How big of a buffer should I create for the longest possible user or group name on Linux?

What is the maximum number of characters that a user name or group name may be on Linux?

I need to allocate a buffer and would like to know how much space I need to allocate to guarantee it is large enough for whatever group or user name my application might encounter.

Answer

(Putting my comment into an answer now that the question has been reopened)

POSIX specifies that LOGIN_NAME_MAX must be >= _POSIX_LOGIN_NAME_MAX. _POSIX_LOGIN_NAME_MAX, in turn, is defined to 9. On Linux it seems LOGIN_NAME_MAX is 256.

For groups, I don’t think there is anything similar. Some kind of upper bound can be guesstimated via the getgrnam_r() and getgrgid_r() functions, which take a user supplied buffer for the char* entries in struct group. The maximum needed size for this buffer can be retrieved via sysconf(_SC_GETGR_R_SIZE_MAX) or the macro NSS_BUFLEN_GROUP. On Linux, NSS_BUFLEN_GROUP seems to be 1024.