How is gcc adding debug symbols by default?

Help
7

I got to the bottom of it. Thanks @jonringer and @futile for your pointers.

gcc does not add debug information related to the program I compile. The ELF section headers I see are actually from other object files which get linked into the final executable, such as Core OS C Runtime objects (crti.o, crtn.o, etc.).

The following snippet shows that gcc does not indeed add the debug symbols during compilation (-c), unless asked to with -g:

$ cat test.sh
#!/run/current-system/sw/bin/bash

echo "int main() {}" > main.c

test_case() {
    option=$1
    if [[ $# -eq 0 ]]; then
        msg="Without option"
        gcc -c main.c
    else
        msg="With $option"
        gcc $option -c main.c
    fi


    echo -n "$msg: "
    if objdump -h main.o | grep -q .debug; then
        echo "debug symbols"
    else
        echo "no debug symbols"
    fi
}

test_case
test_case -g
test_case -g0
$ ./test.sh 
Without option: no debug symbols
With -g: debug symbols
With -g0: no debug symbols

We can similarly observed that passing -g does increase the size of the .debug_str section from 0x163 to 0x1b5 and that -g0 is the same as not passing anything:

$ gcc main.c && objdump -h a.out | grep .debug_str
 27 .debug_str    00000163  0000000000000000  0000000000000000  00003388  2**0
$ gcc -g main.c && objdump -h a.out | grep .debug_str
 27 .debug_str    000001b5  0000000000000000  0000000000000000  00003481  2**0
$ gcc -g0 main.c && objdump -h a.out | grep .debug_str
 27 .debug_str    00000163  0000000000000000  0000000000000000  00003388  2**0

And when looking at the content of the .debug_str sections, we see the CRT-related information when not passing -g, and the addition of some symbols such as main when passing -g:

$ gcc main.c && objdump -s -j .debug_str a.out                  

a.out:     file format elf64-x86-64

Contents of section .debug_str:
 0000 2e2e2f73 79736465 70732f78 38365f36  ../sysdeps/x86_6
 0010 342f7374 6172742e 53002f62 75696c64  4/start.S./build
 0020 2f676c69 62632d32 2e33312f 63737500  /glibc-2.31/csu.
 0030 474e5520 41532032 2e33312e 3100696e  GNU AS 2.31.1.in
 0040 69742e63 00474e55 20433131 20382e33  it.c.GNU C11 8.3
 0050 2e30202d 6d74756e 653d6765 6e657269  .0 -mtune=generi
 0060 63202d6d 61726368 3d783836 2d363420  c -march=x86-64 
 0070 2d67202d 67676462 202d4f32 202d7374  -g -ggdb -O2 -st
 0080 643d676e 75313120 2d666e6f 2d737472  d=gnu11 -fno-str
 0090 6963742d 6f766572 666c6f77 202d6650  ict-overflow -fP
 00a0 4943202d 66676e75 38392d69 6e6c696e  IC -fgnu89-inlin
 00b0 65202d66 6d657267 652d616c 6c2d636f  e -fmerge-all-co
 00c0 6e737461 6e747320 2d66726f 756e6469  nstants -froundi
 00d0 6e672d6d 61746820 2d666d61 74682d65  ng-math -fmath-e
 00e0 72726e6f 202d666e 6f2d7374 61636b2d  rrno -fno-stack-
 00f0 70726f74 6563746f 72202d66 746c732d  protector -ftls-
 0100 6d6f6465 6c3d696e 69746961 6c2d6578  model=initial-ex
 0110 6563005f 494f5f73 7464696e 5f757365  ec._IO_stdin_use
 0120 64007374 61746963 2d72656c 6f632e63  d.static-reloc.c
 0130 002e2e2f 73797364 6570732f 7838365f  .../sysdeps/x86_
 0140 36342f63 7274692e 53002e2e 2f737973  64/crti.S.../sys
 0150 64657073 2f783836 5f36342f 6372746e  deps/x86_64/crtn
 0160 2e5300                               .S.             
$ gcc -g main.c && objdump -s -j .debug_str a.out

a.out:     file format elf64-x86-64

Contents of section .debug_str:
 0000 2e2e2f73 79736465 70732f78 38365f36  ../sysdeps/x86_6
 0010 342f7374 6172742e 53002f62 75696c64  4/start.S./build
 0020 2f676c69 62632d32 2e33312f 63737500  /glibc-2.31/csu.
 0030 474e5520 41532032 2e33312e 3100696e  GNU AS 2.31.1.in
 0040 69742e63 00474e55 20433131 20382e33  it.c.GNU C11 8.3
 0050 2e30202d 6d74756e 653d6765 6e657269  .0 -mtune=generi
 0060 63202d6d 61726368 3d783836 2d363420  c -march=x86-64 
 0070 2d67202d 67676462 202d4f32 202d7374  -g -ggdb -O2 -st
 0080 643d676e 75313120 2d666e6f 2d737472  d=gnu11 -fno-str
 0090 6963742d 6f766572 666c6f77 202d6650  ict-overflow -fP
 00a0 4943202d 66676e75 38392d69 6e6c696e  IC -fgnu89-inlin
 00b0 65202d66 6d657267 652d616c 6c2d636f  e -fmerge-all-co
 00c0 6e737461 6e747320 2d66726f 756e6469  nstants -froundi
 00d0 6e672d6d 61746820 2d666d61 74682d65  ng-math -fmath-e
 00e0 72726e6f 202d666e 6f2d7374 61636b2d  rrno -fno-stack-
 00f0 70726f74 6563746f 72202d66 746c732d  protector -ftls-
 0100 6d6f6465 6c3d696e 69746961 6c2d6578  model=initial-ex
 0110 6563005f 494f5f73 7464696e 5f757365  ec._IO_stdin_use
 0120 64007374 61746963 2d72656c 6f632e63  d.static-reloc.c
 0130 002e2e2f 73797364 6570732f 7838365f  .../sysdeps/x86_
 0140 36342f63 7274692e 53002f68 6f6d652f  64/crti.S./home/
 0150 6464622f 746d702f 6763632d 64656275  ddb/tmp/gcc-debu
 0160 67006d61 696e006d 61696e2e 6300474e  g.main.main.c.GN
 0170 55204331 3720392e 332e3020 2d6d7475  U C17 9.3.0 -mtu
 0180 6e653d67 656e6572 6963202d 6d617263  ne=generic -marc
 0190 683d7838 362d3634 202d6700 2e2e2f73  h=x86-64 -g.../s
 01a0 79736465 70732f78 38365f36 342f6372  ysdeps/x86_64/cr
 01b0 746e2e53 00                          tn.S.
3 Likes