If a process ignores signals by setting one to SIG_IGN instead of SIG_DFL, it will propagate to processes across exec*() system calls. If a process sets an explicit signal handler, it will be reset to the default. This is a demonstration program of this.
The parent process prints the results from sigaction(), then calls sigaction(SIGINT, ...) to set the action to SIG_IGN. It then executes itself again, which prints the status in child.
parent starting (num_args=1)
parent before sigint_sigaction.sa_sigaction=0 (SIG_IGN=1, SIG_DFL=0)
parent before sigint_sigaction.sa_flags: 0
parent called sigaction to set SIGINT to SIG_IGN; spawning child ...
child starting (num_args=2)
child before sigint_sigaction.sa_sigaction=1 (SIG_IGN=1, SIG_DFL=0)
child before sigint_sigaction.sa_flags: 0
child exiting
parent exiting
Note that the parent sees sigint_sigaction.sa_sigaction=0 == SIG_DFL but the child sees sigint_sigaction.sa_sigaction=1 == SIG_IGN.
This is documented, but it the man pages must be read very carefully.
man 2 execve:
All process attributes are preserved during an execve(), except the following:
* The dispositions of any signals that are being caught are reset to the default (signal(7)).
The key words there are "signals that are being caught". Ignored signals are not being caught.
man 7 signal:
A child created via fork(2) inherits a copy of its parent's signal dispositions. During an
execve(2), the dispositions of handled signals are reset to the default; the dispositions of
ignored signals are left unchanged.
The key part here is "the dispositions of ignored signals are left unchanged".