GCC- what does "-undefined dynamic_lookup" do?

Question

I'm reading the open-source code for RBENV, and I came across this line of code. When I generated the Makefile using the configure script and ran make, I saw the following:

$ make
gcc -fno-common     -c -o realpath.o realpath.c
gcc -dynamiclib -dynamic -undefined dynamic_lookup   -o ../libexec/rbenv-realpath.dylib realpath.o

I wanted to know what each of these commands is doing, so I started Googling each of the flags. I found many of them, but -undefined dynamic_lookup initially stumped me. Eventually I found this Github issue, which contained the following sentence:

...the option -undefined dynamic_lookup must be passed to the linker to indicate that unresolved symbols will be resolved at runtime.

This makes sense to me, given what I know about the purpose of this particular Makefile (it allows RBENV to use a faster, more performant version of the realpath program as a dynamic library).

However, I was unable to confirm this independently, since I don't see any official GCC docs which describe the -undefined flag or the dynamic_lookup option. I Googled gcc dynamic_lookup site:gnu.org, but the only results which turned up were Bugzilla tickets, email threads about patches, etc. I also searched man gcc and help gcc in my terminal, but got No manual entry for gcc as a response.

Questions:

Is the Github issue's description of -undefined dynamic_lookup correct?
Am I going crazy, or is -undefined dynamic_lookup actually missing from the gcc docs?

`man gcc` ... `-undefined ... These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.`. — n. m. could be an AI, Dec 03 '22 at 14:03

score 1 · Accepted Answer · answered Dec 04 '22 at 14:10

Turns out I was getting the No manual entry for gcc error because "gcc isn't installed anymore by Xcode, it really installs clang and calls it gcc". See this StackOverflow post and this answer for more info. Once I realized that, I Googled "man gcc" and found the docs from man7.org.

As user @n.m. mentions in the comment above, the -undefined option (along with many others) is forwarded to the Darwin linker, therefore it is those docs that I should have been looking at. Once I found the Darwin linker docs, I found the info I was looking for:

undefined < treatment >

Specifies how undefined symbols are to be treated. Options are: error, warning, suppress, or dynamic_lookup. The default is error.

GCC- what does "-undefined dynamic_lookup" do?

1 Answers1