I think it was first introduced in 4.3 BSD Tahoe (released June 15, 1988): https://www.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Tahoe/usr/src/lib/libc/stdio/doprnt

I think it was first introduced in 4.3 BSD Tahoe (released June 15, 1988): https://www.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Tahoe/usr/src/lib/libc/stdio/doprnt.c https://www.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Tahoe/usr/... This was an update to the earlier 4.3 BSD (1986) which still implemented printf() in VAX assembly instead, and doesn't support the %n feature. So %n may have originally been implemented in 4.3 BSD Tahoe and made its way into SVR4 subsequently.