C/bash: signal one process on the death of another - c

Question

Think of this as a continuation of the good advice here:
https://stackoverflow.com/a/56780616/16739703
except that I am hoping not to modify the child process.
Edit: I have written code which minimises to:
#include <errno.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
int main(int argc, char *argv[], char *envp[]) {
int init_flags=fcntl(0, F_GETFL, 0);
if (fcntl(0, F_SETFL, init_flags | O_ASYNC)) {
perror("fcntl...F_SET_FL....O_ASYNC");
exit(1);
}
if (fcntl(0, F_SETOWN, getpid())) {
perror("fcntl...F_SETOWN...)");
exit(1);
}
if (execve(argv[1], argv+1, envp)) {
perror("execve");
exit(1);
}
return 1;
}
and this makefile:
all: morehup
CFLAGS=-g -D_GNU_SOURCE
LDFLAGS=-g
so that, with this procedure:
parent> export TMPDIR="$(mktemp -d)"
parent> mkfifo $TMPDIR/fifo
parent> sh
# you get a new shell, probably with a different prompt
parent> exec 7<>$TMPDIR/fifo
# must be both input and output, or the process stalls
child> TMPDIR=... # as other shell
child> ./morehup <$TMPDIR/fifo /bin/sh -c "while true; do date; sleep 5; done"
# you get a list of dates
parent> exit
child> I/O possible # followed by a prompt, with no more dates
the kernel will kill the child when the parent exits.
The more configurable version is here:
https://github.com/JamesC1/morehup/blob/main/morehup.c
I have two questions:
What are the chances of adding modest amounts of code, so that this will mostly work for most of the common *nix?
Is there a posix utility that already does something like this? ie am I reinventing the wheel, and if so, what is it called?