commandlines originally comes from my autotools Makefile. Thank you, in advance, for your help. Kind Regards, -- Alexandre BEAUGY Product Engineer at Egis Avia (Toulouse, France)
