What happens during a “relocation has invalid symbol index” error?

Question

Here is a test reproducing the problem:

$ echo \"void whatever() {}\" > prog.c
$ gcc prog.c

This produces the following error on GCC 4.8.4:<

Answer 1

We can break this down into two parts:

Undefined reference to `main'

/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status

This is simply because the C Runtime library (crt1.o) is trying to call your (missing) main() function. There is a good overview of the various C Runtime files here / here.

Relocation X has invalid symbol index Y

Note: This has been a bit of mission (learning opportunity) for me - I've taken a few days to research and understand in the limited free time that I have. It's something that I've wondered about for a long time but never looked into... Hopefully my understanding is correct (though clearly not complete - I'll update it if I can).

This is a little more complex, and hidden away.

Just by reading the message, we can gleen that it's related to debug information (the .debug_info and .debug_line sections of crt1.o's debug file is mentioned). Note the /usr/lib/debug/ path, which just contains debug information, the other crt1.o is a "stripped" file...

The format string is found in the binutils project, specifically bfd/elfcode.h. BFD being Binary File Descriptor - the GNU way to handle object files across a number of system architectures.

BFD is an intermediate format used for binary files - GCC will employ BFD before it finally writes an a.out, ELF, or other binary.

Looking into the manual, we can find some interesting snippets of knowledge:

[...] with each entry in the hash table the a.out linker keeps the index the symbol has in the final output file (this index number is used so that when doing a relocateable link the symbol index used in the output file can be quickly filled in when copying over a reloc). [source]

The standard records contain only an address, a symbol index, and a type field. [source]

This means that these errors are issued due to relocations that are related to specific (missing?) 'symbols'. A 'symbol' in this context is any named 'thing' - e.g: functions and variables.

As these 'invalid symbols' appear to be resolved by simply declaring main(), I would guess that some (all?) of these symbol indexes are derived from main(), its debug information, and/or relations.

I can't tell you what is supposed to be at the symbol indexes mentioned (2, 11, 12, 13, 21), but it is interesting that my tests yielded the same list of symbol indexes.

Running ld with crt1.o alone gives us similar output:

$ ld /usr/lib/x86_64-linux-gnu/crt1.o
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 0 has invalid symbol index 11
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 1 has invalid symbol index 12
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 2 has invalid symbol index 2
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 3 has invalid symbol index 2
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 4 has invalid symbol index 11
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 5 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 6 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 7 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 8 has invalid symbol index 12
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 9 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 10 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 11 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 12 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 13 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 14 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 15 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 16 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 17 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 18 has invalid symbol index 13
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 19 has invalid symbol index 21
ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_line): relocation 0 has invalid symbol index 2
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x12): undefined reference to `__libc_csu_fini'
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x19): undefined reference to `__libc_csu_init'
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x25): undefined reference to `__libc_start_main'