c - GCC is generating binaries filled with zeroes

Question

I'm trying to figure out why the binaries generated by GCC are so large.

Consider this empty program:

int main() {
    return 0;
}

Now I build it with GCC 9.2.1 20190827 (Red Hat 9.2.1-1) and glibc 2.29 without any additional parameters:

gcc -o test test.c

The resulting binary is 21984 bytes (~22 KB). Looking at the generated file with xxd, there are long runs of null-bytes in multiple places:

00000370: 006c 6962 632e 736f 2e36 005f 5f6c 6962  .libc.so.6.__lib
00000380: 635f 7374 6172 745f 6d61 696e 0047 4c49  c_start_main.GLI
00000390: 4243 5f32 2e32 2e35 005f 5f67 6d6f 6e5f  BC_2.2.5.__gmon_
000003a0: 7374 6172 745f 5f00 0000 0200 0000 0000  start__.........
000003b0: 0100 0100 0100 0000 1000 0000 0000 0000  ................
000003c0: 751a 6909 0000 0200 1d00 0000 0000 0000  u.i.............
000003d0: f03f 4000 0000 0000 0600 0000 0100 0000  .?@.............
000003e0: 0000 0000 0000 0000 f83f 4000 0000 0000  .........?@.....
000003f0: 0600 0000 0200 0000 0000 0000 0000 0000  ................
00000400: 0000 0000 0000 0000 0000 0000 0000 0000  ................
<3040 bytes of zeroes>
00000ff0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00001000: f30f 1efa 4883 ec08 488b 05e9 2f00 0048  ....H...H.../..H
<not zeroes>
00001190: f30f 1efa c300 0000 f30f 1efa 4883 ec08  ............H...
000011a0: 4883 c408 c300 0000 0000 0000 0000 0000  H...............
000011b0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
<3632 bytes of zeros>
00001ff0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00002000: 0100 0200 0000 0000 0000 0000 0000 0000  ................
00002010: 011b 033b 3400 0000 0500 0000 10f0 ffff  ...;4...........
<not zeroes>
000020e0: 410e 2842 0e20 420e 1842 0e10 420e 0800  A.(B. B..B..B...
000020f0: 1000 0000 ac00 0000 98f0 ffff 0500 0000  ................
00002100: 0000 0000 0000 0000 0000 0000 0000 0000  ................
<3376 bytes of zeroes>
00002e40: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00002e50: 0011 4000 0000 0000 d010 4000 0000 0000  ..@.......@.....
...

So the resulting binary has around 10 KB, or almost half, of nothing in it.

Looking with size -A, the size is more like what one would expect from a program doing nothing else than returning an exit code:

test  :
section                 size      addr
.interp                   28   4194984
.note.ABI-tag             32   4195012
.note.gnu.build-id        36   4195044
.gnu.hash                 28   4195080
.dynsym                   72   4195112
.dynstr                   56   4195184
.gnu.version               6   4195240
.gnu.version_r            32   4195248
.rela.dyn                 48   4195280
.init                     27   4198400
.text                    373   4198432
.fini                     13   4198808
.rodata                   16   4202496
.eh_frame_hdr             52   4202512
.eh_frame                192   4202568
.init_array                8   4210256
.fini_array                8   4210264
.dynamic                 400   4210272
.got                      16   4210672
.got.plt                  24   4210688
.data                      4   4210712
.bss                       4   4210716
.comment                  44         0
.gnu.build.attributes   4472   4218912
Total                   5991

When cross-compiling for PowerPC using GCC 9.2.0 and musl 1.1.23 it's even worse. Size of the binary grows to 67872 bytes (~67 KB), and looking with xxd, there is a continuous run of 64074 bytes of only zeroes.

Still, size -A reports even smaller sizes for this version:

test  :
section              size        addr
.interp                26   268435796
.note.gnu.build-id     36   268435824
.hash                  36   268435860
.dynsym                64   268435896
.dynstr                39   268435960
.rela.plt              12   268436000
.init                  28   268436012
.text                 496   268436048
.fini                  28   268436544
.eh_frame_hdr          28   268436572
.eh_frame              80   268436600
.init_array             4   268566284
.fini_array             4   268566288
.dynamic              216   268566292
.branch_lt              8   268566508
.got                   12   268566516
.plt                    4   268566528
.data                   4   268566532
.bss                   28   268566536
.comment               17           0
Total                1170

I also tried to compile the program with an old version of GCC which I happened to have handy: GCC 4.7.2 with uClibc 1.0.12. With this combination, the resulting binary is only 4769 bytes (~4 KB), and has no apparent runs of null-bytes in it.

Just to make sure that this doesn't only happen on tiny programs that do nothing, I looked at some real programs that I have cross-compiled with GCC 9.2.0 and musl 1.1.23. For example, tcpdump binary, compiled using -Os and stripped, contains a 32628 byte long continous run of null-bytes. So, why are zeroes trying to consume all of my disk space?

Answer 1

Recent binutils defaults to -z separate-code, which adds additional PT_LOAD segments to the program which need further alignment.

You can override the default like this:

gcc -Wl,-z,noseparate-code -o test test.c

Due to alignment requirements, some zeros will still remain with this change.