On running subsync under Fedora 42, I get an immediate core dump:

$ subsync -2000 < *srt > tst.srt
Segmentation fault (core dumped)
$ coredumpctl info
PID: 58955 (subsync)
UID: 1000 (poc)
GID: 1000 (poc)
Signal: 11 (SEGV)
Timestamp: Tue 2025-08-26 13:05:56 BST (6s ago)
Command Line: subsync -2000
Executable: /home/poc/bin/subsync
Control Group: /user.slice/user-1000.slice/[email protected]/app.slice/[email protected]
Unit: [email protected]
User Unit: [email protected]
Slice: user-1000.slice
Owner UID: 1000 (poc)
Boot ID: 9fe75d368226401a89c5dd409335bb2d
Machine ID: cdc7d96c826a4d44911c53889f442c90
Hostname: Bree
Storage: /var/lib/systemd/coredump/core.subsync.1000.9fe75d368226401a89c5dd409335bb2d.58955.1756209956000000.zst (present)
Size on Disk: 23.4K
Message: Process 58955 (subsync) of user 1000 dumped core.

            Stack trace of thread 58955:
            #0  0x00007fbeb8cbef35 __gconv (libc.so.6 + 0x3f35)
            #1  0x00007fbeb8cbe9b5 iconv (libc.so.6 + 0x39b5)
            #2  0x0000000000401b06 utf_readline (/home/poc/bin/subsync + 0x1b06)
            #3  0x0000000000401d02 retiming.isra.0 (/home/poc/bin/subsync + 0x1d02)
            #4  0x0000000000400f3f main (/home/poc/bin/subsync + 0xf3f)
            #5  0x00007fbeb8cbe575 __libc_start_call_main (libc.so.6 + 0x3575)
            #6  0x00007fbeb8cbe628 __libc_start_main@@GLIBC_2.34 (libc.so.6 + 0x3628)
            #7  0x00000000004011e5 _start (/home/poc/bin/subsync + 0x11e5)
            ELF object binary architecture: AMD x86-64