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I'm trying to find out when errno changes.

At first, I tried "watch errno" in gdb, which led to the error

Cannot find thread-local variables on this target
I was able to fix this by compiling with "-pthread". However, it still doesn't work and I now get the error
Cannot find shared library `/usr/lib/debug/lib/x86_64-linux-gnu/libc-2.13.so' in dynamic linker's load module list
when I type "watch errno". What do I need to do such that setting a watchpoint on errno works?
errno is not just a static variable anymore. Here's how it appears to userland apps on Linux (from my local /usr/include/x86_64-linux-gnu/bits/errno.h):
#   define errno (*__errno_location ())
This is to get error state per thread.
                Thanks, I now added "int *errno_p = __errno_location()" as the first statement in main to my program, and can now use "watch *errno_p" to detect when errno changes. Directly using "watch *__errno_location()" did not work for some reason. Are watchpoints on functions not allowed? Probably it is forbidden because it would lead to false behavior when they have sideeffects.
– Hermann Speiche
                May 18, 2012 at 21:41
                Maybe something changed over the years, but watch *(int*)__errno_location() works fine, as long as you set it after glibc initialization (e.g. b main) and in the right thread (since the returned address is thread-local)
– fzbd
                Dec 28, 2020 at 14:48
        
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