Using pipes works, but is unnecessarily thick
for a one-shot application like this,
and leaves 2 largely-useless pipe fds in the process permanently

sem_post(3) is explicitly allowed to be used in signal handlers,
this is practically the target application of POSIX semaphores

readme_example/glibc before:
  3735433 rt_sigaction(SIGBUS, {sa_handler=0x56412e4350e0, sa_mask=[], sa_flags=SA_RESTORER|SA_ONSTACK|SA_SIGINFO, sa_restorer=0x7f57bb5dd050}, NULL, 8) = 0
  3735433 pipe2([3, 4], O_CLOEXEC)        = 0
  3735433 fcntl(4, F_SETFL, O_RDONLY|O_NONBLOCK) = 0
  3735433 rt_sigaction(SIGINT, {sa_handler=0x56412e40b410, sa_mask=[], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7f57bb5dd050}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
  3735433 rt_sigaction(SIGRT_1, {sa_handler=0x7f57bb627720, sa_mask=[], sa_flags=SA_RESTORER|SA_ONSTACK|SA_RESTART|SA_SIGINFO, sa_restorer=0x7f57bb5dd050}, NULL, 8) = 0
  3735433 rt_sigprocmask(SIG_UNBLOCK, [RTMIN RT_1], NULL, 8) = 0
  3735433 mmap(NULL, 2101248, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0x7f57bb39d000
  3735433 mprotect(0x7f57bb39e000, 2097152, PROT_READ|PROT_WRITE) = 0
  3735433 rt_sigprocmask(SIG_BLOCK, ~[], [], 8) = 0
  3735433 clone3({flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, child_tid=0x7f57bb59d990, parent_tid=0x7f57bb59d990, exit_signal=0, stack=0x7f57bb39d000, stack_size=0x1fff00, tls=0x7f57bb59d6c0} => {parent_tid=[3735438]}, 88) = 3735438
  3735438 rseq(0x7f57bb59dfe0, 0x20, 0, 0x53053053 <unfinished ...>
  3735433 rt_sigprocmask(SIG_SETMASK, [],  <unfinished ...>
  3735438 <... rseq resumed>)             = 0
  3735433 <... rt_sigprocmask resumed>NULL, 8) = 0
  3735438 set_robust_list(0x7f57bb59d9a0, 24) = 0
  3735433 write(1, "Waiting for Ctrl-C...\n", 22 <unfinished ...>
  3735438 rt_sigprocmask(SIG_SETMASK, [],  <unfinished ...>
  3735433 <... write resumed>)            = 22
  3735438 <... rt_sigprocmask resumed>NULL, 8) = 0
  3735438 mmap(NULL, 134217728, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0 <unfinished ...>
  3735433 futex(0x56415f72c028, FUTEX_WAIT_BITSET_PRIVATE, 4294967295, NULL, FUTEX_BITSET_MATCH_ANY <unfinished ...>
  3735438 <... mmap resumed>)             = 0x7f57b339d000
  3735438 munmap(0x7f57b339d000, 12988416) = 0
  3735438 munmap(0x7f57b8000000, 54120448) = 0
  3735438 mprotect(0x7f57b4000000, 135168, PROT_READ|PROT_WRITE) = 0
  3735438 sched_getaffinity(3735438, 32, [0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23]) = 8
  3735438 sigaltstack(NULL, {ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=0}) = 0
  3735438 mmap(NULL, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0x7f57bb7b5000
  3735438 mprotect(0x7f57bb7b5000, 4096, PROT_NONE) = 0
  3735438 sigaltstack({ss_sp=0x7f57bb7b6000, ss_flags=0, ss_size=8192}, NULL) = 0
  3735438 prctl(PR_SET_NAME, "ctrl-c")    = 0
  3735438 read(3,  <unfinished ...>
  3735433 <... futex resumed>)            = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  3735433 --- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
  3735433 write(4, "\0", 1)               = 1
  3735438 <... read resumed>"\0", 1)      = 1
  3735433 rt_sigreturn({mask=[]} <unfinished ...>
  3735438 futex(0x56415f72c028, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
  3735433 <... rt_sigreturn resumed>)     = 202
  3735438 <... futex resumed>)            = 0
  3735433 futex(0x56415f72c028, FUTEX_WAIT_BITSET_PRIVATE, 4294967295, NULL, FUTEX_BITSET_MATCH_ANY <unfinished ...>
  3735438 read(3,  <unfinished ...>
  3735433 <... futex resumed>)            = -1 EAGAIN (Resource temporarily unavailable)
  3735433 write(1, "Got it! Exiting...\n", 19) = 19
  3735433 sigaltstack({ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=8192}, NULL) = 0
  3735433 munmap(0x7f57bb7b8000, 12288)   = 0
  3735433 exit_group(0)                   = ?
  3735438 <... read resumed> <unfinished ...>) = ?
  3735438 +++ exited with 0 +++
  3735433 +++ exited with 0 +++

After:
  3864619 rt_sigaction(SIGBUS, {sa_handler=0x55d614829ed0, sa_mask=[], sa_flags=SA_RESTORER|SA_ONSTACK|SA_SIGINFO, sa_restorer=0x7f8750f22050}, NULL, 8) = 0
  3864619 rt_sigaction(SIGINT, {sa_handler=0x55d6148008c0, sa_mask=[], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7f8750f22050}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
  3864619 rt_sigaction(SIGRT_1, {sa_handler=0x7f8750f6c720, sa_mask=[], sa_flags=SA_RESTORER|SA_ONSTACK|SA_RESTART|SA_SIGINFO, sa_restorer=0x7f8750f22050}, NULL, 8) = 0
  3864619 rt_sigprocmask(SIG_UNBLOCK, [RTMIN RT_1], NULL, 8) = 0
  3864619 mmap(NULL, 2101248, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0x7f8750ce2000
  3864619 mprotect(0x7f8750ce3000, 2097152, PROT_READ|PROT_WRITE) = 0
  3864619 rt_sigprocmask(SIG_BLOCK, ~[], [], 8) = 0
  3864619 clone3({flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, child_tid=0x7f8750ee2990, parent_tid=0x7f8750ee2990, exit_signal=0, stack=0x7f8750ce2000, stack_size=0x1fff00, tls=0x7f8750ee26c0} => {parent_tid=[3864624]}, 88) = 3864624
  3864624 rseq(0x7f8750ee2fe0, 0x20, 0, 0x53053053 <unfinished ...>
  3864619 rt_sigprocmask(SIG_SETMASK, [],  <unfinished ...>
  3864624 <... rseq resumed>)             = 0
  3864619 <... rt_sigprocmask resumed>NULL, 8) = 0
  3864624 set_robust_list(0x7f8750ee29a0, 24) = 0
  3864619 write(1, "Waiting for Ctrl-C...\n", 22 <unfinished ...>
  3864624 rt_sigprocmask(SIG_SETMASK, [],  <unfinished ...>
  3864619 <... write resumed>)            = 22
  3864624 <... rt_sigprocmask resumed>NULL, 8) = 0
  3864624 mmap(NULL, 134217728, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0 <unfinished ...>
  3864619 futex(0x55d64f9de028, FUTEX_WAIT_BITSET_PRIVATE, 4294967295, NULL, FUTEX_BITSET_MATCH_ANY <unfinished ...>
  3864624 <... mmap resumed>)             = 0x7f8748ce2000
  3864624 munmap(0x7f8748ce2000, 53600256) = 0
  3864624 munmap(0x7f8750000000, 13508608) = 0
  3864624 mprotect(0x7f874c000000, 135168, PROT_READ|PROT_WRITE) = 0
  3864624 sched_getaffinity(3864624, 32, [0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23]) = 8
  3864624 sigaltstack(NULL, {ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=0}) = 0
  3864624 mmap(NULL, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0x7f87510fa000
  3864624 mprotect(0x7f87510fa000, 4096, PROT_NONE) = 0
  3864624 sigaltstack({ss_sp=0x7f87510fb000, ss_flags=0, ss_size=8192}, NULL) = 0
  3864624 prctl(PR_SET_NAME, "ctrl-c")    = 0
  3864624 futex(0x55d614867988, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 0, NULL, FUTEX_BITSET_MATCH_ANY <unfinished ...>
  3864619 <... futex resumed>)            = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  3864619 --- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
  3864619 futex(0x55d614867988, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
  3864624 <... futex resumed>)            = 0
  3864619 <... futex resumed>)            = 1
  3864624 futex(0x55d64f9de028, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
  3864619 rt_sigreturn({mask=[]} <unfinished ...>
  3864624 <... futex resumed>)            = 0
  3864624 futex(0x55d614867988, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 0, NULL, FUTEX_BITSET_MATCH_ANY <unfinished ...>
  3864619 <... rt_sigreturn resumed>)     = 202
  3864619 futex(0x55d64f9de028, FUTEX_WAIT_BITSET_PRIVATE, 4294967295, NULL, FUTEX_BITSET_MATCH_ANY) = -1 EAGAIN (Resource temporarily unavailable)
  3864619 write(1, "Got it! Exiting...\n", 19) = 19
  3864619 sigaltstack({ss_sp=NULL, ss_flags=SS_DISABLE, ss_size=8192}, NULL) = 0
  3864619 munmap(0x7f87510fd000, 12288)   = 0
  3864619 exit_group(0)                   = ?
  3864624 <... futex resumed>)            = ?
  3864624 +++ exited with 0 +++
  3864619 +++ exited with 0 +++

…ly and officially broken

Increasing the minor version as the implementation of unix side was
basically rewritten by changing pipes to semaphores.