Add the packed attribute "__attribute__ ((packed))" to where you are instantiating these structures, if not the structures themselves. This should override the aligning that is automatically done on it. > -----Original Message----- > From: gcc-help-owner@xxxxxxxxxxx [mailto:gcc-help-owner@xxxxxxxxxxx] On > Behalf Of Alexey Neyman > Sent: Sunday, July 13, 2008 2:28 PM > To: gcc-help@xxxxxxxxxxx > Subject: Alignment of large structures in GCC > > Hi, > > I ran into the following problem using gcc: I am using some structures, > which are put into a dedicated section. The linker concatenates these > sections from all files; I have a linker script which assigns symbols > to the start and end of this section. When I need to traverse all these > structures, I then use the following loop: > > struct somename *p; > for (p = &__start_section; p < &__end_section; p++) { > ... > > All worked well when the size of the structure was below 32 bytes. When > I added an additional field, GCC suddenly started aligning each > structure to 32 bytes - so the structures in this section are padded to > 32-byte boundary. As the size of the structure is 36 bytes, though, the > loop above breaks on the 2nd element: it tries to access it at > &__start_section + 36, while the structure is actually at > &__start_section + 64. > > I narrowed it down to the following example: > > <<< > struct { > int xxx[NINT]; > } aaa __attribute__((section(".foo"))); > <<<< > > When compiled, GCC selects the following alignments: > > $ gcc -o - -S gg.c -DNINT=7 | grep align > .align 4 > $ gcc -o - -S gg.c -DNINT=8 | grep align > .align 32 > $ gcc -o - -S gg.c -DNINT=9 | grep align > .align 32 > > That's especially strange since __alignof__ reports the alignment of > this structure as 4. It seems natural that the size of the structure > should be a multiple of its alignment. > > For now, I circumvented it by adding __attribute__((aligned(4))) to > these structures. However, it may not be good if this structure gets a > new member which would have a 8-byte alignment. > > The question is, why does GCC perform such 32-byte alignment and is it > possible to turn off such behavior globally? > > P.S. GCC version: > > $ gcc -v > Using built-in specs. > Target: i386-redhat-linux > Configured with: ../configure --prefix=/usr --mandir=/usr/share/man > --infodir=/usr/share/info --enable-shared --enable-threads=posix > --enable-checking=release --with-system-zlib --enable-__cxa_atexit > --disable-libunwind-exceptions > --enable-languages=c,c++,objc,obj-c++,java,fortran,ada > --enable-java-awt=gtk --disable-dssi --enable-plugin > --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre > --enable-libgcj-multifile --enable-java-maintainer-mode > --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-cpu=generic > --host=i386-redhat-linux > Thread model: posix > gcc version 4.1.2 20070925 (Red Hat 4.1.2-33) > > Best regards, > Alexey.
