Simple differences - Missing APIs Platforms often have missing APIs Perhaps the most common difference Some random examples from OpenSSH: arc4random getopt prctl setresuid b64_ntop getpeereid pstat setsid b64_pton _getpty readpassphrase setvbuf bcopy getrlimit realpath sigaction bindresvport_sa getttyent recvmsg snprintf clock glob rresvport_af socketpair closefrom inet_aton sendmsg strerror dirfd inet_ntoa setdtablesize strlcat fchmod inet_ntop setegid strlcpy fchown innetgr setenv strmode freeaddrinfo login_getcapbool seteuid strnvis futimes md5_crypt setgroups strtonum getaddrinfo memmove setlogin strtoll getcwd mkdtemp setpcred strtoul getgrouplist openlog_r setproctitle sysconf getnameinfo openpty setrlimit tcgetpgrp